18W EF86/TMB noise problem with gain/master volume maxed

[paddes53]

Hi,

I've got a little problem with an 18W amp that I built recently. It has the added EF86 in the front end "en lieu" of the normal half ECC83 channel, as such the second half of the first tube for the TMB channel remains unused. The values of the circuit are all taken from the Trinity sIII schematic with the exception of a solid state rectifier (because I don't trust EZ81's), which schould be known by a lot of people, but I'll attach it just in case. The values are all the "original" ones, so there is no added gain, just the standart circuit. For trannys I use a Hammond 370FX for power and a Shinrock SOT-M18R for output.

Now to the problem :

The EF channel dimed doesn't present any problems at all, but when i switch to the TMB channel I can't max out both pre and master volume, at some point the sound has a "glassy" (sorry for the lack of a better word) distortion on top of all the notes which is, franky, quite irritating and not good sounding at all. I was thinking of oscillation at this point.

As of now, I've tried alsmost everything that comes to mind for curing unstable amps:

I've changed tubes around endlessly, put a grid stopper (8,2K) in front of the second gain stage of the TMB channel even if it already has the 100K to ground right after the preamp volume control, pushed around wires with the amp running to determine the culprit, put shielded runs of wire in nearly every place you could imagine and even connected a 50pf cap across the phase inverter plates, all to no avail.

The only thing I can imagine as of this point is the phase inverter tube sitting quite closely to the OT, but on the other hand, the EF86 channel doesn't act up and both Marshall and Fender have done so for years without problems. Might be a bad component somewhere in the TMB channel, but up to say 8-9 on the volumes, the amp sounds absolutely fantastic so I doubt that a bad component will come into play just at those settings. Or, worst case scenario, a bad OT ...

Anyway, I'm all out of ideas and was hoping someone would have some clue as to what I could still try to get this amp stable with both volumes maxed.

Many thanks in advance,
cheers,
Patrick