SLM AMPEG SVT-II Pro

[shiner555]

Hi Guys,

I just had a friend bring me his SVT-II Pro to take a look at. He's had it at several shops and nobody has ever been able to correct the issue, so now I'm getting a crack at it. I suspect the others didn't have the power of the ampage board behind them! A couple of questions to start.

1) Does anyone have a complete set of schematics for this? On an old post from Enzo on here, I found several of them, however a few of his uploads I wasn't able to download for some reason.
2) The amp has had all of the tubes replaced, I'm still going to re-check em, and the amp powers up and plays fine.......so what's the problem?
3) ISSUE: So there is something that I would suspect to be thermal related....after about an hour of playing the amp starts to make crackling/popping sounds...what my friends refers to as "ghosting". These can be the worst to track down as the amp normally works fine on your bench. After looking inside this thing last night, I remember why I hate to work on SVT's, let alone the modern ones. What a hassle. However, does anyone have any suggestions for me to take a look at off the bat?

I have a heat gun/freeze spray I can try to use, however with the thing packed so tightly PCB wise, I'm not sure how I'm going to get real access to it while it's working. In the past this type of thing has been a cap issue, however I'm hoping people on here who have SVT experience can point me in the right direction. Any help you can provide me, the awesome people of ampage, would be much appreciated!!

[Enzo]

We had a server crash and lost a lot of schematic files. Go to any post link that no longer works and post that you need it there. That will wake up the old thread and someone will repost the drawings.

The tubes may be new, but are they right? There are three small tubes on the power amp, two 12AU7 and one 12AX7. Make sure they are not all 12AX7s or somnething. And the two 12AU7s are in the sockets tight behind the two bias controls, make sure.

Does the bias adjust LEDs set up OK?

Does it make this noise with nothing playing? With nothing plugged into the inputs? AMp is noisy all by itself? If so, isolate the problem. PLug something into the power amp in jack, preferable a shorting plug. Noise stop or continue? That will tell you power amp or preamp.

For that matter, run a cord from the preamp out jack to some other amp for a listen, noise coming out of that?

[shiner555]

Enzo,

Many thanks for the quick reply.

Tubes -> I'm going to test all of them tonight to totally verify, when looking them they did seem to have the right ones in the right sockets.
Bias -> I didn't verify the bias LED's, as I'm not familiar with the circuit, but I'll check that out tonight as well.
Noise -> The amp made no noise what so ever last night, and I ran a 100Hz sine wave through it for about an hour trying to get it heat up. Again, one of those things that only seems to happen when it is not on your bench. Great suggestion about the pre out's. Now I just need to get it to make the noise in-front of me.

[bob p]

Contact Ampeg Tech Support with the amp's serial number and tell them what schematics you need. They will send you the right schematics that match the serial number of the amp. Also search the board here -- there is a post in the schematics archive that also includes some service updates from Ampeg for that amp.

[bob p]

be wary of testing tubes and pronouncing the amp as OK.
the 2 Pro pushes the AU7 to their limits, and IME testing "good" on a tester is not the same as working properly in the circuit.

[Fragger]

This one?

[Enzo]

You don't need to be familiar with the bias circuit to follow the simple bias procedure. the lights tell you when you are balanced.


Remember your amp normally works in a live sound setting. I bet you were not listening to 100Hz for an hour, so loads do not vibrate, speakers do. Yoiu could have a mechanical sensitivity. So... with speaker connected, turn the amp up to a normal sort of setting, ball up your fist and whack the top hard. Does the amp react? It should sit there and ignore your whacking. (make your own joke here) If the amp reacts to the whack, there is a loose connection somewhere inside.

Likewise, get a wooden chopstick or other insulated probe, and while running and listening, start poking and pushing everything you can, looking for anyone who is sensitive.

[bob p]

> this one?

incomplete

[shiner555]

Hey Guys!

Finally got some time to get back to work on this. I found some bloated lydic's in the PS section, and since I had a bunch on hand I did a total re-cap. Amp seems to be happy now...........except for if you place it on top of a bass cab and hit an E. Then it starts to really make some distorted sounds if you let the note resonate. If I grab the first or second pre-amp tube, the noise will stop. If I flex the pre-amp PCB ever so slightly, the noise will stop. Sounds like I have a cold solder joint somewhere or another part that's cracked?

Anyone have other mechanical related issues on these guys?