Ampeg SVT2 Pro problem

There HAS to be some kind of specification in the service manuals?
Input voltage before clipping or similar, or the total gain through the amp. There are some similar data on other Ampeg gear in their homepage.

 

I think you need to find a qualified amp tech. Not sure who these guys are you've been using, but a good amp tech can get to the bottom of this in no time. Call Ampeg, and ask for a certified tech.

 

That's a good observation.
Something easy you could try yourself is replacing the preamp tube located closest to the input jack. If its supposed to be a 12AX7, make sure that first tech didn't substitute a lower gain 12AU7 or 12AT7...

 

Preamp tubes you can just sub in easily yourself. No need to work with them as long as you use the same type.

Make sure the PI and driver tubes are good as well, those should be the three small tubes near the power tubes (a 12ax7 and 2 12au7s). Make sure the power tubes are biased right, you can check the user bias controls on the back of the amp (instructions should be in the manual available at ampeg.com).

 

Yep. This'd be the 1st thing I'd check.
Pretty easy. Just unscrew the top & sides & the chassis should slide out(or the top just lifts off-can't remember with this particular amp)
The pre amp tubes -small tubes are really lifted out.
Check what are supposed to be in there & make sure you've got the correct ones.
Also try swapping some known good ones in/out. Even swapping what you've got already should let you know if it's a pre amp tube

 

I've been working with tube electronics since I was about 13. I have built quite a few amps and repaired many more. Still I would be very wary about working on an SVT. This class of amplifier is not something that a beginner should be touching. Please leave it to your tech.

 

If i remember correctly on the inside of the SVT-2 they should be labelled on the chassis. Check the label on the tube to be sure they're the right kind and swap the ones as needed. You shouldn't have to go near anything hazardous to do this with the 2pro but still be cautious with any tube amp. The three are the back near the power tubes are your drivers, the rest up front are your preamp. The first tube closest to the input jack in order is your V1 gain tube and of that's bad you may have low gain. If the 12ax7 near the power tubes is bad you may get weak signal into the power section.

 

Easiest thing would be to just sub in a known good tube. Pick up a new 12ax7 and test out the preamp slots and see if it makes a difference. You can just get any new 12ax7 for this purpose. You can test preamp tubes but unless you have the equipment or want to pay someone the fastest and easiest thing is to just swap the tube.

 

Operating voltage checks and scope the the grid and plate. Neither of which you safely be instructed to do over the internet. Substitution is your only safe way to do it yourself.

You said " A while ago the tubes went (quite obviously so)". Can you elaborate some here? If you had a spectacular failure of an output tube or two there may be damage that was overlooked.
The amp can make 300 watts at a higher distortion with unrepaired problems. It should easily output around 300 watts with <5% THD. Don't let a lazy tech walk all over you, YOU know how the amp performed before he/she doesn't. Where are you located maybe we can suggest a good tech.

 

Nothing posted so far covers the obvious, so here it is:

1) Have you tried using a different bass?

2) Have you tried using a different instrument cable?

3) Are the basses you're trying active, and do they have known good batteries?

4) If it's none of those, then I would check the input jack on the amp and make sure it's clean. I've heard of equally weird problems that were just from dirt in the input jack.

 

Blade3dge, it would be easier to recommend somebody to take it to if your profile (or post) said where you are located.

 

Have you tried hooking up some other kind of preamp to the Effects in? If that works okay, then you'll know the power amp section is good and the problem is in the Input/preamp section of your amp.

Have you tried hooking up DI out or Slave out to another power amp? That will let you confirm if the input/preamp section is working correctly or not.

Check all this obvious basic stuff before you go opening up the amp and messing around inside.

 

Just remember that the difference in amp gain I read in a forum for ampegs is ten times the difference from 12ax7 to 12au7...something about voltage drop but if those tubes r mismatched in the wrong sockets u could go from very low input gain levels to frying the thing. Have u tried different basses to make sure ur not losing gain from the guitar side?

 

BTW if a tech says "sounds loud to me" ask them if they are paying the bill.