Congratulations on your build.
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When I try to jump the channels, I get hum from the normal channel.
Havent sen this before but try the opposite jacks i.e. Hi Normal to Lo TMB channel and just check the grounds arent reversed in the jacks/jumper cable. Should work.
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The amp is very quiet while playing, even with the volumes dimed on the TMB channel, and it isn't bad even with the boost engaged. But, when I roll off the volume on the guitar to zero I get a lot more hiss. Any idea why?.
If the amp is dimed and you roll off the volume, you still have the amp on max. The carbon comp resistors do have inherrent hiss which can be reduced. there are resistors to change in the signal path to do this if you want but I think this sounds 'normal'.
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I'm getting a strange effect when I push the output stage hard on either of the two channels (but it's worse on the TMB channel, probably due to the much higher gain). It's a high buzzy/sproingy sort of noise. Hard to describe really. It's most noticable on the lower notes (E & A strings mainly), especially bad on partially muted notes...they don't decay "cleanly". It almost sounds like something is overloading somewhere. .
This similar sort of thing sounds like it has happend to me and more than once. It was always a loose ground connection. We always assume that when an amp works, we did it right, but when I hear that sort of sound, I start looking for an unsoldered ground wire.
Do me a favor and check that
every ground connection is soldered.
If it's in a combo, there is a whole raft of vibrations to track down.