Peavey Windsor - Hum and White Noise Hissing

[Helmholtz]

Yes, meanwhile I found that V2b circuit is identical to the cold clipper in a Soldano SLO (who copied?).
Acc. to Kuehnel the plate current is 0.1mA (103ľA): https://www.ampbooks.com/mobile/clas...s/soldano-slo/
So the voltage drop across the 100k plate resistor is 10V only.
Your low B++ explains the voltages.
Question is why is it that low.

[TomCarlos]

I believe I still have an issue somewhere....

In the typical amp, we inject a signal, V1 has some gain (as I do), V2 might have some additional gain, but I do not.

If I start with 20mVrms at V1, Pin 2, I see 1.0Vrms at Pin 1 - a gain of 50.

At the junction of R53/C37 and Vr1, that voltage drops down to ~825MVrms.

As I start to increase the Pre Gain pot, it seems like the max ac I can get going into V1b Pin 7 is around 60mv.

I checked the pot and it seemed to be ok. So that is the first road block - why so little AC signal at V1, Pin 7.

If I look at V1b, Pin 6, I get a clean signal (with 60mVrms at Pin 7), I see a clean 3.3Vrms on Pin 6. So that is a gain of 55 (give or take). At that point in the circuit, I do not know if this is excessive AC signal.

Going down the line, I do not see any AC gain with V2. In fact, it I look at V2, Pin 6, I see a paltry 270mVrms. I seem to have signal loss.

The way the amp sits right now, if I adjust both the Pre Gain and Master Gain, trying to adjust for the cleanest sine wave at the output, (with my scope on the output jack, the ground to chassis, I see only 9Vrms. And with a 4 ohm load, that is only 20 watts.

So I am trying to go through stage by stage, wanting to know and needing to see that I do not have signal loss.

Stumped.

[Helmholtz]

I believe I still have an issue somewhere....

One of them is B++ being too low by 70V. Didn't you measure 368V before (post #24)?
(First thing I do with a bad amp is checking all supply voltages.)

As I start to increase the Pre Gain pot, it seems like the max ac I can get going into V1b Pin 7 is around 60mv.

How only 60mV now? How do you measure?
Scope pic in post # 33 says 555mVrms.

[Helmholtz]

A typical bypassed ECC83 gain stage has a gain of 50 to 60 (depending on external load).
Gain can only be determined with a clean signal.

[TomCarlos]

I apologize for the confusion.... Let's focus on Supply and Tube voltages first.

I tossed one of my volt meters and labeled everything now. I installed new 12ax7 tubes on V1, V2, V3 and have them marked. I'll do my best to keep a consistent environment.

I have checked these voltages with three DVMs.

Screen @ P10, 429 Vdc
B++, @ P11/P12, 292 Vdc
B+, @ P13, 225 Vdc

AC Heater voltage to V3, V4, V5, V6, V7, 6.4 Vac
DC Heater voltage to V1 and V2, 6.29 Vdc
Bias P29: -41.22 Vdc

V3, Pin 1, 131 Vdc
V3, Pin 3, 1.1 Vdc
V3, Pin 6, 213 Vdc
V3, Pin 8, 77.5 Vdc

V2a Pin 1: 176.4 Vdc
V2a Pin 2: 0 Vdc
V2a Pin 3: 1.20 Vdc

V2b Pin 6: 280.1 Vdc
V2b Pin 7: 0 Vdc
V2b Pin 8: 3.5 Vdc

V1a Pin 1: 171.5 Vdc
V1a Pin 2: 0 Vdc
V1a Pin 3: 1.43 Vdc

V1b Pin 6: 136.0 Vdc
V1b Pin 7: 0 Vdc
V1b Pin 8: 1.0 Vdc

[Helmholtz]

Looks consistent within normal tolerances now.

[TomCarlos]

Phew !!

Now that we have that squared away, I am back to the original problem - Hiss, some Hum, and no clean output.

If I plug my guitar in the Return jack, I have what I would call ample volume and a clean tone. We tested the output section and everyone appears to be in agreement, 70 something watts RMS and we call it good.

When I plug the guitar into either the Hi or Low inputs and barely turn up the Pre Gain volume and barely turn up the Master Volume (with the Texture at Full power Class AB and all other controls at 50%), all I hear is distortion. It is nearly impossible to get a clean tone out of this amp.

So I am trying to determine how best to "test" the amp to see if and where there are any issues with the AC signal path.

And while I do this, I am updating my notes and lessons learned thus far.

Thanks.

[Helmholtz]

For loudest clean tone always turn the master fully up.
Otherwise clean headroom gets limited. The lower the MV the less clean is available.
The MV is for players who want a distorted sound at a lower sound level.
For a clean sound a MV makes no sense. Lower preamp volume instead.

[g1]

The previous measurement of 555mV at V1 pin7 was with a scope, so defective meter does not matter. Are you no longer able to repeat that condition?
If you can only get 60mV there now, something has changed.

[TomCarlos]

Now that I am working out the kinks with my test equipment... I think I want to do the signal testing in steps.

Step 1 - Testing V1a and V1b

I am using my Scope and built in DVM for measurements, also have an HP 400E AC Voltmeter – they track fairly close.

