Peavey Windsor - Hum and White Noise Hissing

[TomCarlos]

MEF members...

The other day, a friend brought over a Peavey Windsor. Using the advice from the other thread I posted to (Can I Use 6L6GC tubes instead of EL34 tubes ???? Peavey Windsor Head), I purchased and installed a quad set of El34s. They are biased at 65%.

The problem with this amp seems to be White Noise Hiss and Hum. If I pull V1 and V2, turn the amp on, I get the white noise hiss for about 10 to 15 seconds, it subsides, and I then hear a continuous hum on the output. If I pull the Phase Inverter tube, all is quite. (More on the PI tube below...).

The other oddity... If I use a dummy load, leave the PI and Output tubes installed, but no power applied, I connect the Windsor's "Send" output to another amp's Return, the slave amp has lots of hum. If I disconnect that line from the Windsor's Send and leave the instrument cable open, it gets quite. Very odd.

So there may be two different issues going on. But I want to start at the Phase Inverter. Please refer to the Output image attached....

For V3, the Phase Inverter .....

I checked all resistors connected to V3 - within tolerance.
R45 and R66 go to B + + (apprx 295 volts).

V3A, Pin 1- 136.6 vdc
V3A, Pin 2- 0 vdc
V3A, Pin 3- 1.05 vdc (~.7ma)

V3B, Pin 6- 217.0 vdc
V3B, Pin 7- 26.5 vdc
V3B, Pin 8- 66.0 vdc (~.62ma)

So it is Pin 7, how is it possible for Pin 7 to be getting 26v? Could it be that C17 is a leaker? I'll pull the board out tomorrow and check C17. But it just seems odd to be getting that voltage. And yes, I have tried different tubes.

I guess I am a bit rusty at my debugging skills. If I can resolve the P.I. issue, I might be halfway there. Thanks in advance...




Peavey_Windsor_Schematic_20060718.pdf

[Jon Snell]

If you check the voltage on the second grid of that valve, it will drop considerably as there is a very high impedance holding it to just below the cathode voltage.
Don't think into things too deeply, you will confuse yourself.
Removing V2 does what?

[Helmholtz]

V3B is wired as a cathodyne PI.
Your plate and cathode voltages show that it's working correctly.
For proper tube operation the grid voltage must be about 1V lower than the cathode voltage (as always ).
So the grid will sit at around 65V.
The bias voltage of 1V is the difference between grid and cathode voltage, which is caused by the voltage drop across R31 (being the cathode bias resistor).
As there is no grid current, the voltage at both ends of R72 must be the same.

Now if you connect a meter having an input resistance (of say 1M) to the grid, two things happen:

1) the meter input current causes a voltage drop across R72 thus lowering the grid voltage,
2) as the cathode voltage must follow the grid voltage (and vice versa), grid voltage drops even more.

Always best to find grid voltage by measuring at the lower end of cathode bias resistor (R31) as this a rather low impedance point which will not be upset by connecting a meter.

Same with a LTPI which also has elevated grid voltages.

[TomCarlos]

Thank you gentlemen....

I have placed an order for some extra 12ax7 tubes and will replace C7. For some reason, it does not give me a reading on my Cap meter. If it is open, I do not know how a signal could pass.

Good advice on measuring the voltage around V3b - thank you Jon and Helmhotz. I will be back at this one next week and will report back.

[Helmholtz]

The other oddity... If I use a dummy load, leave the PI and Output tubes installed, but no power applied, I connect the Windsor's "Send" output to another amp's Return, the slave amp has lots of hum. If I disconnect that line from the Windsor's Send and leave the instrument cable open, it gets quite. Very odd.

That could be a ground loop effect.
Interconnecting any 2 pieces of class 1 (safety grounded) equipment causes a large ground loop via the cable shield and the earth wires of the components.
This ground loop picks up magnetic hum fields from PTs etc.

[Helmholtz]

and will replace C7. For some reason, it does not give me a reading on my Cap meter. If it is open, I do not know how a signal could pass.

What is the lowest cap value your meter can read?
Always know your meter specs/limits.

[TomCarlos]

What is the lowest cap value your meter can read?
Always know your meter specs/limits.

I have two LCRs, both show the same results. C7 = 47nf, c16 = 100nf, C7 no reading. But a signal must have been passing through that cap somehow.

I'll do additional testing next week, including running a signal directly into the Return jack. I will also follow up on Jon's ask to pull just V2 and see what happens.

[Helmholtz]

I have two LCRs, both show the same results. C7 = 47nf, c16 = 100nf, C7 no reading.

Confused.
C7 = 47nF or C7 "no" reading?

A good LCR should at least read something like 2pF even with an open cap.

[TomCarlos]

Helmholtz, you are correct ... C7 = 28pf.

[Helmholtz]

Even if C7 is open the other half of the power stage will be driven via C15/C16.

[TomCarlos]

Gents....

I'm back in town and back working on the Windsor amp. I have made some progress.

On the Power Output board, I replaced C7, C16, C17, and C23 - all for good measure and since the power board was out. C35 measured ok. So doing this eliminated the Hiss and (excessive) Hum at the output. The output Hum is what I would call "reasonable" for a tube amp. You can hardly see it on the scope. The DVM calls it 2.5 mvac. If I plug my guitar directly into the Return jack, I get a clean yet low level output.

Now then, the remaining problem of excessive Hum on the Send output remains. That is odd because I do not hear that type of Hum on the output. I have tried using more than one slave amp and it is still an issue.

The other thing is this - I know this amp is designed to be a crunchy overdrive sounding amp. But one would think with the Preamp setting to low and then some Master volume, you should get a fairly clean sound. Not on this amp! It is loud but crunchy. I turned the guitar volume controls to halfway, plugging into the low gain, Preamp down to 1 or 2, Master volume around 3, and maybe, just maybe you get a clean tone.

I pulled V1 and V3 and and checked the resistors around them. Everything seems to measure within tolerance.

I have been viewing some YouTube videos were guys are doing demos on this amp. There is one video and around the 1:45 mark, I hear some clean tones - https://www.youtube.com/watch?v=arF2uHdTTp4

Short of putting in lower gain preamp tubes, I'm not sure how you get a clean tone out of this amp. I'll keep looking.

[g1]

This one does not use the typical series FX loop, so a noisy Q1 would be heard at the send only.
(edit: never mind, noise at send is hum, not hiss)

[The Dude]

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.

[Helmholtz]

Now then, the remaining problem of excessive Hum on the Send output remains. That is odd because I do not hear that type of Hum on the output. I have tried using more than one slave amp and it is still an issue.

See my post #5.
The hum is most likely not produced by your amp but by the ground loop between the 2 safety grounded amps.
No wonder it shows with other slave amps as well.
Ground loop hum is likely between any 2 interconnected class I amps.
To really hear the hum from your amp connect to a battery operated amp.

[TomCarlos]

Thank you all...

I have the feeling my friend does not care about the Send and Return... so no need to waste more time on that. BUT... if it is an issue, I will look to see if I can find the SKA-3919 or equivalent transistor.

I'll scope this out tomorrow and visually look at V1 and V2 to see what is going on.