I finally got my tube preamp circuit built(well, been a few days but been tweaking) but I'm having some problems. My scope tells me that I'm getting an amplification factor of about 90x.
But when I hook the output up to an audio mixer I have I end up with about the same loudness. I noticed that when I had the output hooked up to the mixer the amplification factor was only about 10 or so but when I disconnected it it went back up to 90.
It seems the mixer is overloading the output stage? Surely the mixer should have *infinite* input resistance? I've added an additional 100k ohms to it and it's the same(even a little worse it seems)
The same issue exists when I use my sound card.
I know the triode doesn't have much of a power output(.5W max) but shouldn't I expect more?
I also have a problem with a sligh dc offset on the output of about 2V that slowly decreases over time to about .1V. I think it has to do with the coupling capacitor. I guess it's leaky? Although it seems to have a 60hz signal on it of a few mV. Don't know if thats the input that has it or the output. When I turn off the power to the amp the DC on the output seems to jump back up to 1V or so then slowly decrease(discharing).
My only guess is that the triode cannot drive the low resistance of the mixer and once I add the power stage it will work as expected? I just figured the mixer would have a relatively high input resistance so I'm not sure whats going on.
Also, my output seems to have a bit of noise on it. Sounds like a TV station(white noise?) but isn't as "full". I'm guessing it's the tube as it's 50+ years old.