I think I'm kicking a dead horse here. Maybe the tube preamp is so sensitive, it's jsut picking up crazy hum when the bass is turned up and especially when the mid is turned up with the bass.
People who have only done solid state are usually astonished at the hum pickup abilities of tube equipment. This is partly because tubes work at (in general) much higher impedances, and are prone to capacitive pickup, and partly because tube circuits come from a time when modern techniques for power supply immunity were not used. That LM3886 you're using pretty much does not care if there is a few volts of ripple on its power supply as long as its ground is clean.
Other than that, I'm at full blast and all I have is some weird intermitent hissing.
That would be parasitic oscillation being kicked off. Capacitive feedback is getting something going. An "angry" sounding hiss in audio is often the symptom of RF oscillation back-modulating the audio path. Thermal hiss is more polite sounding.
It's surprisingly quiet for how loud it is. This is a nice chip. I wonder if I really shoulda got a hotter transformer to make it really loud. Maybe I'll do that later.
You will be surprised how little going from where you are to the highest power supply voltage you can put on that chip helps with loudness. You're well out of the region where small power increases make the speakers sound much louder. It will, for instance, take ten times the amplifier power to make the perceived volume be twice as loud.
But yea the dead horse...When I turn the volume control down the hum goes away completely (with the tin foil tube shield).
And this tells us that the hum is being picked up and passed on before the volume control, right?
I tried shielding every signal wire in that area. And there was no differece. PRR originally gave me his drawing with a 250k volume pot. I put in in a 1 M every time I built it cos I liked the extra volume. It was never an issue until I was amplifying the preamp so much!!! So I'm letting it be. Everyone said hum is just part of a tube amp.
Hum is an ever-present threat. It is possible to get an almost hum-free tube amp. But you have to not only know what you're doing, you have to work hard at it. There are many ways for hum to get in.
I'd like to hear the hum on a real tube amp. I just don't feel like going to guitar stored and plugging guitars in and not really playing anything.
Why not? I'm sure the store people would love someone who just wants to audition hum and hiss levels instead of yet another rendition of "Stairway to Heaven".
Tho, i'd really like to know if those spring thingies pictured above shield the tubes like a normal tube cover or if they're just there cos of just because because.
They're there to hold the tubes firmly in place. Retainers. No significant shielding.
