5F6A Noise problem

[philster]

Hi Gents
I've been gigging my Bassman build for the last year and a half (after you good people helped me troubleshoot it). But recently she's developed a problem which has reached the point where I have had to stop playing her and I am at my wits end trying to sort it out.
The issue is this - originally, after an hour or so the amp would make "rustling sounds". I though it was a problem with a dodgy lead but over time the problem has been occurring more and more. Then I started getting "gunshot" sounds - in low light every time the sounds happen the power tubes would flash blue. I swapped the power tubes to a new pair, rebiased and everything was fine. For a little while.......
After a couple more gigs the noises started again, usually after an hour of being switched on. I assumed this was a thermal issue and have been checking for burnt out resistors or caps. Anything that looked even slightly dodgy was replaced. It then reached the point where the amp would make the noises almost immediately it was powered up and the tubes reached temperature. I've swapped out the preamp tubes to no avail. If I pull the PI tube the noises stop. If I replace the PI and pull V2 it stops, replacing the PI and V2 but removing V1and the noises re-occur.
I've checked the voltages and everything is within spec. I've replaced most of the electrolytics and the voltage dropping resistors with new and there's no change.
Is there anything obvious I am missing? Is it a transformer that is going faulty? I'm pulling my hair out - if it was a new build I would understand it but it is an amp that has been performing faithfully for 18 months.
Thanks in advance,
Phil

[Stevem]

I have just had a popping issue like this with a Marshall major and it was preamp tube socket and tube pin related.

You did not say how old the amp was but it would not hurt to replace all of the plate load resistors in the preamp section and also how old are the filters?

[philster]

Thanks Steve - she was built from scratch, so it's all 18 months old. I've replaced the plate resistors with new, also some of the electrolytics. I'm going to do the bias electro's next and see - I've cleaned the tube sockets and re-tensioned the power tubes sockets. I'll do the preamp ones in the morning and see if that helps.
Cheers
Phil

[Milkmansound]

Try to isolate which gain stage it's from. I've had mica caps cause that by leaking, tubes cause it, and also plate load resistors. Could be cathode resistors too.

Heat the amp up, get the noise to start, and then start grounding out the low voltage sides of coupling caps until it's gone. Once you have the gain stage nailed down, there's only 3-4 parts that could be responsible and then your work is much easier!

[philster]

Thanks for the help, Gents. If I pull V2 the noise stops - I've chopsticked everything and the only noise I get is when I tap the plate wire to pin 2 of V2. I've reflowed the joints and swapped out most of the caps for new - I reckon it could be the tube socket. Is it worth replacing this or should I check something else first?
Cheers
Phil

[beasleybodyshop]

If the problem is the tube socket, then wiggling the tube in the socket while it is on should make the noise you are describing.

[philster]

If I wiggle the tube it doesn't make a noise - I'm thinking it may be an electro on the way out. I have new caps and a new socket on their way - hopefully with these parts swapped out it should be resolved. Hopefully!

[Firestorm]

So it seems like the problem is V2. What voltages have you got there? There's a cathode follower so it may be a problem with the H-K voltage limit. A lot of modern tubes hate being used in that position.

[philster]

Pin 1, 7 and 8 I get 180V, 334v on pin 6. I've swapped half a dozen tubes in there with no change.

[philster]

Latest update....
Have replaced all the electrolytics in the amp with new and replaced the tube socket at V2. No change...still the same noises blue flashes from the power tubes All the voltages seem fine - as I have previously said, I've been gigging the amp for over a year so its not like she's a new build in need of trouble shooting.
Final thing I can think to check - could the volume pots need replacing? I've sprayed them with cleaner and they work smoothly with no crackling. But they are the final stage before V2. Or the 270K mixer resistors?

I notice that the jewel light flickers and varies in brightness all the time the amp is on - could this be helpful in tracking down my gremlins or is it just a bulb on the way out?

Cheers
Phil

[drew]

Is the heater voltage steady, or is it jumping around in sync with the light's flickering? (It's the normal Fender 6 volt indicator light connected to the heaters, correct?)

[philster]

Yes, normal Fender 6V heaters. Just checked and the voltage is steady, even when making the noise. So that's a good thing, yes?

[philster]

I unsoldered the wire to pin 2 (the grid, right?) of V2 and still the amp would make the noise after it warmed up. I pull V2 and it stops - what should I be looking for next? I gotta be close
Cheers
Phil

[JazzGuitarGimp]

Have you tried a different tube at V2?

[philster]

Yeah, a bunch of trusted ones - still no change.