Ampeg SVT3-PRO issue

[panteraa]

Hi,

since a few days my AMPEG-head is acting weird, when I turn it on, I have to wait for about 10 secs before I hear the "pop" sounds and works fine(all tubes glows). But after a random time it "pops" again and changes to a situation where the signal is alot weaker and I have to turn up the volume and it sounds rubbish, I don't think that it is oscillation. It's distorted but not in a good way and it keeps on switching between these 2 situations and every time I hear that "plop".

So far I've cleaned all the pots, checked all the solders and repaired the bad ones, but it didn't solve the problem. The only thing I can mention is when I touch the 12AU7 tubes and move a little around it switches quickly between the 2 situations. I hope any of you can help me further.

Kind regards,

Jeroen.

[oc disorder]

Hi Panteraa and welcome to the forum.
My gut feeling is its time to change the valves but maybe lets check a couple of things to make sure.
Check that you have one 12AX7 (ECC83 Euro number) nearest the back of the amp and that a 12AU7 is between that and the front see picture attached.
You could try cleaning the pins of the valve usually Isopropyl Alcohol is used but any evaporating spirit will do for now...don't want the valve base wet.
Plug it in and out a few times this action will help clean the pins if that is a problem. Obviously the amp should be off and unplugged - I better mention that !
The other thing to check (and this fix takes up more space on this forum than any other ....IMHO ! ) is the power amp in switching jack socket
see attached mini schematic .. 'cause this can interupt the pre-amp sound
getting to the power amp. I know you mention wriggling the valves exacerbates it but we should eliminate this possible fault just in case...!

Another thing to try is feed the power amp in with a suiable signal ie a cd or mp3 player with a volume control .The front panel volume controls do not affect anything plugged ito the power amp in socket so anything plugged in here will come out FULL volume !

This should tell us if the fault is in the pre-amp or the power amp section so by gradually turning up the volume see if the fault still exhibits after its been on for a while.

If the valves have been there for a while I would try two new ones as its very likely one of them is faulty.

Lets know how you go good luck and take care !

[Enzo]

The very most common repair I have to make to these latter day SVTs is replacing 12AU7 driver tubes.

[panteraa]

Thanks for the answers and welcoming me.

I've cleaned the sockets of the tubes and replaced the 12AX7 but I don't have any 12AU7 so I'll have to order that one. I found a new clue; when I turn down the tube gain and wait for about 20 secs the problem is solved even when I move the 12AU7 around. When I turn it to maximum, and wait for another 20secs the issue appears again. It is only that pot that affects the situation. So it seems very likely to be a tube problem.

[Stoffie1974]

Hi there,
OK i'm a little late for the reply, but i'm new to the forum ...
I had the same problem with mine too, swapped all the tubes and this didn't fix it.
And the damn symptom occured only ON STAGE (Thanx a lot Murphy !!)
So i started poking around in the amp, and found out that when i pushed the large connector on the amp board, the popping/scratching sound was there !!
So i started to wiggle each cable at a time ... pin 11 was the perpetrator !! And yes, that is the cable to the plate supply voltage pot.
Cleande the pins (on board and connector) with a sharp pointed tool and it never came back.
Hope this might help

Grtz