My Kenwood TL-922A RF PA,  ham linear Power AMP. (and mods)  

I will cover both inspections and modification's. (and myths) and more (this page is UNDER )
Stay out of the Insides, and the LETHAL 3000 volt DC if not a trained tech, for HV certifications (see this  first)  Please be safe 1st. (btw, Lethal means zero 2nd chances!)
Rule #1 Do not run any PA with out a real and proper case ground. (if you float the case the case may go to HV and kill you) ok? is this clear enough? (I use double grounds, line cord and bench)
To use a PA make sure all ground to the shack bench and the 3rd round pin are good all the way to your AC power service entrance. (a shack local ground stake is a good idea)

Photos first: (click to zoom them) RF PA , Power Amplifier.  (some call it just a linear)  Spec.: 2 KW P.E.P.-SSB ! 1KW CW.   wired strapped on the rear for 120VAC.  Circa 1978 manufactured. (no QSK mod.'s)
Basically a HIGH Q, Heathikit, SB220.  "A" version does 10M band
If you must go TUBEs (no $3000 for MOSFET modern low voltage PA, this one is PRIME CUT ! Make sure the fan works first, just after making sure the AC power strapping is set right for your SHACK AC  line voltages..

Near mint.?  mine.

Photos by a  licensed trained and skilled technicians never by granny lacking a Teflon suit /shoes/gloves and face mask.  (HV PPE)
(telephoto Camera telephoto lens zoomed safely) TUBES 3-500z ($240 each at   My guess is RFP buys 12 China tubes, load tests them and throws 1/2 of them away and then warranties them.
Last ones  made real were in USA or France   ,  and ended in 1995 (or near)
This BELOW 1 PHOTO is not mine, but is very close to same.  USA/France tubes best. (not from China direct sales )
DO NOT TOUCH 3000 volts it is 100% lethal every time!

Every time I state  SS this means modern Solid State gear, 1974 and newer, even my Heathkit SB104 fails with all PA's this old ,  on both the  KEY and ALC lines  (and cured)
In careful reflection of all things doing less wins out on my PA. I will do the first 5 mod.'s, (only what is truly needed)

The PA needs no modifications, as many think, for sure using TUBE exciters  . (mixing TUBES with SS gear does not work) {mod 3 and 4 below protect your SS exciter (transceiver)
We call Vacuum tubes , Hollow State (a HAM joke) in England (UK) they call them Valves (Fleming fame) I'm a De Forest guy admirer ,  so valves to me fill the bath tub with water. (they really are electrostatic transconductive modulators, sorry !

To use this PA on my Heathkit SB104 or my ICOM IC7200 it must be modified just a bit. (but not my SB401, tube rig from 1969) What makes Heathkit's so good, super documentation. (and common parts not custom endless as seen on new rigs)
  1. The  Gas/flash over protection mod) HV Glitch resistor?  or this 1amp  HV fuse.  Seen here fuses HVNO, not for me, I USE only GDT/TVS clamp protectors  that are vastly faster and better. .(seen here and below)
  2. Mod Power Surge (in-rush)  is called soft power on  In-Rush fix.? , to protect the old AC main  transformers  (I use a $1 fix for this called IN-RUSH class THERMISTOR) the most easy fix of all is this. (in series in the AC line)
  3. Mod ALC,  is to allow this PA to use ALC on modern SS (Solid State) transceivers. this PA has 70k ohm source impedance that is no good for any SS gear. (the fix is here)
  4. Mod   SOFT KEY( link is mine) is  too protect your SS transceivers,    (My ICOM  spec. is 16v max! this PA reads 118vdc !)  ( there are many expensive Softkey cards sold, but mine is greatly more simple and  fraction of the cost)
  5. Mod. Meter protector is always a good idea, a simple Diode across CAP C65 to ground. (yes for sure do this...) The 1N5408, diode fix.  1kv PIV, 3amps and  a whopping 35cents
  6. RE-CAP, HV section. for sure on 1960s Gear, but not newer rigs (1978) but  risk burning up that rare transformer is real.  My HV caps all test and look perfect, and I think are very top quality Japan factory made. (but can be replaced easy)
  7. Upgrading old tungsten panel lamps with LED, Not needed.
  8. PFC mods are not needed,  if you lost your  Plate Cap  coiled  inductor /parastic resistor,  then make your own. No need for magic Nichrome wire ever.
  9. Hard grounded Grid mods?, never.
There are 2 classes of protection, #1 prevent the PA from internal self cascaded damage (T2 ouch)  from Gas/ARC $$$ and #2 external damage to very expensive to  SS Transceivers and what not. 

History :  
You never want to send 100v or more backed up in to any Sold state transciever. (on any path)
The word solid state (SS) means transistors and diodes of silicon or even germanium.   Heathkit and others, started the SS  era about 1974, SB104 about when I left Vietnam that year,  & mil gear back to 1968) Tubes were gently phased out in TV's (CRT last and HV diodes last too.)
The SONY company made many early SS TVs 1969(TV110) and latter amazing Color Trinitron 1968 !  Sony was a world leader in quality like this then and even better later.
All transistor TVs (CRT excepted and is tube too) began in mostly in 1967 with first color, TV by Motorola. (except CRT and the HV rectifiers/regs) The hard part is doing HV and rectifiers then,  GOOD HV diodes and regulators came later, and were poor (until later)
TVs large back then had NTC thermistors on the power line to protect the TV  from surge. (and are even cheaper now and better)
These facts hold true on HAM radios too.
The POWER  MOSFET changed all that. (and now 48vdc power supplies run it)
Most RIGs in the 70s avoided all this 60s junk parts with  (caps leakage (R-leak) was horrible then as were nasty leaky HV diodes, then, and many have horrible reverse bias leakage !)  ~1975+ year is good year for parts, that do no leak at HV.

Now some cures: AUTOMATIC LEVEL CONTROL (OR a better name ANTI-IMD controls) Tube ALC will fail,  the impedance is dead wrong is why.
If your transceiver puts out less than 140W-PEP, the use of ALC is useless due to the 3-400z pair can not be overdriven , the rule of  HAM AG6K

I made this and it works great !

ALC is dead ! to my ICOM-IC7200 and any modern SS Transceiver
ALC is Automatic Level Control, this is the magic wire from Exciter to the PA that works as a team to keep the PA out of non linear PA tube regions and not create IMD (inter modulation distortion) or Intermod for short.

Problem child #1 ,  see the  signal is adjustable @ VR1 , most old rigs like this expect your exciter to have 1 meg ohm ALC input impedance, back 50 years ago. (SS modern  we see 1000 ohm input Z)   1000 times off and wrong.
 Z means Impedance in Electronics. (AC resistance)
Hark ! , even Heathkit ended that HI -Z inputs design in 1974 SB104 SS. (they cooked there own goose that day, yes progress moves on...)  New ALC spec. bingo. (about 1k ohm inputs not 1meg)
You will see ALC being dead, using (22+10+68k = 100,000 ohms source impedance into 1k ohm, SS transceivers.  The ALC putput should be near, -4vdc but now is  dead as a door nail.

