Ampeg SVT3 NON Pro. Power Amp section is jumpy (loud then soft). Some distortion at times.

[nicksorenson]

I notice that the power amp on this unit seems to jump a lot in volume. I measured the power transistor cases and am not noticing voltage jumps. I need to measure at the emitter resistors and check there.

It's probably a 50% power change that comes in and out. Any ideas where to look for this unit? It does have a turn on/off relay for the speaker. Maybe short that temporarily?

[Jon Snell]

I would look at the effects in/out or sometimes called, Power Amp in socket for tarnished contacts.

[x-pro]

If nothing changes after following this advice from Jon Snell, connect another amplifier to the XLR output. Check the operation of the preamplifier to see if there is any level spike. You need to determine whether the problem is with the preamplifier or the power amplifier.

[The Dude]

I would look at the effects in/out or sometimes called, Power Amp in socket for tarnished contacts.

^^^^^^Yes, that. I agree with Jon. For whatever reason, I see switching jack problems more with Ampeg amps than others- often recurring. In fact, for some professional players, I've just installed a jumper to permanently solve the problem.

[nicksorenson]

I had tested by plugging into the FX return when I first purchased the amp (bought used "good working order" with problems). It was still doing this even in FX in so I figured a power amp issue.

I took one of the PCBs that was in the way out of the unit and probed around a little with the eraser side of a pencil and found that an inductor at the output was where the intermittent issue was coming from. I pulled the power amp PCB and it turns out there's a bad solder joint at the inductor. Resoldered it. That should fix the issue.

But... almost all jacks are bad. I probably should just replace them all with Switchcraft and small jumper wires to the PCB. It's a great sounding amp. I'm not a fan of SLM's (crate and ampeg from the 90s) choice of jacks and pots. The pots on this amp work, but I'm surprised at that. I've owned many SLM crates and the pots are one of the weak points.

[g1]

If you do end up replacing any jacks with non-stock types, pay close attention to whether the originals are isolated from chassis or not. Usually, when people mention switchcraft, they are referring to jacks that are non-insulated. Then you have to play around with insulating washers.

[Leo_Gnardo]

Good on ya, reattaching that output choke.

In these and other similar amps I've replaced jacks with Cliff or ReAn parts with good success. I'm not crazy about PC mounted jacks but I reckon it's way way easier than fiddle faddling with isolation washers to keep Swcr jacks from contacting chassis ground and raising more problems. Would likely have to chop away bits of PC board to get Swcr's to fit, as well. Too much sturm und drang for me. Others may differ, that's OK.