Peavey Valveking 112 question

[pontiacpete]

I have one of these come with the complaint that the power tubes are red plating. I haven't yet got them to red plate but the new ones he put in look like they have taken a lot of heat. But what I can't help noticing is that there is a low hum like a leaky cap. Upon checking voltages in the bias supply I'm find 19vac and -67vdc on the bias diode. So I thinks to myself, oh this is easy it must be the bias cap. Changed the bias cap and still 19vac. Then I think, could the diode be failing or leaking? Changed that and no change.
Then I lift one leg of C217, no change.
My question. Is this a meter reading issue or is there some other source of the AC in the bias supply or is this normal?

As always many thanks for any input to help me out.

[Jazz P Bass]

19Vac is certainly an odd number.

I would look to see that R205 (33K) is in good shape & solidly connected to ground.

Double check the ribbon cable going to the output board.

And the Texture connector.

[Enzo]

Where are you grounding your meter?

First, C217 would have no effect on ripple. You are measuring IC voltage at the rectifier, but the first filter cap is still 7k away R211. What matters is what is at the other end of R211, ie C208. Is THAT clean DC? C208 is where the bias supply comes from, not the left end of R211.

[pontiacpete]

I'll re-check the 33k.

I'm grounding my meter to the chassis at the transformer. At R211 is where I'm taking my readings. Across R211 I'm getting 19vac

[Enzo]

Across C208, ie. across R205 or from th right end of R211 to ground. C208 is the bias supply. At the junction of the diode and R211, the DC is not fully filtered. the fact you get the same AC across R211 as you do from its left end to ground tells me there probably is no AC at its right end.

[pontiacpete]

Thanks for setting me straight Enzo, I wasn't looking at it that way. There's no AC to the right of R211. Duh
Nor is there any on pin 5 of the 6L6s, double duh

On to see if these tubes red plate. They do not at idle.
I soldered the tube board hoping that was the only issue. Some sketchy looking solder joints.

[J M Fahey]

Was chiming in to say there is no way you can have 19VAC across C208 and if you did, it would mean itīs open ... but then you would not have -61V DC.
Not saying those values are impossible to find there, according to malfunction type, but itīs either one or the other, never together.
Of course, Enzo said it earlier

Hey Enzo, if being this alert and fast thinking is "the end" ..... then keep doing it that way , because you are doing fine

Sometime ago I read that people who keeps their mind working in high gear keeps Alzheimer and similar crap away for good ... not surprising since I consider it same as going to the Gym and doing a full workout ... I mean the Mental Gym of course

For non Techs, I guess solving crosswords or puzzles, or reading about History or Philosophy or whatever makes the mind muscles flex, gets the same benefits.

FWIW we know about tons of great minds not only staying fresh but getting sharper everyday until the late 90`s, so itīs time proven

[Enzo]

Not to steal the thread, but it was the end of the shop, not of me. And I do do puzzles. All the number puzzles like Sudoku are really logic puzzles. I read my local paper, but I also buy the Detroit Free Press daily because they have two whole pages of puzzles. Never hurts to exercise your reasoning skills.

[gbono]

Yes let's hijack this thread - want to figure out how to beat dementia (if that is what's on your mind

check out this book The Brain that Changes Itself | Norman Doidge, MD