The ALC cure,
is seen below with my ALC card (board) ALC working is a good thing, for sure yelling into the SSB MICROPHONE. (hihi) or in RTTY mode.
Step #1
Turn the POT below VR1 10k max clockwise and let my board do this job. But wait,  this is wrong, my TL922, puts out -28v at  max CW and the book for lies! (my scopes do not lie and is)
Now that my ALC box works I set this pot to -8 to -10v keyed and mic yelled. See slide show just below for scope views (proofs)
Public tube horror #1 is R10  just this one part kills ALC dead to all SS GEAR.  (that 68k R10 tells you this ALC only for TUBE exciters! (now the cure)
If you
had a modern $3000 SS PA, that PA has a vastly smarter ALC built deep into itself, that prevents IMD even if the old Transceiver has dead ALC circuits and can not team up the this new PA. 
( a $3500  ALS1300 cure? MOSFET PA !) It warns you that ALC is not optional. But in this  case its self ALC works much better.
The new PA EVEN checks VSWR and runs higher ALC to protect it self. (and arcing over tuners and band switches sure)

My ALC  PA output (unloaded to a scope, turned VR1 full up is -28vdc (SSB,mic, yelled PTT) It's too hot this , so at the PA ACL pot see this to -8vdc max.
Then I adjust  my DIY home brewed R1 below for any ALC I need to my tranceiver 0 to -8vdc. (tested now and works)

My fix  will stop IMD in its tracks, using a working ALC buffer board (This also worked on my SB200 PA circa 1964  to both my newer SS transceivers and my IC7200 (2008))
All parts from Mouser and the Wall wart here. ( I collect wall warts, all, and find them off old landline telephones at this voltage)

CURE#1 (my cure) I LIKE ALC so I fix them.  (But, I am told by experts below 140watts exciter input we do not need ALC)  I will use a spectrum analyzer to see if I get any IMD at full 100watt excitation.
You can not use a DC output power pack or D2 goes dead.  
I will do this as an external box .
This PA has very little spare room inside. (and keeping it stock is a goal too, "resale value" matters to many)
If you want to use higher voltage AC power packs, (limit is 35v peak, or 24v RMS) I have used 15vac pack found at a flea market, but the reads 19vac and gives me  25vdc my raw rails., the rails must be 19 to 35vdc, mine is 25v.
I just added D4b a "Transorb" TVS Unidirectional, the band goes down to ground on D4b this protect my ua741 chip from the nasty -28v ALC the Kenwood can do. 
TVS can even eat up 100amps for 8.3 miliseconds and are used today on vast devices,  and expanding use and choices. I really like them and trust them.

A quick check of the box, I use : (and super easy to test live or even using 2 dry cell batteries as input to J1 pin with  -1.5v and -9v. Minus pin to jack  J1   and test with meter at J2. Easy peasy lemon squeezy.
To debug this new mod.card, just a DMM meter is all you need, But I also use my scope ,see slide show below.

Case in point. (references)  Looking how other gear works.
ALS1300 (Manual)
The ALS-1300 produces an ALC control voltage of 0 to –4 volts DC. (the meter dial goes to -7v) (and protects itself via both Forward/Reflected power in the amp "D8 and D9 " that creates ALC output)


I have added o-scope views of ALC out, transmitting full power 1000w + (100w input) into my RF dummy load  at this link: (slide show)

TESTING 1,2,3.
The best test for ALC is RTTY mode, as is yelling into a SSB microphone is hard to get consistant results, but with practice you can.(per slide show)
I last adjust the POT. above so ALC in the transciever just starts to wink as I yell in the mic. (all manuals on any transciver state the same or near)
The TL-922 PC suprised me with -28vdc ALC output @70k ohms.  So I reached behind the PA and turned ALC post to -8vdc output at 1000watts TX.
My ICOM 7200 is 3k input resistance at 700uA current so is easy to drive it with my buffer.  (the ratio is 70k to 3k, or 23:1)  this ratio may work direct?,but I'm to scared try this and to allow flashover in to my ICOM.
The bufferworks  now.  100watt in ,
SSB , yelling in mic, this is what you get peak. (see modulation here)  (not yelling and silent gives no RF,  SSB is subpressed carrier.(LSB here) (nothing like "AM" modulation, at all) PEP= Vrms˛ / R  {Vrms is 283v} 
This is RF OUT.

My SB200 does this in RTTY mode.. A nice comparsion to the above.  The above TL922 is  a very powerful amp. 

End  ALC topic. (Canon EOS T5i, all photos by me)

Next up is:
AC POWER LINE IN-RUSH or SURGE:  (this mod. has only 2 purposes, to prevent flex/flux damage inside the T1 and T2, and save the rocker power switch from damage (hard to buy either now)
If say in one year,  you turn on a PA or many makers and models of PA in your shack, you will witness on some, that the transformer 1 in say 20 times makes this huge load GRUNT sound at 60Hz (USA)
That is you hearing SURGE happen. (and is not good for the transformer)  and mostly happens on cheaper low mass (under sized) PA transformers. (hint better ones do not do this at all due to high reluctance cores)

Rumor has it: E-I lamination core transformers used. (and is)
The probem here is called tranformer GRUNT, if you turn any large device , system or PA on with huge heavy giant transformers you will hear this loud grunt  sound and that is not good for the copper wire cores deep inside any transformer (not epoxy potted)
Even 1 in 20 times this happens (it is random or more accurately is the core already magnetically charged)
I call this FLUX-FLEX.   That is my job is to cure that and only that, or at least reduce its bad effects.
Goal 2 is to avoid expensive complex cures, seen and sold everywhere, in the Ham world.  
The most common fix is adding relays. (for soft power on)

The Soft power ON mod. using LOOK MA' NO RELAYS !; no as seen and common the  6 to  22ohm pairs of surge resistors (you only need 1) nor the expensive PCB    and a complex fit inside any packed solid  PA.
No need for 2 resistors ever ! this smart  guy knows this too.!(and no relays either, he uses a SS relay.
Why in the world use 2 resistors?,  there is no ground lugs on the 2 transformers coil primaries.!   (the same current flowing in one AC wire flows out the 2nd wire so there is no need for 2 resistors ever)
Only 1 resistor is needed for 120 or 240 voltage, the current is the same on both AC input legs.

This topic (Mine) is only about Flux-flex and only about  that knee seen below and the huge power surge current, when it happens.
The fact is the surge happens only in 1 to 20 power on acts. (finger at that power rocker switch and that same switch loves to bounce and make FLUX staturation happen so very easy)
YES the power switch can bounce and is not good that.


Will the True In-rush please  stand up, well there are 3 surges,  FLUX/Flex  ,  CAP charging and dead cold high current warm up.
Turning on power at the knee below is big time worse surge on 2kw up commercial gear, a zero crossing detector (a very fancy cure) avoids this knee, (a complex fix, avoided below)
The filament (heaters) do not worry me at all nor the HV CAP bank of caps (adds to 30uF in series Ct) C total.
The knee photos from transformer designers shows what to avoid , happens when the power rocker switch bounces (as all do)
The saturated current will cause bad  tranformer flex, (call it flux flex) actual  copper wires flex inside and wear happens, and is a bad act.
The first rule of curing this surge, is that you can not make it all go away, all you can do is limit the extremes.

A better test would be to measure both T1 and T2 primaries seperate, and to find the flux knee Saturated, next.

I call this Flux flex (saturated core) and is a form of abuse done on purpose, and hard to find.(if it happens at all, not sure it will , only cheap small IE cores do this)
My readings are here and all photos at the end of this document page. (10 camera tries to get the trace clear)
The peak is 100amp, if done correctly all caps at 0vdc. (below is not done at best low cap charge) see photos at the end of this page.

The turns ratio
of T1 is step up 1:29  to 3000 volts. (SSB mode)
The turns ratio to T2 is reverse of that 24 :1 step down to filaments. (filament current is (5v/14.5a {x2 ] 29amp hot) so primary is 24times less  when hot. (1.2amps)  But cold the tungsen is much higher current.
This heater is 14.5x 5v or  77 watt each or 154w both  " a pussy cat !)We know 100watt light bulb uses 13 time more current cold (testing it) so 13x 1.2 amps is 15amp current on the primary side. (stated for scale) "this is Nothing at all to worry"
Next the 3kv T1 side.
The HV side works safer, set rocker to CW mode, for less power turned on. (do so) and less HV. SSB charges the caps to 3kv.
This T1 Transformer secondary sees all 4 caps per phase (full wave) as 50uF , as a short at time zero AC on. (so this is a huge surge but is limited by the tranformer total impedance (input to output)

I want the most simple device, to stop most of the  surge, even 1/2 is good goal.
I like to use this  INRUSH LIMITERS:

Yes if using 240AC line power strapping a different lower ohm  Device will work better, my page here is 120vac,  that and only data for that.  (my 1 goal avoid transformer flex)
Goal # 2 is low voltage drop across RS1, it standby fully hot PA. (I want the device to warm up fast and go to low ohms while in PC standby just turned AC power on;.
 The 5R020 below is not my recommendation it is me testing it and rejecting it .  (the correct part #  is based on you 120/240vac strapping)<< read this and I am USA 120vac (30amp mains "special here")

