Trainwreck Express clone, power amp troubles

[pietro_moog]

Hi guys. I built a Trainwreck Express but it has some major problem. the preamp is ok. the power amp is a mess. this is no my first amp, so i know what i'm doing. the point is this:
the amp works well with the power soak at low volume, really beautiful amp. but when i turn it up, it starts with a loud noise, feedback-like, on the low-mids. it sends me out out of my mind, i cannot understand what's happening. i mean, the signal coming from the amp is the same. it looks like it depends on how loud the amp is in the room. it is not just the room shaking, but a strange feedback phenomenon. (not squealing)
i don't really understand what i'm going through. i also have some loud electrical noise, but i think it is the heaters tap not elevated.
for starters, i thought it was a bias problem, but the values are about right, and the amp sounds really good with the attenuator at low volume.
any help would be very much appreciated.
thanks guys

[nosaj]

Hi guys. I built a Trainwreck Express but it has some major problem. the preamp is ok. the power amp is a mess. this is no my first amp, so i know what i'm doing. the point is this:
the amp works well with the power soak at low volume, really beautiful amp. but when i turn it up, it starts with a loud noise, feedback-like, on the low-mids. it sends me out out of my mind, i cannot understand what's happening. i mean, the signal coming from the amp is the same. it looks like it depends on how loud the amp is in the room. it is not just the room shaking, but a strange feedback phenomenon. (not squealing)
i don't really understand what i'm going through. i also have some loud electrical noise, but i think it is the heaters tap not elevated.
for starters, i thought it was a bias problem, but the values are about right, and the amp sounds really good with the attenuator at low volume.
any help would be very much appreciated.
thanks guys

How about a link to the schematic your using and some voltage measurements.

nosaj

[g1]

Have you tried reversing the primary leads of the output transformer?

[Jazz P Bass]

You may want to check that the negative feedback loop is not a positive FB loop.

Measure the output voltage & then break the loop.
If the voltage goes up, then it is correct.
If it goes down, then you have to reverse the output transformer primary leads.

Trainwreck Express.zip