popping noise through 1 channel on Radford STA15

[jives11]

Hi,

I have a Radford STA15 Mk III. I bought it second hand about 18 years ago. It had been heavily used and was very noisy. Over the following years I replaced all the resistors (the originals were starting to crumble) The wiring and the caps. Most caps were replaced with either Polycarbs (coupling) or Polyprop Solens (PSU decoupling)
. The result was the amp has been virtually silent in terms of background noise and has given me 15+ years of pleasure. A slight mechanical hum up close is the only noise audible.

Today I noticed random popping noises through the left channel. Not at full volume and not a crackling noise as I've had before with bad joints. It seemed to increase in frequency with temperature, but was essentially random in nature. The pops sounded like little electro drum beats. (no jokes about modern music please )

Anyone know what this might be ? I'll take a look at the board. I think there are a few electrolytics which are about 15 years old in the cathode bypass. The tubes might need replacing, I just wondered if popping is symptomatic of a specific issue ?


Thanks

 

[m6tt]

I had the same problem with a monstrosity of a hammond organ amp which was transplanted into a combo I made...essentially I went through and redid some of the solders on the tube sockets that looked sketchy...I would bet that some of that ancient solder in there has been slowly heated and cooled over and over again for a long time. I would retouch it and then try again, but for goodness sake make sure no accidental shorts were created resoldering the tubes (especially with 7 pin and B9A tiny sockets). Also, try cleaning around all the sockets and b+ wiring with an exacto knife or pipe cleaner or other tool, to prevent arcing. Are any of the sockets charred? Tubes sometimes pop as well, so swap the right channel tubes with the left channel (assuming they are the same) and see if the popping stays or moves. Worst comes to worse it could be an output transformer but I doubt it.

 

[jives11]

I had the same problem with a monstrosity of a hammond organ amp which was transplanted into a combo I made...essentially I went through and redid some of the solders on the tube sockets that looked sketchy...I would bet that some of that ancient solder in there has been slowly heated and cooled over and over again for a long time. I would retouch it and then try again, but for goodness sake make sure no accidental shorts were created resoldering the tubes (especially with 7 pin and B9A tiny sockets). Also, try cleaning around all the sockets and b+ wiring with an exacto knife or pipe cleaner or other tool, to prevent arcing. Are any of the sockets charred? Tubes sometimes pop as well, so swap the right channel tubes with the left channel (assuming they are the same) and see if the popping stays or moves. Worst comes to worse it could be an output transformer but I doubt it.

Thanks m6tt - so very good advice. I'll start with valve swapping, then examine the sockets. I'm sure your right. I guess the PSU is fine as it would affect both channels.

When you clean the sockets with a pipe cleaner do you use any solvents , alcohol or switch cleaner ?

 

[m6tt]

if they're ceramic sockets you could probably use some solvents, but I am not sure what bakelite or other plastics might require in terms of cleaners which won't dissolve them. If they are ceramic, you could carefully use acetone on a q-tip, but be very careful and for goodness sake let the thing dry out really good, acetone is very flammable. That stuff will eat most plastics, so keep it away from insulation and capacitors etc. I just used an exacto knife and carefully scraped dirt and debris off the sockets while trying not to scratch them. Generally though popping is often a loose contact being wobbled some how...on my amp I had an input RCA that was wobbly into the first tube. It sounded like someone was writing a thesis on a rusty typewriter in the next room. If it's a tube the swap will put the popping on the other channel, and if it's arcing, cleaning may help (you will see charred sections of socket in this case), or you may have to replace the socket. I'd still keep my money on a loose connection somewhere heating up and resonating open and closed really fast.

 

[monk2]

Popping is a symptom of a gassy tube. Swapping the tubes is tedious, but
will help you identify the culprit. A tube tester would be helpful.

dpn

 

[jives11]

A big thank you to one and all.

I followed your advice and swapped valves until I traced the problem (a bad ECF82 on one channel). I' now back to total silence with just a hint of analog dither
.
....(50Hz if I hold my ear to the cone)