Tech Assistance- 72 JMP Superlead

[DonaldBort]

I've been chasing a problem for about 2 weeks now. This issue is my 72 metal face just started flaking out a bit.
I will be playing and the EQ goes haywire...Full on Treble and the amp sounds like the guitar pickups are out of phase. No body at all. then from there it will go back to normal and then after a few minutes goes to full on bass, really flubby sounding and no definition to any chord or note played.

I have been through all the obvious; eliminate all pedals between the guitar and input, change cords, check speaker cab connections (swap out cords), check ohm setting. Swapped out 3 sets of tubes (both pwr and pre's) & re-biased each tube change, moved the head off the speaker cab...all to experience the same intermittent issue.

Put the head on a scope and found a loose connection on a pair of resistors that was fixed, and I thought this was the issue since the amp sounded totally awesome till the flubby bass and treble out of phase issue returned again.

I know it's a shot in the dark to troubleshoot via a forum but at this point I am looking to you guys for a bit of direction.

Thanks
Don

 

[mickeydg5]

If you have an electronics jumper wire, make the problem occur again.
Turn the amplifier OFF and jumper the tone stack. Turn it ON and try to make it happen again. If the problem does not show itself then it is in the tone stack and may most likely be the TREBLE control potentiometer from the description.

Try that and let us know what you find.

 

[DonaldBort]

Thanks for the direction, this was not the issue, however the culprit of the flubbyness was isolated.
For starters, I changed both the small ceramic 470pf caps over to silver mica's. This seened to take care of the treble issue.
Then probing around the wires while playing, found pins 6 & 7 on v1 preamp tube loose and needed a tighter solder.
So far issues seem to be resolved

 

[mickeydg5]

Ok, the treble to bass thing is weird. It is good to hear that it seems back to normal. Keep us posted if something comes up.