I will report which best parts match later  and I have 2 newer more accurate current probe's  and one is new HALL sensors.
What I do is ask the maker of the Thermistors for help. The .50KVA would be good Choice but is rated 10ohms starting cold resistance and  0.23ohms 3amps current is good.(6amps max and steady current is not very good and temp 177C max)
The trick here is to find max ohms cold and min. low ohms at 1/2 power, and not overheating the device, but  120vac and 240volt usage is first, I will show 120vac only (I have 30amp service in the shack)ametherm 
The key factor is your PA standby AC line current at 120vac here. ( my PA is 2.2 amps AC line current at CW and 2.2 amps SSB not keyed) key power is est,12amps current. (1200watts:?)  I must choose a part that is best fo r2.2amps in standby.
The prime choice is to get the Thermistor near 1/2 the surge at 50amps and that be 25amps or 5 ohms cold (the engineer at the makers site says go to the lower amp rated device for faster heating time!)
I have on hand,   (Ametherm SL22, 4R014 4 ohms cold and , 0.12ohms  hot and 14 amp rated, 214C max. temp.  <<<  I  think this is good too.  The tricky part is not overheating the device (inrush limiter) (I am still testing it)
The best way to do this is try different ones and be sure the device measured (across it)  goes to a low voltage drop in PA standby.   (2amps must keep it hot enough full  time  and not overheat when transmitting for sure RTTY mode.)
My PA surge is 50amps, and standby current is 2.2amps,  so I need to keep it warm in standby (means not keyed)
This chart is some  makers crude examples: (just on transformer class alone) not PA.

I have SL32-5R020 but is way too big, may not get hot enough in standby, Still testing me.  Max is 20amps ,  and 5ohm cold. They have vast choices, and are #1 in this domain, I think.  50amp is my  surge.
Last comment make sure the device has air flow , not confined and not agains cheap vinyl wire, use Teflon wire, if limited on space. Do not block the bottom vents of this PA ever.
In all this testing I made sure filament voltage is 5vac, as close as possible,  to that, and that the wall AC plug voltage does not sag as mine did , until I DIY installed 30 amps 120v service, and now the PA gets full 120v line, at full power.
My point here is if wall power is weak you need a resistor with with maybe 1 ohm less cold resistance. OK  (but no lie 240vac services is always best, if you can do it. )
All my test date is above and at the end of this html page.

KEY LINE FIX   (my way)  DONE OUTSIDE THE PA. (best is this)
This is only for SSB and is not QSK and for sure hot switching your TR relay in the PA.
  (PTT push to talk or SOFT KEY{tested and now working great} This is super easy to do. (and in an  external box this time)
There is a ham called 
JAMES  COLVILLE.  K7RY $9.99 cheap empty PCB, who makes PCB like this and is seen here. (but mine is all MOSFET'S)
The PROBLEM is 3 fold as is the cure. (mine not his)
  1. Keep in that nasty 118vdc key line (PTT) voltage out of my very expensive SS transciever.
  2. Keep that 35mA current on the same line much lower, in fact at 1mA now, I= 12v/12,000 ohms (super safe for all gear)  Do not arc your transceivers relay contacts, for sure. (very hard to repair that)
  3. Fear (justified) of PA TUBE  GAS discharged (a.k.a Flashover) out of PA to your exciter (Transceiver) {or that nasty VHF Parasitic oscillation surge}
The TL922a  has +118vdc key line and at 35mA the 2 relays (RL1/2 combined current) is too much for most SS gear.
See  many PA's  that  will blow up my ICOM (SS 7200)   there are may key buffers (my name) that flat do not work, some are low voltage input only.
Using and optical coupler (3000v isolation is very great idea) but has problems, it needs to be tuned to your input voltage (from 12 to 120vdc ) and many sold like that do not work at all at 120vdc.
How ever one could put such a device OC between Q1 and Q2,

I did this with an external box, and with one cheap 12vdc WALL WART. ( I have spares for my IP web cams that I have )
With D4 below removed and jumpered. and no CUT. (the cut below is for inside PA installations)
I used 2 BNC jacks and one  Japan (style) DC power socket jack to the box. J1, is keyline Send, , J2 is RLcont out to Kenwood 922, and J3 is DC 12vdc wall wart ( or hide all this in side the PA, the Kenwood has no real space inside for this...)
I had to use BNC to RCA(phono) adapter to fit my PA RCA RL_cont. and RG58 cables.
I used 2 MOSFETS, My current to the exciter is now 1mA, 0.001 ampere's,  (nada) at 12vdc. safe and sound.  The Icom send line uses a weak voltage PTT relay.  D1  can be a better  15v TVS.
My PA this NO FLASH-OVER  mod too that helps me.
(in effect this is a custom SSR, solid state relay.) All parts common as nails, (electronics world common) and I used no RCA jacks (loath  them I do, but audio they are ok) The use of only MOSFETS (N-chan-enh-mode) is best, GDT and TVS devices, yah!
The below is not for QSK modes.(full break-in) Do not do the cut below if using my external box build. (like mine)
I like the MOSFETS, they have a nicer grate voltage Vgs , gate threshold and can be poor and still work. (for weak logic low, not perfect 0v but at near 1v and bipoler (BJT) fail easy. Yes one can stack a diode on the emitter to cure that (BJT bad act up)
See my PCB version. (lay out for PCB )  I tested this and it works below.
Click to zoom.                   

MOST SS GEAR have weak send key lines (the relay used is weak , in the world of TUBE PA weak only)

The below is ICOM IC7200 actual and Send wiring and parts details, the manual warns you "not for TUBE amps". (and crude cures adding yes more stinking relays)  I offer one better. 
Key line is 0 to 4.7v and diode clamped and goes CPU in Radio..  NO HV must land here.
Done: deal: Softkey works, next is ALC card DIY to make.
No PCB needed , this works just as well. Note antistatic mat for MOS

Caps checks: (more here)
I do not have a full spec on these caps,  no diameter or length , nor % accuracy, +80 to -20% is and was common, so if true that, the  200uf can read, in fact. 360uF  or 160, if Caps are ± 20% spec then  160 to 240uf is normal.
The new caps can be had in 25mm diameter (gringo 1 inch) and for sure are shorted TODAY, (evolution of caps) and will fit if you make up cap shims in the rack below, use high density foam, or custom machined Teflon blocks? (do what you must but non conductive)
I did  not reCAP my  HV Capacitor BANK,  mod.'s not shot gunned, unless the gear is from mid 1960s, (junk caps  were and are (R-leak huge !) 
I test the caps with my ESR meter and make sure all resistors related are in spec. (not ruptured, electrolyte spills from caps, no bulging or burns or any thing odd , seen on any CAP)  Eyeball inspections are very good and first.
If you see the 8 caps there the total series capacitance is 25uF (200/8) and the resistors must be lifted 1 by one to do ESR tests.
I do close inspections and tests.  ( ever see relic 1960s parts?)
The caps here are $35 a set. ( do not let others charge you 10x that cost. oK? ( I dislike scalpers) 25mm diameter caps will fit ! modern and are now SMALLER ! (evolution happened) New caps are smaller and do not leak anywhere like 40 or 50 years ago.
Those resistors run very hot, some Hams do exhange them with 100k ohm versions lowering heat greatly (P= I˛  x R) and increases future life of CAP.'s.
the Kenwood caps 40 years old now , most are Panasonic (NEC) and are of super good quality, unlike this 60s junk.
OOPS:   failed HV caps.

Looks bad is bad, looks good, can still be bad, testing solves all these kinds of problems. (cap meters, ESR meters and live testing solve this, or ripple tests,  even scopes used to measure for noise or ripple and more)
Hot useful links : ( tops my list of useful.) I do not get paid to say that, ok?
Mouser mica caps.220pf example ,vastly more.

C14-C22 HV CAP bank rebuild?  200uF at 500vdc. 220uf is more common,,, use that.(the diameter is a secret but 25mm would be my guess.) Mouser 25mm diameter so does digikey
I'm told 500vdc caps are now on the resurge, due to modern SMPS power supplies for PHOTO CELL power systems, many are using them for Green lower carbon earth (90% efficiency).  GOOD! NEWS ( see this list the tall ones and skinny may fit)
This TDK cap comes  in 3 sizes (in mm). DIA/length as you can see modern day caps are vastly smaller. In Japan they use mm and never yankee, imperial inches.  25.4mm = 1.0 inches. (easy no?)
25×55( 2.2" tall is very short and very modern...)
×40, mm all.

