Rare I get a item that has me stumped but given it's a tube amplifier made some 60+ years ago and I haven't played with tubes for over a decade this one seems worth asking about.
I don't even know where I got this one from exactly but I have had it a long time and never so much as turned it on until today.
It powers up but makes all sorts of fine crackling sounds and as the tubes first warm up it puts out fair sound but as the continue to warm up after about a minute the sound gets distorted and fades out. The cracking stays but no further sound comes through.
I've swapped out all the tubes with good spares from other old amplifiers that do work and they all checked out good so what am I looking for that would cause this?
I'm guessing I have a tube bias voltage drifting up or down on one but I can't seem to catch it since I don't know what tube it would be or what would be normal to begin with since I have yet to track down a correct online schematic so far.
12ax7 pre amp, 6AV6 ?, 7247 output tube driver and 2 6L6 outputs.
I don't even know where I got this one from exactly but I have had it a long time and never so much as turned it on until today.
It powers up but makes all sorts of fine crackling sounds and as the tubes first warm up it puts out fair sound but as the continue to warm up after about a minute the sound gets distorted and fades out. The cracking stays but no further sound comes through.
I've swapped out all the tubes with good spares from other old amplifiers that do work and they all checked out good so what am I looking for that would cause this?
I'm guessing I have a tube bias voltage drifting up or down on one but I can't seem to catch it since I don't know what tube it would be or what would be normal to begin with since I have yet to track down a correct online schematic so far.
12ax7 pre amp, 6AV6 ?, 7247 output tube driver and 2 6L6 outputs.