What's going on with my Ampeg SVT-3 pro?

[AndyPanda]

What's going on with my Ampeg SVT-3 pro?

I was expecting to hear a cover of Marvin Gaye's "What's Going On" played thru your SVT-3

 

[Jim C]

I'm a big Ampeg fan for over 40 years.
Still have an SVT and SVP-CL pre.

I had a 3-Pro; it was mostly use at rehearsals and wasn't moved at all.
It went to the shop 3 times to touch up bad solder traces.
This amp had terrible QC and was I happy to sell it.
Also had a 7 Pro that were sometimes called hand grenades.
That amp worked perfectly although I sold it for a Shuttle 9.2 and never looked back.
Hope yours is an easy fix.

 

[Mark_70]

Shot in the dark here.... have you biased the mosfets? There's plenty of discussion here in TB on the issue.

Biasing mosfets? euh - gonna leave that up to the qualified doctor me thinks

Regards
Mark

 

[agedhorse]

Biasing mosfets? euh - gonna leave that up to the qualified doctor me thinks

Regards
Mark

Especially if that's not the cause of the problem

 

[Hand slap]

Any thoughts on what could be going on here?

My specific SVT-3 pro is a bit of an oldie. Bought new (by me) in '95 so just over 20 years old. Played many gigs with it but a while ago it started seriously acting up. I no longer gig it (can't trust it) but kinda miss the sound.

Here's what's happening

When I power on the amp, it hardly produces any sound. A little distorted gurgling when I play..
But then, when I leave the amp on for 30 minutes or so (I guess something is slowly warming up), it starts to do better. It doesn't exactly sound "as new" but at least it produces some clean tones.

Now - obviously I was suspecting the tubes, so replaced all of those. No difference.
The input jack has been replaced and I've hotwired the send/return effect loop (someone smarter than me suggested this may be an issue). Bass has a new 9V battery and is working fine with my other amp (same cable).

I'm at a bit of a loss here.
Any thoughts on what my old girl may be suffering from ?

Thanks in advance,

Regards
Mark

Send it to me, I will

The preamp out won't drive headphones, the iPod will work with the power amp in... but this may not work if the effects loop was modified.

The preamp out won't drive headphones, the iPod will work with the power amp in... but this may not work if the effects loop was modified.

Not for sure... the preamp out can't driver the headphones without distorting. It MAY be in the preamp, but your test is flawed (like I indicated in my first post).

Take the preamp out and drive the effects return of a known good amp to see if the noise remains.

Is the effects loop modified? If so, that would be the FIRST place to look. IME. modifications often are the cause of problems because a lot of modifications do not look at everything that is going on within a circuit, or mistakes are made.

Preamp output, will not drive headphones, send it to me, I will fix it, was an Ampeg service Cntr. Back in the day, still do a fair amount of repairs, CLUE, probably is a 1 or several leaky caps in the supply, that retain their capacitance as they warm up.

 

[agedhorse]

How come I so rarely see a bad cap out in the wild? I haven't replaced more than a dozen (other than very very old ) caps on anything made within the last 30 years in my whole career. I have seen an awful lot of amps and from what I have read on the internet, EVERY AMP should have bad caps. [/end head shaking puzzlement]

 

[sjors]

At this moment , I'm busy with repairing an Ampeg "SVT-3 Pro".

The problem with this type of amp , is that it is almost impossible for the average bass player to service this yourself . because you need a scope , tone-generator , dummy-load and multimeter
( even if you want to replace the pre-amp tubes ).
The tricky thing is , that you can't use just "any" 12AX7 and 12AU7 to replace the tubes located on the output-power board.
Check TP2 , TP3 and TP4.
This is a very uncommon and sensitive circuit which can easily cause an unbalanced output signal.
Adjust 10K trimpot so that sross-over distortion just disappears.
Check for unbalanced output .

I never should buy this amplifier myself due to that sensitive circuit ( V1/J11 , V2 /J12 )

Friendly regards,

Sjors,
Amsterdam,
Holland.

 

[agedhorse]

Or, you should learn how anplifiers work if you intend to work on them yourself. Then it's no problem.