Ampeg SVT-3 Pro issues. Preamp schematic needed.

[Bishop]

Does anyone have an Ampeg SVT-3 Pro PREAMP schematic?

The power amp schematic is of course easily available, but I can't find the preamp one.

I have one here (I think it's an older one) that the tube gain doesn't seem to be working and also the graphic EQ is always ON, even though I'm fairly confident the switch is OK.

[nevetslab]

Ampeg SVT3-Pro Preamp schematics

Does anyone have an Ampeg SVT-3 Pro PREAMP schematic?

The power amp schematic is of course easily available, but I can't find the preamp one.

I have one here (I think it's an older one) that the tube gain doesn't seem to be working and also the graphic EQ is always ON, even though I'm fairly confident the switch is OK.

Yes....here they are:

Preamp PCB Schematics (534XXHB).pdf
PWA Preamp Assembly PCB Schematics (534xxpb).pdf
Preamp Assembly Notes (07-534-03).pdf

The Tube Gain pot controls the plate voltage for V1/V2 on the power amp assembly, and tends mostly to be a headroom or degree of tube overdrive control ahead of the MosFET power amp stage.

The EQ switch has the foot switch involved with it's action, so that rear panel jack could be giving you grief. Take a look at the solder joints along all of the rear panel jacks to make sure there aren't solder joint fractures.

[Bishop]

Thanks a ton. The foot switch jack was the first thing I checked out. From what I can tell, it's not bad solder joints, unless they are really badly fractured. Unfortunately you have to flip the main board to check it out, which I'll probably end up doing, but I wanted to go over the schematic first. I really appreciate it.

[Enzo]

Don't rely on vision for cracked solder joints, the solder can fail without looking like it. If in doubt, ssimply resolder teh joint, only takes a couple seconds.

But in the case of FS jacks - and others - MEASURE the resistance of the cutout contacts. They should each measure less than half an ohm. If you measure like 1 or 2 ohms, it might work at the moment, but that higher resistance means the contacts are not clean electrically.