Most the parts I need are at mouser, huge selection or at (both are good) Mouser has BOM support and even shared !
Make up a  bill of materials "BOM"  you need first then buy , or shipping gets too high) order 20 things, or for 2 or more projects at once, they have a BOM on line builder at mouser, there to do all that.
Take your time on lists. and order LAST.  (do not buy one 3 cent resistor and pay $7 to ship it, that be so wrong. (if you have 3 projects going buy all 3 project BOM parts at one time)
Keep a good perspective, you are not  buying a $3000 PA,  or $8000  that for and ALPHA Amp. so spending $100 is wrong? on OLD PA? How can this be?

Inspections are next.:  (first is to consider removing MODs others have done wrong or with wrong parts, or poorly.) (4 decades old , expect anything) look first, do not turn it on day 1.
  1. Open the rear case 120/240AC access port (pull rivets easy) and strap it for your power feed (AC) ) { never hook 240AC strapped to 120 or BOOM you just blew it up} <<< first things first !
  2. All grounds in the case and screws related tight and clean.
  3. The PA AC chassis ground must be properly grounded to the HOME power service entrance ground at the ground stake.  Never let any PA float ungrounded for any use or testing. (3kv is lethal)
  4. Check all solder joints for cold solder, or some dweeb used low temp Pb free,low temp , junk class solder,  if the joint does this on a band switch it burns up (or other rare parts) do not cry later. (suck out the old solder and use 60% tin lead solder. (costs more , is more)
  5. The Tubes  Plate(anodes) have  cap  and with PFC  VHF suppressor,  the resistors are bad now this old , warped ,cracked  and read 47/2  (23) ohms, and are not sold now, carbon comps are dodo bird now.  See below for sources of parts.
  6. Check visually that no seen damage, no missing parts, no variable cap fins  bent,  nor tank coils bent, for sure band switch cap.'s crossed over and shorted. Look for hacking gone wild.   See the 3 tests here.
  7. Look at parts inside for obvious burn damage. (look for 30minutes all hatches open (no AC line connected)
  8. Do the NO HV BULB CHECK below this is for Gross OXYGEN gas in the tube (if any Tube has white frosting inside it is bad)
  9. Make sure the PA HV meter shows HV correctly and if  turned off the HV drops to below 100vdc in 2 minutes or less , if not the bleeders inside are bad. (get the PA serviced if this is not clear what to do or how to do this work safely )
  10. Once turned on  make sure there is no seen TUBE gas, or boom happens or blows up you expensive SS transceiver. Do this test standing far back from PA with hands in pocket, (ok this is a test with interlocks, bypassed) if not sure, ask your HV TECH for help.
  11. Never  work on any PA with tubes lacking a working  chicken stick. ( and it grounded to PA case metal)
  12. The T2 no blow up mod.
Most of my mods are to keep GAS /ARC from killing dead other expensive things, like my ICOM IC7200  ! (I have zero fear of grid inductors L7/8 blowing up, as they are sacrificial.)

The so called Bulb tests (3-500z): (and first a crude and safe Oxygen test is first things first) then the HV gas test real is last.  Bad gas looks like this.
No distractions , TV , cell phones,  tired or drunk. (no kids near by either! or wild Pets?)
This first will fail if oxygen is present in the tube and oxygen will kill  filament dead fast. (the test is short , seconds)
  1. The test is this (with NO AC line connected first) Big top lid off, then the screened cover removed next. (HV is not present now but, if you don't understand this say the heck out of the dog house or use a CHICKEN STICK)
  2. Remove both the 2 PLC1 and 2 cap HV devices totally removed both, so the tubes can not get HV, (that tank coil in the center there is lethal,  with HV present turned on and after turned off , lingers and is lethal)
  3. Put the V1/V2 BULB cage screened lid back on with at least 2 screws, in center.Left and right, to activate no death safety switch #1
  4. Now hold the 2nd safety switch engaged (press it) on the left,  on case top left side hold it down and... 
  5. This test is very  dangerous done wrong , HV is present but not to the 2 tubes. V1 or V2.  (this device COIL  will have HV present now , DO NOT GET NEAR THIS AREA WITH YOUR BODY WHEN HV IS PRESENT)
  6. Push the power on  rocker switch, and see the tubes glow like this. (only seen with eye through the lid screen , only see normal filament heater glowing)
  7. If one tube is dead, it is dead. (or Filament wiring bad,   under socket is bad or pins damaged. (etc)
  8. If both glow the  same way,  that is very good sign , if not then one tube has gas. (if heater glow looks odd){means gross oxygen present"  We do not want oxygen inside the tube. (EVER) Do not proceed if this test fails. !
  9. Power off the rocker switch and pull the AC line from the wall., let the PA rest for one hour, or chicken stick it. (or both if you the person living a long time is good )
  10. Put the front panel knob to HV and meter left shows 0v zero volts, if not? wait or fix your bleeders now, death awaits you. (trusting my life to 1 meter is folly. I use a 2nd tool and the chicken stick)  <<  a real tech has seen meters stick all his life, moving coil meters do that !)
  11. Next we put back #2 above PLC1 and 2. and last put back all panels but the one large painted panel
  12. All is safe now only the top huge no grill  panel is still missing, so we can again see through the grill. (top right side)
  13. The PA is ready with AC power and you turn on the power and meter set to HV and CW.
  14. You turn it on and you see 2000v to near 3000vdc in that HV meter needle , next  set rocker to SSB and see HV is max and last look in the grill for this. (ignore this odd blue glow here, It is Video camera lamps or shop black lamp present, just look in the grill holes.)
This poster shows the safe way to look ,grill in place is that.

Here is SB1000 with bad gas. HV is present now. ( this tube was abused,  over powering it got it too hot.)
The unsafe way is here HV turned on, but  a very good photo  GAS Blue is Oxygen , purple is Nitrogen ionization a mix here(a.k.a AIR). (as seen here.)
GAS 101:  (no tube on earth is 100% gas free  , and if  I do a crazy  high voltage test at say 10kv ,  this tube must be gas free at 3000volts. (only)
Factoid #2 is that the PLATE  deep inside the metal hs gas lurking, waiting to sneek out  (TRUE) I am told the plate has top surface getter coating !
The #1 cause of gas is owners overheating the PLATE (anode) over 500watts and migrates out of the metal, (so don't do that, so run you AMP under driven a bit ) Some versions of this tube  rated at 600watts.(plate)
#2 is buying fake tubes, from China or Russia?. (with bad glass or very poor factory full deGasifaction technique and no GETTER , and not trival to do right (RCA can teach you!) USA and French are best up to 1995 made.
I think the Russian tub rumor is just that, a dream they had, never executed.
VACUUM TUBES, Parked on a  shelf tubes mostly do not gain gas, but the getter inside can fail (seen on cheap receiving tubes , that huge silver coation on the glass, mirror like, bingo Getter)  This 3-500z does no leak gas, for fun or age, sorry.
(I have vast tubes in boxes, here, all gas free (means not effecting normal operation) back to 1960 , most 99% are gas free at 60 long years,! and counting)
GAS is ABUSE ! period. (or sure cheap poor grade tubes)

The best book on this is from old Collins radio man,  Walter H. KOHL, 1961 seen here, find all words (outgassing easy and learn )
The glass in tubes are not like window glass or drinking glass or even a light bulb glass,  (there are 1000s of grades of glass , this is  commerial grade PA TUBE very special glass)

All tubes made use a very special type of  Pyrex Borosilicate GLASS and KOVAR seals,  that does not let AIR gas walk past the molecules inside this glass.(or at the pins) {great pains and engineering went in to any tube designs USA/EU,  but China I bet not)
The Pryrex codes are 1500 types. (only few work with vacuum tubes)
The #1 glass then was Corning 7720 called "NONEX" for transmitting tubes ended 1995(hot running)
Some glass rated at 500C . temps.
Not withstanding , dropped tubes or pins bend hard over, and other abuse.(the top hat can fail and for sure overheated (allowed)  (the PCF plate cap device, on mine acts as a heat sink too, so don;t throw that away)
The overheating PLATE will cause gas, the illegal temperatures, cause liberated deep gas inside the plate, call  Occluded gasses and if you overheat the plates (over 500watt) you forced out that hidden gas. ( called COCK PIT ERROR).
Some Hams bake out the gas with 50vdc on the plate, even making up a  bench bake out test bed station (socket , transformer and 50vdc PSU to plates , 5v to filaments )

No GAS , good glow , just so you know what good glow looks like.

