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Looking for help with noise problem in 20-year-old Williamson amp

Hi,

I've had a couple of 6GU7/7355 Williamson amps for many years. They're probably getting on for 20 years-old.

One of them has developed a noise problem that is quite noticeable during quiet passages or music. Or it could be that an existing noise problem has got more noticeable.

I have a couple of other larger Williamson amps and no problem there either which leads me to think something other than interference.

Anyway, it sounds like pffft pft pfffffft etc. I think it might be more evident just after turn-on. It's not continuous; there are relatively quiet periods. On the scope it looks like this:

Output 3.jpg

The big spikes correspond with a "pffft" sound culminating in a "tick" from the speaker.

There's no feedback in this amp. I can trace the noise and spikes all the way back from the output to the output of the front-end voltage amp.

There is spiky noise on the input whether the amp is on or off but the the spikes don't correspond with the spikes in the output when the amp is on.

Input and output.jpg

There's no RF filter on the inputs of any of my amps. The power rails look clean enough.

So, any suggestions please?
 
If the first tube is removed, the noise will go away if it's a component in the first stage, tube, plate resistor, etc.
If carbon comp, the plate resistor is a likely culprit.

Thanks. Great troubleshooting suggestion.

I can confirm that the noise is still present with the first tube removed.

Attached are pictures of the resistor around the driver and output tubes. They don't look like carbon composition resistors although the 2 47k anode resistors on the driver triodes might be.
 

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rayma

Member
2011-04-29 8:37 pm
I should have guessed that, since if it were in the first stage, it would probably be louder than it is.
Ok, remove both the first and second tubes. Is the noise gone now? If not, is there a third tube
before the outputs? (Guess what's next.) Can you post the schematic?

Once I found a carbon comp dropping resistor in an amp's power supply that caused audible noise.
You never know.
 
Last edited:
Thanks all for your help!

Here's the schematic.

Schematic.png

There's a full wave rectifier driving 2 voltage doubler capacitors (270uF/450V) followed by a choke and 330uF/450V capacitor driving what you see in the schematic.

I removed the driver tube and the problem was still there.

IMG_1242.jpg

So, I removed the output tubes and the problem wasn't there. Tiny spikes only:

no tubes.jpg

Here's the resistors around one of the output tubes and an overall view. Do any of the resistors around the output tubes look like I should replace them?

output tube socket.jpg

IMG_5430.jpg

Thanks again!
 
Perhaps short R12 and R15. And bypass each 7355 cathode.

The noise is still present with R12 and R15 shorted and the cathodes bypassed (I tried 63n and 270uF) the noise is still there.

Here are a few screen captures showing the output (top in yellow) and 400V HT (bottom in blue) with R12 and R15 shorted and the cathodes bypassed.

The HT does seem a bit noisy but I don't see any correlation between the output noise and the HT noise.
 

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If R12 and R15 are shorted and the noise is still there, it is in the output stage. Confirm it by removing the driver valve, V2.
The noise could just be worn output valves or a problem with the cathode bias tree. Check the pot has a good wiper connection and it is wire wound.
This is not a Williamson design. He would not use oversized capacitors or over done cathode resistors as that shows the designer has no idea.
 
And as a last resort to include/exclude the output stage valves (and valve base terminal connections) from the fault tree, you could remove those valves and temporarily link the anode to cathode terminals with power resistors of equivalent dc operating resistance. Eg. for 20W dissipation, a temporary 4k7 resistor of at least 30W rating should be ok.

PS. Such a test doesn't rule out a bad screen resistor.