Voltage variation on test points 12 and 28- Seductor

[tomn3]

Hello, I recently replaced a blown screen resistor (the tiny 237 ohm) on a channel at D7 (lost left channel on audio), and also replaced the MJE 350 transistor at 28, 29 and 30 as a precaution.

I ran through the resistance checks and voltage checks again, and only one voltage point was a little high- values at 12 came up at about 68V instead of the normal 30-55V.  The other counterpart at 28 was right at 56V.  I swapped tube sides and was now getting the same 68V on 28, while 12 read 56V.  I recall there was a tube variation when testing the voltages while completing the kit, at that time it was about 63V. 

I'm guessing it's the 6AU6 (when 6AU6 was swapped from one channel to the other channel, the higher voltage variation followed.

I was wondering, is the tube starting to vary too much, or is still OK at that variance (23%)?

Thanks for your help!
Tom

[Paul Birkeland]

The circuit will vary the 6AU6 screen voltage in order to set the EL84 cathode current.  If you have 100V at pin 3 of the EL84 (or very close to 100V), then the circuit is working properly and there's no need to chase anything down. 

[Paul Joppa]

At 68 volts, it's still within the servo bias control, so the amp is (as PB said) operating correctly. But the high screen voltage indicates the cathode emission is getting weak. You might want to look around for a replacement pair, just to have them on hand - hard to tell whether you have months or decades of life left, but they are inexpensive and it's always nice to have a replacement ready. You can try replacing the tube to see if you can hear any difference - that's the ultimate test!

[tomn3]

I measured 103V at pin 3 on each channel, so it looks good. 
That is great advice on keeping a replacement set ready.
I learn something new every time I visit. Thank you both!
Tom