Test for GAS 2 ways, first with no plate caps, and power for 1min max, see if filaments glow right, or act odd, (zero plate voltage, done carefully.  BBI, does the AIR BOTTLE TEST, hands in pockets or death.
Test two, if above passes !,  put the plate post caps( PC1 and PC2) back and do test 3, no GAS test.
Put the screened top case lid back on, closing safety interlock and look for any clues of gas, while  turned on 3000vdc in HV dial, SSB  mode 
This is BBI again DOING IT LIGHTS OUT (BLACK AMP ABOVE I GUESS) AND SEE THE TUBES GLOW AND NO GAS. (The  below is normal filament glow)

Test 3: Oh my golly gee, see this fast test. 3kw peak out. (200watts excitation)(a short test never long)

One of the longest lasting tubes (electron vacuum tube) was: BBC's main Northern Ireland transmitter at Lisnagarvey. The valve was in service from 1935 until 1961 and had a recorded life of 232,592 hours.
Some transmitter tubes (never abused ) can last 50,000 hours.  (overheated tubes drive gasses deep inside the iron/nickle plate (anodes) and now is gassy. So do not run 100% power.
Quote from Kohl, 1961 page 24 up, in his great book,
Page 27
"for argon, oxygen, and nitrogen the permeation rate at the same temperature becomes less by a factor of 100,000 or more; hence glass may be
considered impermeable to these latter gases for practical purposes, "  (he is talking glass tubes USA made using top grade glass not China made trash.)
I say myth busted, that this  glass leaks gas at all. (abuse or china knock offs will fail)
The tube makers, worked directly with Corning glass to discover and control that this fine glass does not leak.  (if the fan in dead the tube gets to hot and that is abuse or shack at 120degrees F ambient?)
The trick used by many makers of PA, paint the dog house inside flat black. The tube emits IR, energy, (heat ) and lands on the black shields and they get hot,  cooling the tube, not just the air flow, this is something to consider at rebuild time. (cheap flat black spray paint is amazing)

The French tubes are marked 3-500Z/8802. and need up to 20% more drive to get full output.
The prime supplier now is RF PARTS  or RFP,  if you can not find a good USA/French tube go there.  Buying direct from China, will not work well and no easy returns if allowed at all.
Cheap consumer radio receiving tubes were  $1 then (Dumont,etc) and were of  softer and cheaper gass, leaking glass, I am only talking about this one commercial TX TUBE by Amperex.
On all tubes with the getter inside dies,  the tubes life is near the end, for sure overheated.  The PA tubes many have the GETTER covering the anode plates, looks like black paint.
Note no  GETTER seen inside our tube.   (no getter silver mirror coating on the glass (barium),  but I am now told it is a coating over the PLATE.)

The production plant heats the tube very hot at pump down time, getting all abnormal gas out of the tube. (and even fushes the tube with Hyrdrogen gas using and inductive furnace.
One expert told me this: The gray coating on the tube's anode surface is a so-called "active getter". When the tube's anode gets hot enough, any gas molecules that get close enough to that surface become chemically bonded to it."
When the tube was made they....  (watch this video, it covers the induction RF heater clean out very well,, the words second inductive heater is the Getter activation step "Barium loop seen here".)
Considering all that one way to get rid of huge gas is run only filiments, then if possible run low HV on the plate , in  test jig 500v for 6 hours then 1000vdc for 6hr. more  and work up to full power only if the blue or purple glow is getting less.

Next, up, kids do not do this at home, but this is super ON TOPIC !
Sees this Video too.(thanks Mr. Carlson !) (he tells you how to clean up gas a the end)

One other manufacturing trick is the  baking a new tube with hyrogen inside to eat all that oxygen for breakfast. (there are many methods, all very good)
Not only that trick but  the envelop (glass ) flame baked, the metal inside is INDUCTION heated and run super hot to get all gas out of the metal, as you can see this is non trivial how they do that. What matters is not abusing the tube NOW.

PRC (jelly beans) You must buy 12 , min. order.( direct buys from PRC)

More abuse is letting the plate cap get too hot +250C, or by throwing away the cap PFC coil that works as a heat sink too. oops , why do that?
Or using too much torque on the cap screws.
end gas.

GRID folly & myths and
the like (all for gaining 50watts , really , but I read others don;t like burning up the inductors with bad gassy tubes)? 
There is nothing at all wrong with this factory design, if you have gas deal with gas.
HAM AG6K )read this first, how the grids really work.

If you work on  GG (grounded grid) Amp.'s. PA read what Collins radio said.,
"To change the grid connection from inductive to capacitive, provide a measure of negative feedback, and to neutralize the tube"

Facts: the 221 cap's put the tube grid at RF ground. (at 30MHz and below) The COIL inductors put it at DC ground.
Do not guess, do not put in mods that have  no purpose.
If worry about CASCADING GAS flash over is the motivation to take actions then why not use GDT and TVS devices that only cost $2 a set !
Yes blowing up T2 would not be fun, if that is a goal?, then address that.(click link above bingo)
Just to be clear, here is an UN MOLESTED 922a below (there is nothing wrong with Kenwoods design !) The yellow wires are heater filament and the other 3 are grid.

One of my Amperex tubes is 1974  date coded and has no gas. (46 years old today) "think about that " (or Eimac tubes)
The real tubes,  have very very special GLASS envelopes that does not leak gas, (something those guys in China do not understand !)
Look to at the fantastic , aluminum ground plane there, and the spot welded ground studs, why on earth wreck this beauty.?
Then cry that the grid grounded (hard strapped to case)cures gas? (I see many folks do have gas and is cause by running the PLATES too hot) Self inflected pain.
This PA has 10 grid components, not for fun or giggles and was on purpose. (by clear intent and design and everyone knows, 10 extra parts costs more...)

Each of the 3 grid pins have there  inductance you can not change. (the tubes each have 3 grid pins and will look like 3 tiny parasitic inductors and parasitic, capacitance to cathode and plate..
This is GG (RF) Called a Class AB2 amp.  (grid BIAS on all tubes,(Fleming valves, De Forset tubes) comes in 2 classes.  Fixed and SELF , or a mix of the 2)
This PA uses FIXED bias, by placing a plus +100vdc voltage on the cathode that causes the tube to be more near cut off in standby. (this is indirect fixed grid BIAS)
Then when you key the RL-cont. jack the RL2 relays energize, and R7 is un-shorted and R7 now sets the new DC BIAS. (again not by accident but clear intent)

Do not go to the dark side and hard metal strap a Daisy chained Grid pin bar, to the grids and then last  to 1 case ground (each tube). < doing  this makes ground even more inductive. (why do that?)
The grids are tuned, to prevent parasitic non HF , VHF self oscillations. Leave them be ! Please. and leave the normal grounds there alone,  the chassis is a ground plane (best),adding or doing some wild daisy chained inductive bus bar ground is not wise at all.
The folks claiming all this voodoo have bad tubes, china cloned graphite plate with non spec. Mu tubes and  gassy, bad sockets bad relays, etc ,etc., ( its only 50 years old: LOL !) bad socket pins, loose screws? oops, and bad grounds in this area. (Carl @KM1H   nails voodoo .)
Some folks do fight VHF 110MHz parasitic oscillations in this amp for sure higher mu tubes.

On this PA I say, leave the grids alone.
The only grid bias is done indirectly to the cathode. The Kenwood manual states " a zero-bias Class AB2 ,   
 via L20 cathode  inductor and R7"  
" Is connected to the filament return circuit biases the cathode positive during transmission (makes the grid negative to the cathode !). "

!yes confusing that line in the manual. What they meant is this.

Thus. the grid is biased indirectly and this voltage is low (key'd) to achieve low  distortion."  DO NOT MESS THIS UP ! As Carl; states.
The manual is more clear.
Mode Selecting Circuit
Zener diode D2 connected to the filament return circuit
biases the cathode positive during transmission (makes
the grid negative to the cathode). Thus. the grid is biased
without floating it. Bias voltage is low to achieve low
distortion. Plate voltage is 2.2kV. idling current 100mA
in CW mode and 3.1  kV at approximately 200mA in SSB
mode. Therefore. plate dissipation power always exceeds
In SSB mode. the zener diode is shorted by S3 lowering distortion.!!!

