G8MNY  > TECH     06.04.25 08:30z 80 Lines 3605 Bytes #114 (0) @ WW
BID : 29650_GB7CIP
Subj: Drake L4 PA 3-500z valve repair
Path: ED1ZAC<ED1ZAC<GB7CIP
Sent: 250406/0819Z @:GB7CIP.#32.GBR.EURO #:29650 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                   (Updated Jun 24)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
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THE FAULT
I came across a common fault with a many PAs including the 1960 Drake L4 Series
an expensive valve that goes intermittent "heater to grid short". I read
somewhere this is a common failure of directly heated cathode PA valves.
Symptoms are a very low gain & standing current on Rx, as the +120V cathode Rx
bias is shorted keeping the valve on, making "electron shot RF noise" on Rx.

On testing the separate HT PSU with a 2A mains variac wired straight to
transformer, bypassing the disconnected PA switching. I found several issues
including on the 120V resistor attenuated rail dropped from 2.7kV, missing. But
that was not it!

Still using the variac this time wired direct to the PA heater transformer (no
2.7kV HT), I found the 120V rail line on the PTT Relay was shorted to ground
when the PA was on its side! Putting PA upside down the short cleared, but only
until the heaters were powered up. With the variac I was able to find the
voltage were short just occurred.

Now to do internal surgery inside the valve & not damage it!

 Äo-oÄÄÄÄo/oÄÄÂ                  RF drive >ÄÄ´ÃÄ¿   _³_ 2x 3-500x _³_
 Fuse 1   on  ³                                 ³  /ÄÁÄ\         /ÄÁÄ\
 Trip         ³       ÚÄÄÄÄÄÄÄÄÄÂÄÄÄÄ¿    ÚÄÄÂÄÄ´ |- - -|ÄÂÄÄ¿  |- - -|ÄÂÄÄ¿
       Ú Ä Ä >ÀÄÄÄÄ¿  ³5.3V    ===   ()))))  ³  ³ |  /\ |=== )  |  /\ |=== )
       |       115V)||(______   ÃÄÄ¿ :::::: === ³  \³__³/ ³  )   \³__³/ ³  )
       |           )||(30A   ³ === Á ()))))  ³  ³   ³  ³  Á  Á    ³  ³  Á  Á
    Variac    ÚÄÄÄÄÙ||ÀÄÄÄÄÄÄ)ÄÄÁÄÄÄÄÙRFC ³  ³  ÀÄÄÄÁÄÄ)ÄÄÄÄÄÄÄÄÄÄÙ  ³
    Mains     ÃÄÄÄÄ¿|| __    ³            ÀÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
       |      ³115V)||( 18V  ³                        5V
       |     FAN   )||(_0.5A ³  RxÚÄÄÄÄÄÄÄ<+120V
       À Ä Ä >ÃÄÄÄÄÙ         ³___/ Relay
              ³ 250VA Heater    Tx³
              ³ Transformer       ÃÄÄÄÄÂÄÄÄÄÄÄÄÄ>Anode<ÄÄ  HT return 
              ³ in PA unit.       1R  Grid I     Meter I
 ÄÄÄÄÄÄÄÄÄÄÄÄÄÁ                   Á    Á

THE FIX
I tried several amps at a few volts between the negative grid to cathode, to no
effect.
What did work was to use a 68,000uF charged up to 20V between the heater
transformer centre tap and a hard earthed grid (not to burn out the grid choke
of 2.5 ohms). I then turned up the mains variac until the short reoccurred, I
had to do this a few times to make sure the intermittent short had been blown
off!
The pulse current had to be high enough to blast away the touching grid wire,
but not damage the 15A thick heater wire, & also not crack the glass!

CONCLUSION
Saved a few hundred dollars of PA valve, so worth a try? I see about 800W now.

N.B. Never run or test directly heated power valves of their side!
N.B. Never over run the heater voltage (as measured on valve base).
N.B. Never over temperature Anode (orange) as that can cook the heaters too!

NB. On these unbiased power triodes, the Cathode goes quite -ve with resect to
the Grid with RF drive causing 100mA of Grid current per valve.


See my Tech bul on "ALC line for Drake L-4B PA", "PTT Line for Drake L-4B PA"
"Power Factor of Drake L-4B PA", "Valve Heater delay Drake L-4B PA" & "Using
2 HF PAs".


Why Don't U send an interesting bul?

73 de John, G8MNY @ GB7CIP


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