HT-20 Head problem with volume stability on overdrive channel

[thephantum]

Considering that this thread is two years old and the post count, I doubt the original poster is still around.

Your issue sounds like a bad OD volume pot.

Since the amp is out of warranty you can test the pot with a multi-meter by checking for resistance. See here - http://www.instructables.com/id/How-to- ... /?ALLSTEPS

Best practice is to isolate the pot from the amp, so that downstream components don't impact the measurement. I believe (I can't remember as I have not been inside of mine in a while) the HT-20 has a type of ribbon cable going to the pots. So you might be able to unplug/disconnect it. If not, it will need to be desoldered. Just be sure to drain the filter caps before poking around inside the amp. Even when powered off and unplugged from the wall, tube amps can store lethal current levels in those caps. So they need to be rendered safe first.