So... a little update here. Furthering on my 'chasing ghosts' investigation into this very minimal 'hum' I had through my speakers, I replaced every tube with the NOS backups that I had purchased. I realized it made more sense (to me) to start with a fresh set of tubes all around, rather than using the 'remainder' of the tubes that were shipped with the amp... Since I have no idea how much 'life' is left on those tubes, why not keep those as backups and run with the new tubes instead? I liked this idea, and also hoped it would solve the 'hum' I was hearing.
To recap, the hum was stronger in the right channel, and only appeared after maybe an hour after the amp was on, strongly indicating a heat-related phenomenon. Changing the tubes greatly helped! It reduced the hum by about 50%, but it still remained... Since I was 'on the job', so to speak, I decided to swap my left and right channel tubes to see if the hum would follow. I figured this would tell me if it's caused by the tubes, or something in the amp circuit. Well... after the L/R channel swap, the hum was nearly completely gone, and all that remained was a basically equal amount of very very very low level hum on each channel... Which, again, was only audible with my ear basically pressed up next to the driver. From everything I've read on the subject, all SET amps hum to some degree, by the nature of their circuit (unlike push-pull amps)... and it's just a matter of degree of 'how much' hum there is -- ranging from barely audible with ear-to-speaker, to 'this is annoying even when music is playing'. Fortunately, I seem to be waaaay on the 'quiet' end of this scale, so I'd venture to guess that the amp is working perfectly as intended.
I'm totally baffled why the tube swaps would yield such a seemingly random result though... Logically you would think that either the slight hum that was originally in the right channel would either remain there, or be equally swapped to the left channel... but for it to disappear --?? What voodoo is this? One thought I had, based on reading another thread, is that it could actually be caused by the fit between a particular tube and socket... These old tubes commonly have corrosion built up on the pins, and the only thing I did before inserting them was to carefully clean them with isopropyl alcohol... So, maybe I'm hearing the slight effects of some resistance between the tube pin and the socket sleeve... Shrug, I have no idea. Does this sound plausible, or is there a more likely explanation?
Obsessive compulsive noise hunting aside, this amp sounds absolutely fantastic, and I'm totally happy with my venture into SET-landia.
I recently went on another crazy speaker arrangement spree, changing positions all over the place -- sliding seating location as well. On my last night before I had to leave for a while again, I felt like I had found a hard-fought, hard-won amazingly fine-tuned location. I sat down for a listening session, and *ya*, wow, it really did sound great. Then, after a few tracks, suddenly my left channel just cuts out, with only some slight crackling remaining! I immediately stopped the music, and jumped up to shut the amp off. I haven't turned it back on since.
Oye ve, I've had my fair share of hiccups as I put this new system together, let me tell' ya... But, such is the way. I have no idea on this one, and I know that nobody can help me here -- I just need to get back and diagnose everything to first figure out what component failed in the first place. I didn't notice, in my flurry, if all the tubes were still glowing in the Yamamoto. It almost sounded like a cable suddenly disconnected, but upon checking, everything was snug and secure. I wonder if an internal connecting wire wiggled loose in the speaker -- it sure sounded like it. If this is the case, I worry about the amp for a new reason -- knowing that it can be quickly damaging to run a tube amp with no load / resistance... Hopefully turning it off within a few seconds was sufficiently fast for no damage to occur.
Anyways, I don't know -- I'll come back and update again once I return to my studio and dig in to the problem. Cheers everyone, and happy holidays!
