1993 JCM900 4102 Sound Decaying

[chudsy]

Hi All,
First post so please be gentle!!!
I've had this amp since it was basically new but it's seen very little use unfortunately but it's time to get it back up and running.
The first step was to change all 10 pots as MVs were like on/off switches and most were crackling so all 10 were done.
After doing that I put it all back together and all was well initially - plenty of grunt etc.
I then noticed that after maybe 10 minutes of playing at higher volumes the sound started to get 'farty' so to speak, even woofy could be used to describe it as if the speakers were shot but they obviously run ok to start with.
I've got two new F&T caps coming as mine are originals but they're not swollen or anything just from what I've read that they're probably knackered by now. I've rolled the preamp tubes and tried a different set but it still does the same.
Luckily there's a tech about half a mile from where I live but I just wanted to see if there was anything to try before going to him.
I've cleaned all jack sockets and put a patch cable in the effects loop also and I'll be trying the guitar direct into the return loop tonight to see if it happens again.
Just wondering if anyone has had or seen this before?
Regards
Paul.

[g1]

Look at the tube heaters and see if any go out when the sound quits.

[chudsy]

Thanks G,
I spoke to the local tech last night but he's had some major surgery so we had a good chat and told me to try a few things.
There was a very low volume woolly sound when I pugged into the effects loop so I cleaned the valve seats. I also noticed that when I tapped v2 with a pencil that the sound broke up so I replaced the valve and that seems to have made things better but it was getting late and my wife kept yelling at me to turn it doen so I didn't wake the kids!!!!
More testing tonight so I'll let you know how I get on.
I'll have a read today about biasing also.
Thanks again.

[chudsy]

Just measured the plate voltage at 492 for each valve - is this too high?
At least it's consistent!!!
Thanks,
Paul.

[Jazz P Bass]

Did the problem occur before all of the pots where replaced?

If not, go back & check your work.

[olddawg]

Very odd to have to replace 10 pots unless the amp has been abused. Did you try simply spraying them with Caig first? You should get some flux, a small brush, a good temperature controlled soldering iron and quips and alcohol to clean up afterward. Go over your work in good light and use a magnifying glass. Be aware that you may have caused bad connections anywhere in the amp by manipulating it when you changed all of those pots. Sometimes resoldering the everything is an acceptable approach for intermittent problems. You don't need to ad 10lbs of solder to the amp. You just need good clean, reliable connections.

[chudsy]

Hi,
Amp has been in my possession for the last 20+ years but rarely used so I know it's not had a bad time apart from dust (swapped a JMP50 for it!!!!). I did try a clean but there were some which wouldn't behave so I thought I may as well do the lot as they're not really expensive to buy a full set.
After swapping out the crackling v2 and cleaning off the electrolube cleaner / lubricant things are looking on the up as I've just had it cranked for a good 20 minutes with no break up as before.
Just leaving it on standby for a while then I'll crank it up again and see what happens.
There are a few dry looking joints which could maybe do with a freshen up as well.
Thanks for the responses folks!!!
Paul.

[g1]

Was that 492V on the plates of the power tubes? Do you get that same voltage now when it is working right? Sounds a bit high, but maybe normal. If the power tubes are not working right that plate voltage may read high. This could be a result of lack of screen voltage.
The screens get their voltage through the high/low power switch, so give it a bit of exercise as it may be dirty/oxidized.

[chudsy]

Hi G,
Yes, I measured at pin 3 at high power as I've never switched it down to 50. I'll give the switch a clean and report back.
Many thanks,
Paul.