Hi Mitch,
the same problem with my 18 watt tmb ef 86. Everything in the amp checked out good, but the sound disappears after a few minutes, when the amp gets "operating temperature". My tech said the output tranny probably has a "leak", so I have to find a new one. Do you have any idea where to find one aside from the ceriatone-made (because of the horrible shipping costs)?
Cheers, Martin