SnapcaseZA
So I was busy jamming in band practice last night, and all of a sudden my lead channel on my amp cut out. It just went VERY soft , then came back on for a bit, then off again in an instant until it just wouldn't get loud again. Rhythm channel is fine so to me the power amp and rhythm tubes are fine.
Is this what happens when tubes go bad? I haven't had time to take it apart and look, but its my first all tube amp so I'm a real newbie here with problem diagnosis.
Can I just replace the lead channel preamp tubes? Or even just swap the rhythm tubes in place of the lead ones to check? Do I need to rebias them then?
This is the same amp which I bought off Musiciansfriend and was shipped here, so I'm not sure how long the tubes have been in the amp, or if the shipping busted them either.
arjunmenon
My guess is that one of your output stage valves is going south.
Or like you said, check the preamp tubes that feed your drive channel.
Gearhead
Fault finding is best done by trying things and see if what you find is consistent with what you thought was wrong. My first thought would be to see if the tube doing the lead channel (normally an extra gain stage kicking in) is faulty. Swap it with any identical type tube in your rig and see what changes (even if now neither lead nor rhythm channel sound ok - new information is better diagnosis). It might be several other things from your footswitch to caps to wiring etc but to test those is more work, so why not try the easier test first?
I've yet to come across a guitar amp (schematic) that needed biasing on the preamp tubes - safe to assume that they can be swapped.
In general, when it comes to simple fault finding by swapping tubes on outout tubes, I would even think it's fine putting the bias on coldest and then just swapping tubes regardless of the matching. You're not going to test for more than a minute and you're not looking for great tone either, so what's the harm?
Nicholas-L
I agree with GH. Check the easy stuff first. Swap tubes and see what happens.
SnapcaseZA
Cool, thanks for the help. I'll try that first guys.
So its only when replacing power tubes that you need to adjust/check the bias?
Also, what do you guys suggest for replacement tubes? Should I always keep a few handy? The amp has 5 x 12ax7's and 2 x 6L6 power tubes....I can never go solid state again, what a beast!
Nicholas-L
Yes only if you change the Power Tubes that you would need to re-bias.
Yes keep a few tubes handy and as for type of tubes it all depends on you.
JJ Tubes are usually a good go. I quite like Electro Harmonix Tubes as they are slightly warmer and less gritty.
karoo
this seems like exactly the same problem I'm experiencing with my Mesa's red channel.
