Originally Posted by hitman47
My Mosfet-MAX has been acting up again recently. I have a quiet, but audible hum in the background, and for some reason I can't adjust my V+ with the trimmer - this used to work a while ago. When I gently press the tubes into the sockets while the amp is on, the hum is sometimes reduced a bit but it comes back as soon as I take my hands off.
It sounds like the regulator has blown. There is no hum or ripple present if the power supply is operating correctly. The fact that the trimmer seems to make no difference in the voltage supply sort of confirms this suggestion. What voltage are you reading on V+?
As for the tubes humming, but then not when you press them down into the sockets, it could be one of the following:
1. Dirty tube pins,
2. Faulty tubes, or
3. Faulty sockets.
I think this problem is just tangential. You may indeed have less-than-clean/optimum tube pin contact, but that's not going to cause the hum all by itself. As I said, if you can't adjust V+ with the trimmer, that could mean a bad regulator (assuming the trimmer is OK). If so, the amp will be getting un-regulated DC, which can be quite noisy and full of hum and ripple.
Also, with some of my tubes, touching the tubes (I have 2 sets, 3 of those 4 tubes seem to do this to a certain degree) or even just giving the amp chassis a little nudge causes a strange microphonic effect; the touch is clearly audible on the affected tube's channel. I assume those tubes are faulty and should be replaced? Some also take much longer to start glowing with full brightness...
It depends on whether you can live with the noise when you "nudge" the amp. Microphonic tubes are somewhat common. Unfortunately, microphonics can't be tested with a tube tester - they have to be put into a device that produces analog sound (an amplifier). It can mean the tube is worthless or it may sound pretty good as long as you don't nudge it. It's up to you whether you think the microphonics are bad enough that they must be replaced. If they are Beezar tubes and you purchased them from me, of course - I'll replace them.
As for varying brightness among tubes or some starting to glow quicker than others, it means nothing.
Any help would be much appreciated. If you need more information, I'm happy to provide it.