Peavey Windsor - Hum and White Noise Hissing

[Helmholtz]

I wonder if V3 pin1 is already getting asymetrical when pin6 does.

Agree.
Also I think previous scoping was not done with texture a full power setting

[TomCarlos]

Thanks again for the help with the Power Amp section. We'll call that one done.

There is a problem in the preamp section however. We know the power amp will swing a signal using the Return jack. But when I insert a signal into the guitar input, I get very little signal at the output and it is very distorted. So I need to look at the preamp section.

For this testing, I have removed the Phase Inverter tube (as I do not want to burn up that one or the output tubes.

The DC voltages on V1 and V2 appear to be ok (or is V2b a bit too high)?.

V1a Pin 1: 189.2 Vdc
V1a Pin 2: 0 Vdc
V1a Pin 3: 1.68 Vdc

V1b Pin 6: 148.3 Vdc
V1b Pin 7: 0 Vdc
V1b Pin 8: 1.03 Vdc

V2a Pin 1: 199.3 Vdc
V2a Pin 2: 0 Vdc
V2a Pin 3: 1.30 Vdc

V2b Pin 6: 318 Vdc
V2b Pin 7: 0 Vdc
V2b Pin 8: 3.82 Vdc

Using a low input, I tested the gain for V1a. That measured out to a gain of 50.
20 mVrms at Pin 2, 1.0 Vrms at Pin 7

There is 1 Vrms at the junction of C39 and R53.

There is 824 mVrms at the junction of R53 and VR1. And at that point, the signal is clean.

The Preamp volumn is turned all the way up.

The problem is at Pin 7 of V1b. I am measuring 555 mVrms but the signal is cut off on the top end. See picture below.

So I think I will pull the board and take a look at solder connections and measure parts.

More to come on this one...

[The Dude]

So, the short of it is that signal is clean on one side of C2 and distorted on the other side. I'd definitely check that cap.

[TomCarlos]

C2 is measuring at .047uf (47nf) but it could be a leaker. I did not see any DC voltage on V1b Pin 7. I'll replace that cap for good measure. I am checking the other parts in the section of the board - everything looks ok so far.

[g1]

Why turn the pre-gain all the way up? I think you are just over-driving the grid of V1 stage afer pre-gain.
Reduce the pre-gain setting to before it clips like that and see what you get for signal voltage at V1 pin6.

[The Dude]

Good catch g1. I must have glossed over that part about pre-gain being maxed.

[g1]

The V2 second half voltages don't seem right. If B++ is 368V, pin6 is 318V, then pin8 should be around 20V. Check R65 and R21, and swap the tube out if you have not already done so.

[TomCarlos]

I was thinking that if I had the Pregain all the way up but turned down the Master volume, I might be able to spot a problem in the preamp section. So the goal here is to see how much signal can be inserted until it clips at V1, Pin 6, got it.

I checked R65, looks ok at 100K. I checked R21, measures 38.5k. I also checked C41, 1nf (.001uf).

I will finish touching up solder joints tomorrow and retest. I'll swap out V2 and recheck the voltages on that tube. If B++ is 369, what would you expect to see at V2, Pin 6?

Thanks.

[Helmholtz]

So, the short of it is that signal is clean on one side of C2 and distorted on the other side.

That we don't know and I actually doubt it.
Rather the signal will be clean before R53 and clipped after.
I think what we see is beginning grid conduction at V1b at around 0.7Vp.
The effect is emphasized by the large source impedance due to R53.

[Helmholtz]

The V2 second half voltages don't seem right. If B++ is 368V, pin6 is 318V, then pin8 should be around 20V.

Agree, but I wonder if the triode would even work with a grid bias of -20V. Looks like cutoff to me.
Maybe there's a schematic error and R21 is actually 3.9k.
Or it's really a 39k resistor causing cold clipping but then I'd expect lower plate current.

[TomCarlos]

A quick check in ....

Something interesting about R21... when I looked at the PC board, all of the resistor leads seemed to be formed nicely and the resistor centered between the holes. But R21 looked a bit wonky, as if it were messed with. The PC board screws have the lock type goo still in place so it looks like it had never been removed. So maybe no one touched R21 - I just don't know.

I am going to contact my friend and ask him if anyone has worked on this amp. I think he is the second owner. Who knows, maybe someone messed with R21.

I have searched the internet blogs looking to see if anyone has mentioned R21. There are way too many sites suggesting making mass changed to the preamp and I really don't want to go down that path.

To the best of my knowledge, there are two versions of this schematic which have been published. The version I have (and posted) matches what I see in the amp. There is also the parts list which shows R21 as 39k. But that doesn't make it right.

Who knows, maybe the engineer / designer John Fields will see this post and comment!!

[g1]

Please recheck DC volts at V2 pins 6 and 8 with no signal. If the resistors measured correct, the math needs to add up.

[g1]

Or it's really a 39k resistor causing cold clipping but then I'd expect lower plate current.

Yes, it's a cold clipper and supposed to be 39K there. The voltages don't add up.

[TomCarlos]

Alrighty.... another update.

I pulled the main board, reflowed the entire thing, changed C2 for good measure, and now have the following measurements.

Screen: 431 Vdc
B++ 294 Vdc
B+ 225 Vdc

V2a Pin 6: 178.1 Vdc
V2a Pin 7: 0 Vdc
V2a Pin 8: 1.13 Vdc

V2b Pin 6: 283 Vdc
V2b Pin 7: 0 Vdc
V2b Pin 8: 3.42 Vdc

So, if my calculations are correct, we have 1.16 ma flowing through V2a.

B++ measures 294.6

Flowing through R65, (294 - 283) / 100k = .116ma

Flowing through R21, (3.42 - 0) / 39k = ~.0876ma

I swapped out V2 for a new tube, same results.

[g1]

Those numbers are better, V2 is working fine.
Thats a substantial difference in B++ from the 368V you had before. Maybe resoldering fixed something? Have you tried it to see if it's still weak?