The amp with the triode/pentode switch developed a progressively louder hum over the course of a couple of weeks and along with degraded sound quality from the speakers. I suspected the input transformer and ordered a replacement. While I was putting in the transformer I also put in new tube sockets.
When I started up the amp with the new transformer the amp still hums quite loud. On initial start up the speakers also hum very loud and after 10 seconds or so quiet down. The hum in both speakers is equal. To me it sounds like a 60 cycle hum from both the amp and speakers.
Both channels bias properly. I have swapped all the tubes out with identical tubes from the other amp with no change to the hum. Switching from triode to pentode has no noticable effect on the hum from the amp or speakers.
So, having not properly diagnosed the problem in the first place, the transformer does not appear to be the source of the problem (= waste of time and money )and tubes do not appear to be the problem.
The quad cap is less than a year old and I do not see any leaks. Should I replace the quad cap and see what happens or are there some other diagnostics I can try? Please keep in mind that I am new to electronics so I've no doubt missed something obvious and at the same time I do not really want to electrocute myself.