SVT Fault.... fix it?

[cherin]

i have an early 90's svt cl and replaced the tubes, ran through the recommended warm ups and adjusted bias on the back of the amp. no green lights come on at the bias controls, only red when adjusted too much. i gave it a run to see how it worked and it sounded great for about 10 min then shut off and the indicator light at the power/standby switch was flashing green/red indicating a fault. any ideas on where to begin? i have a V4 and haven't had this type of problem with that. i am technically inclined and fully understand circuits as i have been in the electric field for quite some time, i just have not had much experience with amp repair itself. any help?

[Alex R]

No green on the bias lights means you have a bad tube, or a badly-seated tube.
You can get the manual here:

www.ampeg.com/pdf/SVT-CL.PDF

Page 7.

[cherin]

yea, i got that on the manual but the tubes still light up just fine and it worked for a little bit as well. i checked the tubes again anyway tho, all seam to be seated just fine. after trying it again i notice that 2 of the 3 preamp tubes light up rather bright when i first flip the power switch then dim, but the third slowly lights up as the amp warms, and still no green light at the bias adjust. i appreciate your input. any other ideas?

[Alex R]

Nope, just the same old idea. You do almost certainly have a bad power valve (though it may well have a nicely glowing filament). You will need to get hold of a new 6550 - preferably a matched pair of them - and swap them in till the lights show correctly.

These amps do appreciate matched valves, unlike most guitar amps which aren't too fussed. Once you have the lights starting to behave themselves you'll be swapping valves from side to side to get them just right and to reduce hum. By the way, the three leftmost and the three rightmost power valves constitute the two 'sides'; the sides need to match up in terms of total current draw (which is what valves are matched for) before the lights will do what they s'posed to.

[cherin]

thanks, i'll take a better look at these tubes.

[Amp Kat]

Check your voltage rails on the TLO-74 IC's for +/- 15 volts. If the chips are bad the leds won't lite. It is possible that a tube short took them out as I've seen it before thru the heater circuit which may be why they are lighting up like that. You'll have to take the main board off to find it. Trace the heater wires from the connector as it goes to some resistors also that may be fried. Look for burn marks on the resistors or pcb board.