The grid is at RF ground at HF freq.. 3 caps one for each matching grid pin, not by accident, but to match each and every grid pin internal inductance (fixed). Xc = 1/2πFC and XL = 2πFL (when Xl = Xc they are in resonance)
The grid is at DC ground with the 2 choke coils 470uH. and go to higher XL at wrong VHF freq. XL for 2π fL (as F rises so does XL) F= frequency, L = inductance, π = Pi or 3.14.  C= Capacitance.
All tubes have parasitic capacitance, (from plate to grid and cathode and even out side the tube, to other near by structures, that is why you need to not change things in the design by Kenwood.
this basic parasitics,  it is far far more complex but these 3 are primary.

Do not hard ground strap the grid to ground level the 5 parts alone (10 for both V1/2)   but new grid caps may be needed, Silver Mica.

Plate (anode) Parasitic suppressors:  
THE PFC  device. (PC1/2) VOODOO:  Do not read this , it is my notes to learn what sellers are selling and failed. (me)

The above Parasitic caps (invisable to the eye) the lead in wire inductances (and C strays)  and in phase feed back paths can cause VHV Oscillations a very bad thing to happen even arcing over band switches.
 (read this on now to test for Parasitics 

The black art of VHF ( prevention) and endless black magic cures, some are wild and crazy, I will show some below.
I see lots of web sites, and such using NON copper coils , using crazy ferrous metels, NiCr Nichrom and stainless steel (SST) wire. (400+ permeability over copper silver, gold or even aluminum)
Why do they do this, (my only guess is they are lost.
Not one person reported the OEM or factory inductor spec, on there stock factory PFC. (the nH of stock coil)
Bunk1 and 2:
  SST ?   1990 this 2mm OD Nickel in the wire has 400-600+ times more permeability.(not good that)
Nichrome wire is mostly used to make resistance type room space heaters.
The worst thing of all when you leave copper,(silver,gold,alum or brass) there are no computer models for the above...  for 1/2 loop coil of Nickel wire..
Not only that but it is an alloy of steel and nickel (vast mixes exist and the permeablity(Ur) is not known on mixes,(funny that they pick this no?) and  nickel is on ave. 440 Ur and copper is 1 Ur.(relative Perm)
Iron is Ur=200,000, Cu(copper) is Ur =1, as is silver,gold,aluminum and  platium,but  SST can reach 2000 Ur, and last, the vast  types of Nichome unknown Ur.(for sure impurities )


Unknown materials and reason to do this.

Next brass !
At least the below uses copper based brass. (+zinc) and not a ferrous metal .  5nH? super low nH, why do that? 

r. Brass strap is same as copper /w low permeablity (Ur = near 1) {no calculators cover a 1/2 loop}
 10 nH or less? above , IDK but is a better heatsink than any classic DIY crued copper coils used.
Next up the PFC resistor Rx.
Resistor data sheets cover newer class resistors to use here. (or buy no longer made CC carbon comps from scalpers at $6 each) the cure is MOX.
 The OY series, (20kv rated) and or the MOX series,  now replaces the old crap carbon comp's (infamous for failing and aging poorly)
 OY series show no traces of resonance but....(and used on Ameritron PA) keep leads short ! yes the end caps are metal, and make tiny inductance   (RCL effects matter.

PLC,  Parasitic L&C:
Kenwoods PA, V1/2. 22.5 ohms is PC1/2 Rx that, and Lx is UNKNOWN.
This drawing can be heard to find but here it is now.  Not the low inductance double grid pins.

SUPPRESSION magic (black art PLC1/2) I did not mod this I just replace that carbon comp resistors (If need be)
First off copper wire (ITSELF only) is non magnetic element, and using Nickle wire of any kind is and  has huge magnetic properties and huge permeablity. (and very very hard to design or predict outcomes)
To wind any coil there are upto  8+ variables, (the higher number is at near VHF) with 12pF C-stray coming into play.

By: Serge Y. Stroobandt, ON4AA (THANK YOU !) 

The problem of self inductance causing self resonance...

Turn count, diameter, length or turns per  inch,  and materal  used, and  there is  the coil spacing  and  coil  spacing  capacitance  effects for sure at VHF. (100MHZ )
The calculations are not easy and many formual's are faulty. (see link just above for good calcs)
Here is my inputs for 4 turns of copper or siliver Ag, and silver is not a factor at all. (if you run the calculations many times on material) The only change big was Aluminum with 200 less Qeffective.  (gee aluminum is cheap and easy to find why mess with NiCr)
This jewel here, from Mr, Serge,  Coils achieving the highest quality factor require large diameter wire, strip or tubing and usually exhibit a cubical form factor; i.e. the coil length equals the coil diameter. )
Consider this?, a flat bar coil (Kenwoods) has a very low capactance (coil to coil wound), compared to any round wire standard wire.  ( and very different inductance too for the same reasons but magnetic , not capacitance electro static.
The opposite of this.
Any cap value is a function of plate area and flat bar is the lowest. (coil to coil) and would be great this at VHF,  was Kenwood on the ball, seems YES.
The inductance,  is not the same as normal round coil wire either due to very different mutual induction.
What bugs me is that I see not one Ham testing the stock PLC (PC1/2) to show what it really does 3 to 30MHz and then at 100mhz,  not one. (sadly)
The coil has  good Q at 3-30MHZ and horrible at 100MHz.  In a perfect world.  (why  not first change R from 22ohms to 50 first if you have problems here Parasitic VHF)

MYTH1: busted. (SKIN effect is real, but they forgot to measure deeps of  it first, oops.)
This Kenwood  flat bar would be less and lower mutual inductive effects, (lower nH) but the skin effect on copper is  and near the same for pure silver wire.! (the Silver is bad mantra is not holding up!) Myth busted..
The silver does not matter for this possible parasitic oscillation, the silver prevents corrosion (green copper bad)
The Stock PFC is plate cap heat sink. (matters)
The facts are that 30Mhz and 100MHz copper the skin effect is 6.5 um deep (micro meters) 0.0002559 inches . (are the same)
Then silver (Ag) is at 100MHz is 6.3 um (just .2um less) so 100MHz gains NOTHING here worth beans. lets do that math this  way, the differnce is 0.00000787 inches,  can you argue this, I SAY NO !
No change in DC resistance worth any worry and no change in self inductance using MATH.  (the reason KENWOOD silver was used is to stop corrosion this  in no high band UHF radio OK?)
Silver plating common practice is from 2 um to 10 um, 2 is short lived usage) (I'd guess at 5um)

100MHZ is not UHF,  so at 30 to 100MHz this is not any issue, (there math on sliver at 100MHz  wrong in the myth. )

The silver stops nasty copper oxide and also the nasty copper oxide would wreck he PFC flat bar heat sink effects. (for sure) so Kenwood again wins.
The pure inductance  of a wire is based (divisor) on wire radius,  and flat bar radius is huge, unlike round wire.That means the flux density is very low between flat bar turns. and inductance is way way less. (shocking facts)? wow complex when not using round wire !
The skin effect forces the current flow in photo 1  to be mostly on the outside of the coil, and very tiny currents (the core) and way less  on the inside thin edges. (in the proximity zone of coil design)
One can expect Z and XL to   go super high at 100MHz. (more so than round wire) Seems Kenwood design does work..  See this great page to see why they are not  the same.
It gets worse for very short coils, a new formula is needed.(and seen here under arbitrary pitch)
Photo 1 is stock PA Supressor.  The nH spec, is not known or in the spec..(yet but 20nH seems rational to me.)
Look again at the flat coil the pitch at low HF Freq, it is from center to center line, but at 100MHz, it is from vastly smaller thin edge to edge.  (mutual  inductance effects)
The capacitance does not change on the Lx , coil to coil.

Photo1    (<=20nH?)           C-stray coil to coil is lower see that super tiny strap side edges there?, that is LOW area CAP plates and low capacitance.  (dielectric is air) and only 2.5 turns, not 3, not 4 or more.

