My twin reverb loses volume/gets real distorted after about an hour of jamming

[Jima321]

AB763 stock circuit:  This happens regardless of channel using.  Turn it of for 10-15 minutes sounds wonderfull again.  Could this be a tube going bad?  I must reiterate this amp sounds great until this happens--lots of power.

Background:  I hand wired this amp several years ago, but never played it this much till recently playing at open mic bars.  I am not a EE--just had an amp to use as a reference when I built this one. 

As far as test tools, I have a multimeter and and a tube bias tool for the 6L6s.  Any insight as to what the issue might be would be appreciated.  R/Jim

[John]

Try chopsticking (carefully) all your solder joints, resistors, caps... everything basically, with the amp turned on. It sort of sounds as if something gets hot enough to expand and short out or not connect just right, and then when it cools down you have a good joint again. Could be a lead broken inside a component, anything. Hopefully when you hit the bad spot you'll hear it through the speaker. HTH!

[stratele52]

If you have a bias tool , it would be easier to check bias  and monitoring bias from 15 minutes to distorted output.

Phase inverter cricuit ( Look for bad solder ) and / or PI tube to replace.

If bias is good , look for a bad solder.