To : TECH@WW

By G8MNY (Updated Jun 24)
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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!

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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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