Power Tube on the way out or something else?

[gybe!]

Just got a new amp in today but unfortunately all is not well with it....I am hoping that it is just a bad power tube, but wanted to check with those more familiar. Currently, all symptoms are start after about three minutes of play time and are somewhat intermittent. Sudden changes in tone or volume occur (volume drop and return, sudden dark or bright) and noises (even with guitar volume at zero) best described as the sound of somebody rubbing the top of a live microphone.

All power tubes are evenly lit and none appear abnormal in coloring. Tapping on any of the power tubes does not illicit any particular noise from the amp (turned on, plugged in).
My initial thought was a bad preamp tube but I have rolled through all of the preamp tubes 2x with known good tubes, no change in behavior. It is independent of guitar or cables. Thoughts?

I would just swap power tubes with one of my other heads, but they are all EL34 and this happens to be 6l6. Anytime I have had a power tube go it has taken one of the fuses with it or just been replaced due to age, so this behavior is new to me. Just wanted to get some other thoughts/ideas before I drop even more money on new tubes or bench time...

 

[Chubbs]

Sounds like powertubes are probably the answer. Could also be something like a floppy grid wire with a bad joint.
What amp is it?, I might also try pulling v1 through whatever tube is before the phase inverter and starting it up to see if the circuit is pickup up noise anywhere.

 

[GuitarGuy08]

Just got a new amp in today but unfortunately all is not well with it....I am hoping that it is just a bad power tube, but wanted to check with those more familiar. Currently, all symptoms are start after about three minutes of play time and are somewhat intermittent. Sudden changes in tone or volume occur (volume drop and return, sudden dark or bright) and noises (even with guitar volume at zero) best described as the sound of somebody rubbing the top of a live microphone.

All power tubes are evenly lit and none appear abnormal in coloring. Tapping on any of the power tubes does not illicit any particular noise from the amp (turned on, plugged in).
My initial thought was a bad preamp tube but I have rolled through all of the preamp tubes 2x with known good tubes, no change in behavior. It is independent of guitar or cables. Thoughts?

I would just swap power tubes with one of my other heads, but they are all EL34 and this happens to be 6l6. Anytime I have had a power tube go it has taken one of the fuses with it or just been replaced due to age, so this behavior is new to me. Just wanted to get some other thoughts/ideas before I drop even more money on new tubes or bench time...

You are right to check the preamp and power tubes. I had an amp that exhibited somewhat similar issues. Amp tech checked the normal stuff, said he was sure it had to be something loose but didn't want to dump the time in it. I ended up sitting down with it and just staring at the board to see if anything jumped out at me. I am not experienced at amp repair at all. I found the issue and fixed it myself! Turns out, a leg was not soldered. Odd because the amp worked fine for so long and then just had issues like yours. Look for something like this. Attached a photo.