Ampeg SVT vintage model issue

[Ampzone]

So I inherited a Ampeg SVT in very clean shape MTI era. All original caps inside measuring (out of circuit) 50uf out of tolerance in a couple caps (the 100uf 40uf multi section cap), all caps were off a some to a lot, so I ordered a SVT cap kit. It was working prior but mushy and a bit noisy, a rocker eq switch in the normal channel was not working either. Now after a total recap job the plate volts read good 650v (tubes in) and bias reads well per Ampeg bias calibration specs.
But only hum, no signal passes, hum gets louder as volume is increased tells me a preamp check is in order. All heaters light up.

Obvouly I screwed something up. In the pre-amp I only changed the one 40uf cap. I was planning on doing the 12dw7 conversion but I will wait until I sort out my mistake.

Having never worked on these any Pointers?

[g1]

Check grounds in harness from pre to main chassis. Seem to recall them breaking sometimes.

[Enzo]

When looking at old caps, especially can caps, it really isn;t the value in farads that matters, it is the ESR. Caps of that era had tolerance specs like -20/+80%. Honest. Values could be all over the map, and still be in spec.

OK, you screwed something up, we all do it, trust me. Just forget that you did the work, pretend you just now saw the amp. In other words, don;t try to decide what you did wrong, just troubleshoot the amp to find the problem. Your mistake will become evident when you find that.

If the volume turns hum up and down, then the problem is likely before that control. Your issue might not be the caps at all, it could be that while changing caps, you knocked some totally separate connection out of whack. Some wire broke off a board connection just from flexing, that sort of thing.

[Ampzone]

Check grounds in harness from pre to main chassis. Seem to recall them breaking sometimes.

Good points and checking now'

[Ampzone]

When looking at old caps, especially can caps, it really isn;t the value in farads that matters, it is the ESR. Caps of that era had tolerance specs like -20/+80%. Honest. Values could be all over the map, and still be in spec.

OK, you screwed something up, we all do it, trust me. Just forget that you did the work, pretend you just now saw the amp. In other words, don;t try to decide what you did wrong, just troubleshoot the amp to find the problem. Your mistake will become evident when you find that.

If the volume turns hum up and down, then the problem is likely before that control. Your issue might not be the caps at all, it could be that while changing caps, you knocked some totally separate connection out of whack. Some wire broke off a board connection just from flexing, that sort of thing.

Enzo Im going to check preamp as you suggest. You know its crazy how the new caps are almost dead on in spec. A 40uf cap actually mesures 40uf! I did something stupid but I aporeciate you guys helping bobo here out.

[Ampzone]

Ok so the power amp 100uf - 40uf new cap can was shorted. I put a 50/50uf in to check and its working. Now I don't have another new 100uf 40uf 500v lytic. So I grabbed the original one, here are the esr readings:


40uf side reading actually 58uf is .73 on the esr blue meter
The 100uf side is actually 147uf reading .40 on the blue esr meter.

What do you guys think? Should I put the old cap back and roll with it? How does the esr look to you?

[g1]

No warranty on a DOA multi-cap can?
They should send you a new one with no need to return the old.

[Ampzone]

No warranty on a DOA multi-cap can?
They should send you a new one with no need to return the old.

Got a new one, installed all good now, sounds good powers up fine. The only remaining issue is the mid control on ch1 does not work nor does the middle rocker switch. Pre-amp issue but not sure what the issue us here. Anyways making headway.