[mourip]

I have a "rehabilitated" Single Power Extreme amp that runs 6SN7/5998 tubes. It is ~9 years old. I followed some suggestions on HF and from Kevin Gilmore to replace the transformer and a few other under-spec'ed parts. It has been working great and I really enjoy the sound especially now that I am running my Yggy through it.
 
To the point. It is starting to be a little noisy. This noise is inconsistent and present whether music is playing or not. It does not seem come from one specific channel. It is there when cold or when warmed up. Changing tubes does not help. I also opened up the amp and re-soldered as many joints as I could get to.
 
The noise is like a dull, muted, irregular "rustling". It is never terribly loud but is still distracting.
 
Any ideas?

 

[NA Blur]

Are the tubes high quality and new? Tube amps can take up to 20 minutes to settle. To me it sounds like tube noise.
 
Perhaps try another outlet.

 

[mourip]

Thanks. Pretty much NOS tubes but I have tried swapping them out with others and still have the same symptom.
 
Good idea about the outlet. Will give that a try.

 

[dsavitsk]

That sort of noise is consistent with something oscillating. If it just started recently and happens regardless of the tubes used, that suggests that a part has gone bad -- likely a capacitor somewhere.

 

[NA Blur]

I was thinking a bad cap initially too, but those usually produce high frequency noise in my experience. See any bulging electrolytic caps?

 

[mourip]

That sort of noise is consistent with something oscillating. If it just started recently and happens regardless of the tubes used, that suggests that a part has gone bad -- likely a capacitor somewhere.

 
Caps look good on physical inspection. I also tried disconnecting the IC's and also plugging it into an AC filter. No joy. It has no hum or hiss.
 
I would consider recapping except for the way SP locked the board to the chassis with the tube sockets. It would be a very big job to redo it and I would be half way to buying a new amp by the time I was done, especially considering my time.
 
To bad because when it sounds so good.

 

[NA Blur]

My last advice is to look at the grounding especially from the power supply, relays, and transformers. Any transformers you did not replace?

 

[DutchGFX]

I'm not expert, but I would see if you have a grid leak resistor on the tubes, like 500k, to reduce oscillation as Dsavitsk pointed out. Also might wanna see if adding/replacing the cathode bypass cap helps. Could also maybe be floating filaments. I'm not an expert, so someone can correct me if I'm wrong 

 

[dsavitsk]

I would see if you have a grid leak resistor

You mean grid stopper. Unlikely the issue unless it has always been there.

 

[DutchGFX]

You mean grid stopper. Unlikely the issue unless it has always been there.

I was thinking a resistor from grid to ground and the grid stopper, both of them

 

[mourip]

Sorta in the same position regarding replacing board parts. To compound things Mikhail who ran/owned SP seems to have built inconsistently. I have a schematic that is a very rough estimation of the circuit but not all parts values are fulled in and the positions numbers stenciled on the board have nothing to do wih those on the schematic.
 
Regarding the transformer there is just one. It is the ElectraPrint custom one I put in after emailing Kevin Gilmore who helped a lot of us make our SP's safe a few years ago.
 
There is one resistor on the board that looks a bit over heated. Because of the poor schematic I have had trouble identifying its function. I may test some voltages in either end and see if it might be a dropping resistor in the HV supply. If it is stressed it might be noisy.
 
Thanks for the suggestions!

 

[kevin gilmore]

The dc filament circuit has peak currents in the 25 amperes. So look for any bad solder joints, or overheated parts.
Since the transformer was replaced, the board had to come out, and I did not put back in any of the impossible to remove screws.
 
Filament capacitor, or capacitors, and filament resistor should be changed, the insides of the thing runs brutally hot.
Likely the noise is a bad B+ capacitor. 

 

[mourip]

Thanks Kevin. Much appreciated.
 
Sounds like you rebuilt some of these for others. A few years ago you helped me with specs for the new transformer.
 
Believe it or not I replaced the original transformer without removing the main board. I drilled holes between tracings to get to the transformer bolts.
 
On my unit the tube sockets are attached to the top of the chassis and soldered to small round daughter boards which are in turn soldered to the main board. Classic Single Power engineering!
 
I will see if I can replace some of these parts without removing the chassis. Not sure I am up for a full rebuild...

 

[kevin gilmore]

there were so many different versions, and I do not remember what I did and for who.
 
a few of the units I did, I completely ripped it apart, removed the broken sockets, replaced with yamamoto and the round circuit boards.
 
you might actually have an issue with the tube socket on the input tube, rusting and then going high impedance.
 
you can probably remove all but the filament cap from the front of the board without removing the board.

 

[mourip]

Success. I replace the scorched resistor which I figured to be the dropping resistor just before the power supply decoupling cap for the driver tube. Now I have a quiet amp again with a nice dark background.
 
Much appreciation to those who offered ideas!
 
Best,
 
Paul