Kenwood knows what the are doing. You can Run a PA like this for  decades and never have a problem. (of failing this suppressor) 
My, Lx , I call it.                
Factory coil 2-1/2 turns of copper flat bar, the silver does not matter ! here!, at 100Mhz. 22ohm CC=carbon Comp.
The above flat bar changes the effects of mutual self inductance, I think that the inductance is way lower due to it being flatbar,  than any close wound copper classic coil inductor. (seen below some horrors)
I can not find a computer model to do a flat bar inductor.

Photo2.   Using Mr Serge math. 
Photo 2 ,  DIY   RFC 50ohm 2 times spec of 22ohm,  14AWG house wiring used,  17mm long coil is ok. (above 31mm long will fail and resonate at 100MHz.) 51nH and 280pF of parallel stray cap..
The above looks like a 2watt USA mil spec, RFC spec. resistor, 0.31" OD, and 0.675 long(mine is) (Using Stroobandt calcs)
The Q increase greatly at 100MHZ (from 30) from 400Q,  and Xl= 10ohms 30MHz, and at 100MHz , its 30ohms,Xl., Q=670 and self resonance is 1GHZ.
Parallel stray capacitance @ design frequency           C_p = 280.7 pF 30MHZ drops by 10 :1 atg 100MHZ, to 22pF  This really helps prevent Parasitic ocillation. (and is used widely in many PA of many types for decades)

The below is me running Mr.
The horror of 3 turns copper   is self resonance at 106 MHZz,   (ouch)
3T = Self-resonant frequency =    f_res = 106.707 MHz (3 turns)  see tests below using his great model.
1 turn is 13nH at 2.8GHz (GHz is the self resonant calc)
2  turns is 23uH at 2.2GHz
and 4  turns is 46nH at 1.4Ghz. fails above 30mm long.
The coil length is the most imporant parameter and can cause failure down wrong, seen below..  That is why the hair pin 3/4 turn ferrious wonder device is less than 1 turn. (permeablity is 400)  Cu (copper is ) 0.999991 Ur (perm) and
Input was
INPUT (Mr,  Stroobandt) data:
  mean diameter of the coil    D = 7 mm  or 8mm  {not much change at all} 8 is best if using 2 Watt carbon comp. resistors inside. (sen below Ohmite comments)
  number of turns              N = 3  { tired 3 and 4}
  length of the coil           ℓ = 25 mm (1 inch)   {I tried 15 to 40mm to find self resonance point  and bingo found  it} 
  wire or tubing diameter      d = 1 mm (18AWG)  2mm shows no change,(major still 4turns fails at 30mm and less long)
  design frequency             f = 30 MHz
  Self Resonance at  f_res = 94.724 MHz  (self resonant at 3 turns copper at 24mm and longer, it fails L=24mm to 45mm fail but below L+23mm long passes at 1604Mhz )
  4 turns fails too. at L=31mm & up shows 70mhz F_res   ( Testing from L- 20 to 50mm range. )  30mm passes but 1mm more longer fails
  The key factor here is the mutual self inductance factors. (the coil wire spacing is very important to the final results)
  The coil needs to be wound tight , or rish deadly self resonance. At resonance Z goes way low in a series resonance circuit.  Fo=  1 /  2pi X sq rt of LC.   and Z =  very  low,

The (plating) material is annealed copper. (doen't matter which kind of copper  nor if aluminum or pure silver)
The inductance of a coil depends on the permeability of material used in its construction, and Iron, Cobalt , an Nickel will cause huge changes in inductance. ( for 100mm long wire from 100nH to a 3000nH change)
And the coil form design (turns , length and diameter)
Nickels Permeablity is 400 times more than copper or aluminum or gold or even solid silver.
Now you know why the (HAIR PIN) NiCr PFC is 1/2 a loop VOODOO, this huge change in permeablity mandates this.
Coil Diameter (I.D) can be due to this reason:
One goal of coil winding common then, is to allow a 2 watt CC resistor inside (Ohmite spec or Allen Bradley) at 0.3" OD (or 5/16") that is  7.62  to 7.9mm) so 8mm is best.

Why did Kenwood use this PFC.
As we all know , the Q of the device matters at all HF and all VHF possible parasitic frequencies (100MHz?)
The flat coil also pulls more heat off the plate cap (it is a heat sink! not just a PFC) to avoid 250C max on it the top cap,, my guess is that in Japan the wise engineer wanted to do that first, and went flat bar!. (a wise man IMO)
They didn't make the flat bar coil to save money,  they sure did NOT. (ponder that more?)
They used silver only to stop wild copper oxide corrosion effects. (never good) The used silver as  quality feature too. Not just electrical corrosion effects.
There is not any  fault in their design.

If one studies this carefully standing back 3 feet scratching the head,  one can design vast types of coils for PFC.  (endlessly)
I wonder does your PA have some other  problem not even related to the PFC here? (if still stock) (did you wreck the grid circuits that  Kenwood designed)?
One thing to do easy , is using the Mr Serge equations is to double the coil diameter  and SEE the  C-stray goes from  32pF to 3 and henrys double.  (as will Xl go way up)   would not you want XL very high at 100MHz.? and that makes R highest and Q lowest.
The coil can be made larger Diameter easy. (and not longer and not fit)

Now my wild theory  why this happens, (VHF parastics) besides,  some pie in the sky too high Mu tubes. (moot now,  and  you have to pay $500 a set to replace)
  1. Lost the PFC coils?, bought the amp and they were lost long  ago by someone else, ouch. (and not sold now.)
  2. The 2 resistors there  burned up and you have no silver solder to replace them (high pockets ?) and threw them away.(ouch2)
  3. or read the hype and voodoo masters telling you that stock parts are no good. wow. (the word UPGRADE captures your  attention) " saleman love to invent a market from zero  ,just like the "Pet ROCK"
  4. OK then you make 3 or 4 turn coil  copper DIY coil and is self resonant at 106MHZ<  oops. (next time use this) Use Mr, Serges web page.
  5. You wrecked the KENWOODs grid design or self bias?

Myths: (true or not)
  • SST or Nichrome (NiCr)wire is best, for plate chocks. (fail) copper will ALWAYS work (and the math is 1000x more easy using COPPER)  There are no computer models that can do NICHROME wire nor SST , not yet.
  • SILVER IS BAD ( and outright lie) hell you can use pure  silver wire if you have the cash. (and the math works the same way with pure silver or copper and are both non ferrous and predicatable )
  • Hair pins only work. (no truth here nor proof, nor math )"the only game in town , is a lie" heard in Ham forums endless.
  • Running the PA into no load or shorted antenna, or not tuned up correctly never causes arcing. (true) (Not tuning first at low power, at the exicter ?) True.
  • Not cleaning parts of dust inside the PA, causes band switch burn up. (true, never run any PC dirty, why do that?) use any top brand of oil-less electronics cleaner on all band switches, and safe for platics, and varnished wire.
  • You have gassy tubes and try to reDesign the PA to run gassy tubes, ouch.
  • You wreck the grid cathod BIAS and then wonder why problems pop up. (hard strapped grids to case ) The grids are inductive and kenwood tuned the amp  for that inductance and Cstrays.

PFC Resistors choices, many:
MOX resistors are non inductive. (metal oxide)

First the Bad.
  (even buy one and scrape off the coating and LOOK.)
"Metal film resistors are (can be) trimmed to value by having a spiral track cut through the resistive film" - they can be extremely inductive!  
Better is to use MOX resistors. and not laser trimmed in a spiral. use 10% or 20% values to avoid precision and those wrong laser cuts.
One metal film rated for HighFreq use is Vishay  CCF0247R0JKE36 at mouser, look up all value for CCF02 for what you need.
The Metal Oxide (MOX) resistors are the better class to buy.

Many resistors sold are listed as non inductive but is a lie.  (XL = 2πFL )   Non Inductive at audio or at RF 100MHz is not the same thing.
Most makers of resistors are not even aware that HF exists now. ( what's a radio?)
In a computer the only radio inside is a wifi  chip the size of a babies finger nail (pinky).

SOLDER ISSUES (and heat) (for  the stock Kenwood PFC, copper plate cap. reworked)
Some of these parts get super hot. and some need high temp solder (vast alloys of solder are sold now, complex) but some need silver solder, the plate (cap) seal is 250C max rated and hotter than most common lead free solders rated low temp are useless here .
Some solders go very weak and sag at 250C,
A nice chart here shows what you can buy. (1.5% silver, may be best on plate caps)

60/40 Sn-Pb, which melts at 188 °C (370 °F) (Sn = tin, Pb means lead)  one of the most common solder used for 50+ years, but here is Kester list that is very good and maker famous longer.

