Valve amp volume fading in and out

CHrisP86

My amp (Bogner Atma) has an intermittent issue where the volume will just fade out and then maybe 30 seconds later fade back in again.

Any thoughts on what might be going wrong?  It's had new power amp valves recently, but I guess they could have been bad valves.

Funkfingers

Could be cheap valves. Could equally be the failure of something in the signal chain before the output stage. 

Simple tests include valve substitution (begin at pre-amp valve #1) and disconnecting anything connected via the amp's Effects send/return Loop. 

crunchman

As @Funkfingers said, swap all the preamp valves out one by one.  From a quick Google, all the preamp valves are ECC83/12AX7 types.  If you don't have spares it's worth getting one or two.

If you still have the old power valves, and they are still functional then put them back in to rule out the new power valves being the cause.  The Bogner site says it is cathode biased so you won't need to worry about rebiasing.

There is probably a >80% chance of it being a valve, but if you do swap out the valves and it continues you will probably need to take it to a tech.

CHrisP86

Thanks.  Will give that a go. 

Could it be the rectifier valve? I know I have some other 12ax7 and el-84's but I don't have a spare rectifier valve.

DJH83004

One simple thing to check, when the volume fades out, are the valves still glowing? this will establish if there is a fault in the heater circuit. Because the volume 'fades' it sounds like one or more of the valves are gradually losing emissions e.g. losing their heater supply. This can also point to the rectifier.    

dindude

You've got your tremolo set reeeeaaally slow.

CHrisP86

Thanks, it might be tricky to test as it's so intermittent so I'm tempted to just buy a full new set of valves.

KMusic

CHrisP86 said:

Thanks, it might be tricky to test as it's so intermittent so I'm tempted to just buy a full new set of valves.

Sorry to resurrect an old post. I'm having the same trouble with another amp and wondering if you found a solution to this?

ICBM

As before, fading in and out is likely to be a filament circuit problem - either an individual valve, or the supply. When it fades out, have a look to see which (or all) valves are not glowing.

CHrisP86

I ended up replacing pretty much all the valves and thought that had fixed it. But it randomly came back.

I then tried a different guitar lead and never had the problem again...

Could be a councidence and the new valves were the fix.