Owen wrote: Sun Oct 02, 2022 6:38 pm
Mesa 400+
Play 1/4 of our set, all of sudden it drops about 10% volume (no crackle, no problem sounds), we stop, turn up volume and turn it back to where it was and it is back to normal. Play another 20 min everything totally fine. Start the set again, and the volume drops again, do the same thing and it is fine for 10 min and it happens again, after this time, it is stable for the rest of practice.
What do you think, bad power tube?
If it's one of the 12 power tube versions then one power tube going bad would not lose that much volume. Hell, I bet you could pull one power tube at random and not really be able to hear a difference.
What's happening is that something is causing the signal to drop somewhere in the circuit. Could be a preamp tube, a solder joint, a component, some node of the power suply, etc. But it's probably something well before where the signa splits to hit all the power tubes in parallel.
FIrst thing I'd do is test all the tubes. Then I'd run the line out and/or effects send into another amp or interface and try to get the amp to fail. Basically, any available output will tell you something. An excellent scenario would be to take a send from the effects send, the slave out, the balanced out, and then mic the speaker cab and plug it all into an interface and record all four channels at the same time
Then, when the amp fails and is in the failure state, you can see at what points the problem shows up. I don't know the 400+ circuit by memory, but my guess is that the FX loop is in the middle of the preamp, the slave is at the end of the preamp, and the balanced out could be pretty much anywhere. Since the failure happens at the speaker cabinet, you know it has to be the speaker cable or earlier. However, if the effects loop out does NOT lose level when the speaker loses level, then you know the problem has to be in between the effects loop and the speaker jack.
In short, chop the amp up in to sections and find out which sections break and which do not break
