Solid State Guitar Amp Forum | DIY Guitar Amplifiers

And the factory doesn't need to be there, ANY competent amp tech can repair this.  I bet you have a local shop.

I surely wouldn't assume the same zener needs replacement.  It MIGHT of course, but frankly it is more likely your FX loop jacks have dirty contacts.

Refresh the connections to the DFX card.

I could be way off base, so consider that.  But you have both 15v rails at the zeners, but do you get both rails right at the IC, IC8b?

Just thinking, but normally that IC sits between the two 15v rails and centers between them.  Well, the center between plus and minus 15 is zero.  And whatever appears coming out of that IC is what is going to output the amp.  At least at idle.

SO what if one of the power rails doesn't make it to the IC?  Then the poor thing tries to center between 15v and zero, which is 7.5v, and that is what is on the output.  Obscure, yes, but possible.

It doesn't care about the reverb.

Look up "light bulb limiter" and make one and use it.

I wouldn't think so, maybe across R3 or R4.  Personally I do it by looking at crossover distortion.  Star at the coldest setting, apply a sine wave and watch it on the output.  Now advance the bias until the crossover notch JUST disappears.  Most adjustments by the current spec will be a little warmer than this.  I like it though.

Remember it was getting hot just idling?  That means it is turning power into heat.  SO a quick and dirty close enough method is to monitor mains current draw of the unit.  Start at coldest setting and start advancing the bias control.  At first the mains current stays low.  At some point (in fact, the point the crossover notch goes away) of a sudden, mains current starts to rise with the bias adjustment.  At that point, stop and back off ever so slightly.

people will make arguments for one way or another, but these methods will get you close enough.

For that matter, using the existing speakers in those two combos, a readily available ABY switch on the front end would work too, wouldn't it?

You are right, we need to isolate the speaker grounds.  The Peavey speaker return is NOT to ground, it has a current sense resistor there.  I don't have JC120 handy, but the similar JC77 does reference speakers to ground.  But still, even if both went to ground we might create a ground loop if connecting them.


When you described the project, I immediately got a mental picture of how I would do it, your inner vision is diffrent, but I bet yours will work too, and if it differs by one relay, so what.   I have a very lengthy history in relay systems - think old pinball machines and other control systems - and I think certain ways, but it ain't rocket surgery.

Clean the jacks and especially the cutout contacts in them.