I am using a 1Khz signal, low gain input, Pre Gain turned all the way down.

I am starting out with an input signal at Pin 2 of 20mVrms. I get 1.0 Vrms at Pin 1, a gain of 50, clean signal (and easy to calculate gain of the first stage). Photo Pin 1, 1.041 Vrms.

At the junction of R53/C37 and VR1, I am seeing approximately 825 mVrms. Photo R53 and VR1 unloaded, 831mVrms.

With the scope at V1b, Pin 7 and the Pre Gain turned all the way up, the 825 mVrms drops down to approximately 544 mVrms. So cranking on the Pre Gain pot puts a load on V1a. I seem to be losing 50% of my signal at that point. Photo R53 and VR1 loaded, 544 mVrms.

With approx 544mVrms at Pin 7, when I measure Pin 6, I get 30.62 Vrms with some light clipping. Photo Pin 6, . So that is a gain of approx. 55.

So far so good?

[g1]

Yes that's good. Turn down the pre-gain to around 10 o'clock. What AC volts do you get at V1 pin6 ? V2 pin 1 and 6 ?

[Helmholtz]

I am using a 1Khz signal, low gain input, Pre Gain turned all the way down.

I am starting out with an input signal at Pin 2 of 20mVrms. I get 1.0 Vrms at Pin 1, a gain of 50, clean signal (and easy to calculate gain of the first stage). Photo Pin 1, 1.041 Vrms.

At the junction of R53/C37 and VR1, I am seeing approximately 825 mVrms. Photo R53 and VR1 unloaded, 831mVrms.

With the scope at V1b, Pin 7 and the Pre Gain turned all the way up, the 825 mVrms drops down to approximately 544 mVrms. So cranking on the Pre Gain pot puts a load on V1a. I seem to be losing 50% of my signal at that point. Photo R53 and VR1 loaded, 544 mVrms.

With approx 544mVrms at Pin 7, when I measure Pin 6, I get 30.62 Vrms with some light clipping. Photo Pin 6, . So that is a gain of approx. 55.

As said earlier gain values between 50 and 60 are normal.

R53 and VR1 form a voltage divider. With VR1 at zero and at low frequencies the divider ratio is 2/3. Above 700Hz the impedance of the HP filter (R53//C37) drops so the divider ratio increases.
Turning up VR1 places R13 in parallel with VR1, so divider ratio drops to almost 50%.

BTW, connecting a 1M scope or meter also lowers the signal at high impedance points.

[Helmholtz]

Can't you really get a loud and clean sound from the amp with master fully up and pre gain low?

Did you try a patch cable between send and return jacks?

[TomCarlos]

I appreciate everyone hanging in there with me. Seems like most of my hobby work is replacing jacks soldering, checking tubes, etc. It is easy to overlook things like “voltage dividers” are also applicable to ac signals!! So thank you Helmholtz, G1, Dude, and Jon.

Ok, I am going to try and include some test requests from you all in this post.

First, confirm that my DC voltages are the same as last night. Check !!

Helmholtz asked- “Can't you really get a loud and clean sound from the amp with master fully up and pre gain low? Did you try a patch cable between send and return jacks?”

Tom replies, Yes, but with a caveat. Guitar connected to Hi input jack, guitar volumes at 10, Master Volume on 12, Texture at Full Power, PreGain at 3: This isn’t a Fender Clean, but I will now say it is “Clean.” After all, this was not designed to be a clean amp. But if you look at the G1 request and tests, we can see an issue at V2b.

Something to note, with PreGain at zero, as I increase the Master Volume, the hum and noise increases. That is to be expected. See Photo 1, Output Noise photo.

G1 asked: - “Turn down the pre-gain to around 10 o'clock. What AC volts do you get at V1 pin 6 ? V2 pin 1 and 6?”

Tom replies: Using my same setup as before, 20mv at V1 Pin 2, 1.0Vrms at V1

Test 1 Photo 2 shows V1, Pin 6. I am seeing 1.768 Vrms.

Test 2, Photo 3 shows V2 Pin 1. I am measuring 53.27 Vrms. This looks like a gain of 30 but the waveform looks a bit wonky on the bottom end.

Test 3, Photo 4 shows V2, Pin 6. As you can see, the cutoff is now on the top edge of the sine wave and meter reads 14.14 Vrms. That doesn't look good.

The Dude asked – “If you are down to early distortion, insert a test tone and use scope to see which gain stage clips first. At least you might be able to narrow it down to where clipping starts.”

Tom replies – I guess you can refer to the G1 tests and we can see the problem seems to be at V2b. I don’t think I am overdriving the amp that hard, am I?

Regarding the Return Jack – G1: This one does not use the typical series FX loop, so a noisy Q1 would be heard at the send only. I am betting my friend could live without the FX loop. If in any way this is introducing noise, I would bypass it in a heartbeat.

[Helmholtz]

Can't see anything wrong here.

Test 2: You're missing the 50% voltage divider R54/R55 before V2 pin 2, so gain again is around 60..

Test 3: 53Vrms is huge and even though it's again reduced by 50%, V2 can't handle that large voltage without clipping.
Remember this stage is designed as a cold clipper.
Lower pre gain until the signal is clean at V2 pin 6 and measure power output with MV maxxed.