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

[dch53]

Remove all the tubes and replace them with the tubes from the other amplifier. Is the noise gone now?

Thanks. That was the first thing I tried.

 

[rayma]

Thanks. That was the first thing I tried.

And what happened?

 

[dch53]

And what happened?

The noise stayed with the problem amp. The problem is something to do with the output stage or power supply. Next step is to identify which.

 

[rayma]

The noise stayed with the problem amp. The problem is something to do with the output stage or power supply. Next step is to identify which.

Then try one output tube only, along with the rectifier, testing both sockets. Does noise happen only
with one particular output tube socket?

 

[wiseoldtech]

A signal tracer is what I'd use - going from input to output.
This can be used along the B+ feeds as well to listen for noises.
It's the "stethoscope" us techs use and saves us lots of time.

 

[dch53]

Then try one output tube only, along with the rectifier, testing both sockets. Does noise happen only
with one particular output tube socket?

Woo hoo! The noise is only present with a tube (either of them) in one of the two sockets. The grids are still shorted to ground.

That narrows it down to the cathode resistor chain associated with that socket.

 

[rayma]

Woo hoo! The noise is only present with a tube (either of them) in one of the two sockets. The grids are still shorted to ground. That narrows it down to the cathode resistor chain associated with that socket.

The pot is still not yet in the clear, though. Clean it and exercise it a few times before replacing the resistors.

There's a chance it's a resistor, replace with a good part. A single 1k is fine for replacing the three resistors on each end of the string. The two 1k resistors should be matched to 2% or better if possible. The power rating of each 1k should be
3W to 5W.

 

[dch53]

The pot is still not yet in the clear, though. Clean it and exercise it a few times before replacing the resistors.

There's a chance it's a resistor, replace with a good part. A single 1k is fine for replacing the three resistors on each end of the string. The two 1k resistors should be matched to 2% or better if possible. The power rating of each 1k should be 3W to 5W.

Thanks. The pot is a very old Allen Bradley carbon type so I'll replace them with new wirewound ones while I'm at it.

 

[rayma]

Thanks. The pot is a very old Allen Bradley carbon type so I'll replace them with new wirewound ones
while I'm at it.

There are two screen resistors, look at those if no results.

 

[dch53]

There are two screen resistors, look at those if no results.

Thanks. There's a minimum order quantity to avoid a $24 shipping charge so I'll replace all of them.

 

[rayma]

Thanks. There's a minimum order quantity to avoid a $24 shipping charge so I'll replace all of them.

Those should be 1W - 2W, 100R at 5% or better. That's a pretty steep shipping charge.

 

[dch53]

That's a pretty steep shipping charge.

The cost of not living in the USA!

 

[rayma]

The cost of not living in the USA!

When you put it that way, it sounds much more reasonable.

 

[Chris Hornbeck]

It's a tax on it being late spring there right now. Seems worth it.


All good fortune,
Chris

 

[rayma]

It's a tax on it being late spring there right now. Seems worth it.

Except for the plane ticket.

 

[trobbins]

You could short the pot ends to ground to identify if it was the pot - idle current isn't going to go up much.

 

[rayma]

You could short the pot ends to ground to identify if it was the pot - idle current isn't going to go up much.

That's a good idea. It's a strange balance circuit, never saw that done before.

 

[gem_piano]

I had a vintage amplifier a while back that developed an intermittent, low level rustling noise. I changed a couple of carbon plate resistors to film type, but to no effect. Finally, I pulled out all the tubes and scrubbed the pins with 400 grit sandpaper. Result: rustle gone, now a very quiet amplifier!
Cheers,
Roger

 

[baudouin0]

It usually a poor connection or dry joint where an o/c can produce a large voltage drop. This causes a minute arc which cleans the contact. Hence the rusting noise.

 

[wiseoldtech]

I had a vintage amplifier a while back that developed an intermittent, low level rustling noise. I changed a couple of carbon plate resistors to film type, but to no effect. Finally, I pulled out all the tubes and scrubbed the pins with 400 grit sandpaper. Result: rustle gone, now a very quiet amplifier!
Cheers,
Roger

That's what us old techs have been doing for decades now, but with emory paper.

Along with "pinching"the tube socket connectors to insure a nice tight fit on the tube pins.
Deoxit sprayed lightly in the sockets also cleans things up.
"0" ohms is your friend here.