If it helps anyone, here is the reply I got from Ampeg regarding a query I had with this head shortly after getting it some years ago....
Question(s) to Ampeg.....
Hello there, I have a couple of technical queries to pose to yourselves
I recently acquired a used SVT2 Pro head
During the second time using it, it cut out in me completely, I ran it into a local service centre and they diagnosed a faulty resistor, resistor replaced, full service and health check later, it was given the all clear
My main query is regarding tubes and bias lights on the back, although the amp SEEMS to be working perfectly, there is what appears to be just one (I can't tell if it's reflection off of the others or there is more than one doing this) tube that is glowing purple, the service centre said the tubes were all fine upon testing them and sometime a blue/purple glow happens, is this correct?
Also, the bias lights are both green (as should be so I believe) but as soon as I start playing my bass (or even not playing, maybe just tapping the body or strings) they both briefly flash red, every time, is there a fault or is this perfectly normal?
Reply FROM Ampeg...
Hi Dan,
Your bias lights are acting as they should. At idle they should just be green.
Here's a link that explains the blue glow you are seeing in some of your tubes:
https://www.physicsforums.com/threads/what-is-blue-glow-in-vacuum-tubes.732194/
All the best,