SVT short, OPT suspected.

[antieatingactivist]

Hi. I have an SVT that is popping fuses, so I plugged it up to a light bulb current limiter. The Bulb glows bright even when all of the tubes are removed. The only way the amp will turn on is with the center tap of the output transformer disconnected. Is it safe to assume that the OPT is the culprit? I didn't get any off the wall readings on the primaries, and there didn't seem to be a short from the center tap to ground. I am going to test it using the filament transformer tomorrow. I was just checking in because there is an SLM era OPT on ebay for cheap. I understand the dangers of working on this thing, however, it is a rather intimidating amp.

by the way. The damage to the amp was caused by an electrical problem at a practice space. It even took out speakers, and the Output IC in a solid state head.

[Steve Conner]

Unfortunately it does sound like the OPT has shorted between the primary and the core.

But before buying a new one I'd check that it's not just one of the power tube sockets that has arced and broken down. (Remove the OPT primary leads from the tube sockets, insulate them, reconnect the centre tap to B+, and try the light bulb limiter again.)

[antieatingactivist]

it was NOT the transformer. The 3 diodes that connect the plates to ground on one side were shorted. They were also different and much larger than the ones on the other side. I looked them up and they were rated at 3A 200V! Looks like some tech didn't realize that s/he was repairing a 600+ volt amp. Replaced them and fixed some improperly routed wires and the amp works fine. Thanks for the advice!

[melvin]

btw, SVT's always make a light bulb limiter shine pretty bright. lots of heaters.

[antieatingactivist]

i found that out too!

[J M Fahey]

1) SVT are very hungry amps; a 100W bulb is the *smallest* one to use with them, if the tubes ere in their sockets. I have a 200W one just for such beasts.
2) 3A200VPI diodes? I guess the poor guy has "a lot" of experience repairing SS amps.
Thanks God he did not replace a 6550 with a MJ15024