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SVP Pro question


no-logic

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I just got my Ampeg SVP Pro today from Ebay. Some of the pots and switches are scratchy which I knew. And the mid range pot and frequency selector don't have any affect on the sound at all. Not being a "tube guy" is it possible that this might be one of the tubes? It sounds great otherwise but the only midrange adjustment that does anything is on the graphic EQ. Any ideas?

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I just got my Ampeg SVP Pro today from Ebay. Some of the pots and switches are scratchy which I knew. And the mid range pot and frequency selector don't have any affect on the sound at all. Not being a "tube guy" is it possible that this might be one of the tubes? It sounds great otherwise but the only midrange adjustment that does anything is on the graphic EQ. Any ideas?

 

That's pretty strange. I know that on my SVP, the mid pot and frequency selector don't make a big difference, but they do make a difference. I'd look into the tubes or maybe a bad pot/solder joint.

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It COULD be in need of new tubes, but that wouldn't be my first guess. Mine was really hissy and of course the usual snap, crackles, and pops of worn tubes before I retubed it.

 

I hate to say it but you might want to look up Jon Blick's number and take it in to him to have it gone through. :(

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It COULD be in need of new tubes, but that wouldn't be my first guess. Mine was really hissy and of course the usual snap, crackles, and pops of worn tubes before I retubed it.


I hate to say it but you might want to look up Jon Blick's number and take it in to him to have it gone through.
:(

I was actually going to do that anyway. It has some scratchy pots and switches that I wanted Jon to clean up anyhow. I will proably retube it too right away. But it actually sounds really nice with whatever is in there for tubes now. I'll have to pull the cover and see what's under the hood.

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I was actually going to do that anyway. It has some scratchy pots and switches that I wanted Jon to clean up anyhow. I will proably retube it too right away. But it actually sounds really nice with whatever is in there for tubes now. I'll have to pull the cover and see what's under the hood.

 

Be careful not to spend too much on tubes for the SVP. I've used everything from cheap Chinese tubes to uber-expensive JAN-Phillips and I could never tell the difference. Now I just retube mine with the cheapie Chinese.

 

Also, if you find that it has a little too much gain for your tastes, swap out the 12AX7's for 12AU7's or even 12AT7's. You can swap as many as you like (1-4 as there already is a 12AU7 in there) and adjust to taste.

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The tone controls don't have anything to do with the tubes other than telling the tubes what to amplify.

That's what I thought too. But I also thought it was wierd that the mid controls don't work at all but there is still a signal coming out of this thing. I thought maybe it was a bad solder joint but then there would be a signal lose somewhere.

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I just got my Ampeg SVP Pro today from Ebay. Some of the pots and switches are scratchy which I knew. And the mid range pot and frequency selector don't have any affect on the sound at all. Not being a "tube guy" is it possible that this might be one of the tubes? It sounds great otherwise but the only midrange adjustment that does anything is on the graphic EQ. Any ideas?

 

 

I have one to , it sounds liek an issue with the pots, but I dunno, is yours the new one of the old one BTW

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Be careful not to spend too much on tubes for the SVP. I've used everything from cheap Chinese tubes to uber-expensive JAN-Phillips and I could never tell the difference. Now I just retube mine with the cheapie Chinese.


Also, if you find that it has a little too much gain for your tastes, swap out the 12AX7's for 12AU7's or even 12AT7's. You can swap as many as you like (1-4 as there already is a 12AU7 in there) and adjust to taste.

 

 

SVP pro is class A? Forgive my ignorance

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The block diagram (page 5) shows the Mid control and Freq switch going though the same tube or opamp so it could well be a tube or it's signal shorted across the socket somehow.

It sure would be nice if it was a tube. The pots and switches I can handle myself. What's a opamp?

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It sure would be nice if it was a tube. The pots and switches I can handle myself. What's a opamp?

 

 

It probably doesn't have any. In the drawing, the triangle figure represents either an opamp (trasistor based amp) or a tube. Since it looks like it is aserial from the previous controls section, it could jus be shorted across the tube as well and would give the same result.

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It probably doesn't have any. In the drawing, the triangle figure represents either an opamp (trasistor based amp) or a tube. Since it looks like it is aserial from the previous controls section, it could jus be shorted across the tube as well and would give the same result.

Thanks Lug. I think I'll replace the tubes and see what happens. I was going to retube it anyway. Might save me a trip to the repair guy.

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Well I took the cover off today and found 2 Mesa tubes and the other three are unreadable but look like a couple of differenrt brands from what I can see. I tried switching some of the 12AX7's around and it made no difference on the mid mid controls working. So I guess I will have to take it in.

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Well I took the cover off today and found 2 Mesa tubes and the other three are unreadable but look like a couple of differenrt brands from what I can see. I tried switching some of the 12AX7's around and it made no difference on the mid mid controls working. So I guess I will have to take it in.

 

Careful switching tubes around - the 4th tube from the left is a 12AU7, not a 12AX7 :D

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12Awhatever7s are completely interchangeable. No harm will come from interchanging them (unless you count your ears).

 

This is because of cathode biasing, which is the way preamp circuits are made. (see the earlier, 'is it class A' comment). Simply put, cathode bias is a way to make a tube 'self biasing'; it won't demand anything it doesn't need/want or is more than the system is capable of providing.

 

I'm gonna agree with Lug to a point: If you have a bridged pin or solder joint this COULD happen, but since I have not seen the insides of one of these I can only guess. I still find it a bit unlikey unless its an internal short in the tube itself (and I have never found a tube yet that had such a fault, (even though I can and do have the equipment and have checked hundreds.)

 

The law of averages indicates something else than a faulty tube.

 

Retubing is never a bad idea, and is certainly affordable in this case, but it may not correct the problem. I'm inclined to think that it WILL not correct the problem.

 

If you swap the tubes from one place to another, the problem should move with it, IF the tube is at fault.

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