[AMRadio] Johnson Valiant Audio Driver Stage Question

w0ng at comcast.net w0ng at comcast.net
Sun Oct 14 14:15:57 EDT 2007

In my eternal quest to bring some life back into my (once total basket case) 1957 kit-version Valiant transmitter, I now have a "gremlin" of sorts living in the 12AU7 audio driver stage (V15).

The 1k, 1/2w cathode bypass resistor R45 (in parallel with a 10uf, 35v electrolytic cap C99) connected from pin 3 & 8 of the 12AU7 driver tube to ground "lets all of its smoke out" after the power is on for about 8-10 minutes. 

No high voltage is applied, just sitting there on the bench idling and I can watch the resistor burn in the middle. It's weird (to me anyway) that it only happens after the rig is powered up for about 8-10 minutes.

On the off chance the resistor was bad, I replaced it and the electrolytic cap but it burned out again anyway. Checked the chassis grounds and solder joints, all OK.

I took some voltage measurements and discovered that there is ZERO voltage at the 12AU7 cathode after a brief warmup. The manual says there should be 14 volts at pin 3 & 8. When I turn on the filament switch (with the rig cold) there is a slight momentary + voltage on the cathode as the heater warms up but after a few seconds, ZERO voltage on the cathode (???). All other voltages at the other tube pins are in spec. I'm using 117vac line voltage from a Variac. 

I've gone thru the wiring to look for any obvious problems, swapped known good 12AU7's but for the life of me don't know why the cathode bypass resistor keeps overheating (until it is actually smoking) and why there is no cathode voltage. 

I'm hoping some of you more experienced folks can lend a suggestion or two to point me in the right direction. I'd like to avoid encouraging anymore grey hair on my head if I can, hi.  Thanks, Bill, w0ng

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