Classic Anniversary something whatever, about 10 years old, hardly ever gigged, stationary in a dry room - friggin' thing breaks down all the time, has drop-outs.
I herewith declare all people that claim their "old Ampegs never have anything" outright liars!!!
Yours isn't an "old" SVT, not even close. The Anniversary Series was a sales gimmick introduced by Loud Tech and even if your amp may say "made in the USA," it probably wasn't. Loud had concurrent production in Arkansas while they negotiated with Chinese, Korean, and Vietnamese contract production factories and even legitimately US-made Ampegs were being assembled from cut- rate parts obtained from overseas. All of the Anniversary Series tube heads were simply SVT CL's, a later design from SLM that differs significantly from the real SVT, with cosmetic changes. The SVT CL, which was introduced in the late 80's, was never a particularly reliable design to begin with and became much less so once Loud started making them. The real SVT was the model Ampeg designed in the late 60's that stayed in continuous production until 1986 though three different owners, first Magnavox, who made most of the ones sold, then MTI, who shifted production to Japan and changed the design a bit, and finally SLM, who made a limited run to near-original specs when they first acquired the Ampeg name from a defunct MTI. After that run ended, they introduced the SVT CL, which is what you have
The problem with yours is probably related to the current sensing relays which SLM designed into the SVT CL. I worked on many SLM SVT's that had nothing wrong with them, but the protection relays would keep making the amps cut out or shut down. Removing the relays is major surgery, but even if you did, your amp still isn't going to sound like a true SVT. It has a completely different preamp and output drive section. SLM was very conservative in the design limits of the CL because the available tubes they had to use were all over the place in quality, and those annoying relays are supposed to be protection circuitry in case the output section suffers a tube short. However, even on a completely healthy CL, if the amp is biased for full rated clean output, the relays will shut it down.