The winner here is Sn5Pb93.5Ag1.5  (Sn = tin,Pb = lead, Ag = Silver ) top of Kester list. 301C rated. if rich get the 2.5% Ag.
 Lead is SAFE not licked and used/worked at, with a simple fan on the bench. (no need for commercial EPA vent hood)
Always wash your hands before eating after playing with solder.(lead)

Silver (Ag) solders  for jewelry can be crazy expensive. (AVOID THIS)

Do not use acid core solders ever.(in electronic work)

Conundrums, by Mr, Kenwood. (solved)
R7 is named 2 times, one in the meter box and the R7 bias resistor near, D20 and worse  the R7 shows 100v (100k is true) on the schematic,  R7 is the BIAS resistor.  (this practice is common as they use same names on many modules)
The 100vdc (118v on mine no key) you never want this to go down the KEY line to any SS RIG  of any kind (most).  This is what I call the Virtual GRID BIOS voltage. (and IS via cathode bias)
R7 is cathode BIAS that becomes grid bias. ( does standby Ig rise with this mod?) (I will test for that soon) Ig means "current grid" In ohms law I = current. E = voltage, and R = resistance and Z = impedance. 
D2  btw is 10watt stud mounted zener.  7.5v zener. 1S265 ( Nippon #) Toshiba , common maker.  I use GDT1 and  TVS device in parallel both.( and only costs $2 to add)
I do not use the X cut,  I added 2 parts, non subtracted.   TVS and GDT in parallel and wired from ground to D1 cathode.
T2 blow up mod1:    This is all to protect the Transformer from damage if tubes go gassy and flash over.
  RL2,and R7 100k, in all its glory. The BIAS relay, All I do is add device called a GDT1 or flashover  CLAMP.

I did not use the X cut wire hack, the reason they do that is to protect T2 from flash over,  but clamps work too. GDT +TVS paired in parallel. The clamps do not change the BIAS (so wins in my book) Do not change the BIAS, leave it alone.

Next up A.C. line current Surge testing. (100amps possible here)

My photos of TL922 current using the CT013-000 (100amp, 50mA out) sensor device.   (the performance plots are here)  the device
PA in operate , CW mode, turned on. After about 1 full second current is 2.2amps on 120vac line RMS current 60Hz.  The surge is same in CW and SSB at turn on. 50 amps.
This sensor has bandwidth of 1000 Hz.

See Hall sensor data here.

I am pretty sure this is not a true in-rush, current seen here,  it is cap charge followed by filament loads  not FLUX FLEX Saturation knee.
I do not know if this PC  does this huge core saturation problem like my SB200 does for sure.
The worse case is a bouncing power on rocker lever. the 2nd bounce can be horrific currents into the 2 transformer primaries.

SSB: surge (captured dead slow HORZ. the using memory knob ,  I go BACK IN TIME (like Michael J Fox ?)

The below is with PC turned off for 12 hours,  and I captured worst case cap charge current on the AC line.

If I do the turn on PA with no line cord connected then play nasty tricks with the line cord plug , I can in fact get a 150amp surge. (here)( to do that 20 more times to get the FLex Flux pulse, I  go chicken.) and stop here.


Rad line is math (theory) and blue is actual as the tool saturates its mag core.
Math is easy 50mA precision wound sensor is 100amps, CT013-000, on a precision 22ohm Burden. E = I x R   1.1v = 100amps peak.  (R is fixed at 22ohms so we read the voltage and solve for amps,)
50 amps would be 1/2 that. or 550MV out. 25 amps would be 1/2 that at 275mV out to scope;Scope reads pack.  .707 time peak is RMS. (a.k.a, true power)
Next is worse case SSB line current, (2.2. amps is normal ,with no modulation) The max measured was 50amps Peak. on the AC line. or 6000watts peak or about 4242watts RMS  (the average was about 40AMPS)
Seen here: 100mv =10Amps. 
 So to make 3200watt peak it takes 5000w off the line for a  1800watt heat loss.  (MATH:  n= 100% x Pout/Pinput   so 3200/5000 =  64% efficiency. (peak, not average, or P.E.P)
The line fuses are 15amps (ave RMS) The below never averages 15AMPs.  1500wPEP is an  average, power. spec. or 1060w RMS and that be 8.8amps of line current, RMS current not peak. The below is max max peak current.
THIS PHOTO SHOW AC line current only. (hint how much it costs to talk , yeah)

Transmitting on to huge 1500w watt dummy load, and tuned for SSB 40meters, 120vac line 60Hz, 30amp RV class service, (super powerful dedicated AC power feed, to the PA. Input is 100watt from ICOM, excited.
My power feed 120vac  30amp, service ,  NO voltage sag here. (seen on cheap 15amp power feeds in homes)

End mag sensor start Mr. Hall.

HALL sensor data:   AC current scope photos to come soon:
It does 20mV/amp . direct measuring hall device.
The Allegro™ ACS758
I have a Hall effect sensor coming soon.( it is very fast device and $4 cost, got it and am making a box for it now, for safety and future use) This Hall sensor has huge bandwidth at  120 kHz (more later back this mag sensor)
I made a schematic of the HALL device,  (maker has none so made my own)
The DIY box.

Space heater (test 1, basic)

PA power on event. I did not turn the PA on 10 or 20 times to see the 100amp event.

Parting shot: If it is not broke, don't break it.  (but in truth this PA is NO APPLIANCE IS IT?) not by a country mile. In fact this amp is in a  42 year time warp.
Most modern rigs sold 40 sold years later, (year 2060)  into  the future will be door stops with no parts found., to fix them. (look inside if you doubt me) The golden age of ham radio has ended.
If you over load any PA and get the anode, too hot, the anode out gases.  So don't do that. No tube ever made is 100% gas free but is at 3kv, but most gas (occluded) is hiding deep inside the Plate (anode)do not exceed grid or plate current specs.
The getter is claimed to be on the outside surface of the Plate/anode.  (so running the tube at low power keyed up can burn up gas, but in truth best done in gas burn off bench jig, with 5vac and 50vdc supply and pulling plate current.)
If the PA still fails replace this next. (hihi)

Goals (this page and rig)
Get the full RL1/2 relay spec. (coil V and Ma)  118vDC mine unkeyed, and at 35mA keyed, SOLVED: I'd say the 2 relay coils are (mine stamped on side as 100vdc) 3000 Ohms in parallel or 6000ohms each.
Gen up my new RL_cont,  Soft key, schematic (done)
Get best RS1 thermistor P/N for this PA  only.  (soon) 5ohms would work.
Lots of scope photos of surge (mine) and RF power out and a nice AC current scope view with me doing max SSB modulation.
Build and test my 2 external protector boxes,  SOFT KEY and ALC.   SOFTKEY DONE,  ALC  done.

Dutycyle,  this is no commerical amp, or broadcast class,
SSB. continuous for 30 minutes (FogHorn-leghorn is in trouble)
CWR and TTY. key-down continuous for 10 minutes

The Heathkit SB220(read about heat here and black paint cures)
SSB: continuous voice modulation.
CW: Continuous (maximum key-down 10 minutes).
RTTY: 50% (maximum transmit time 10 minutes).

Overheating, avoidance.
  • Keep the AMP bottom side on to say  a grill stand to allow air flow to the bottom. Get as much cool air to the bottom side.
  • The fan working right and grill behind  not packed in lent.
  • even paint the (mods) HV cage inside walls in flat black paint,  really helps any PA,  the heat is IR, (infraRed) energy and best for sure to land on black surfaces, heating the case is good first , then air flow gets the rest of the heat.
  • Do not block fan exit to a say a wall. (or other large object)

rev 6   ++++ 12-25-2019 (day1 +)  This website  is not affiliated with or sponsored by Kenwood or any other named device or product. (we do not track you ever) 
Do  not work on any PA if you lack HIGH VOLTAGE TRAINING OR FOR SURE RESPECT OF IT.  (If unsure? ask for help or get your rig fixed by a real tech, that knows HV power supplies or join a HAM club and find a HV Elmer.)