Preamp working but not

Jon Slaughter · 2008-10-28T14:09:57+00:00

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.Any ideas?

Oct 28, 2008 22 Replies

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Jim Thompson 17 years ago

I think Jon's vain attempt at humor :-)

Jon should go hang out at sci.electronics.basics since it's pretty obvious he is clueless about electronic fundamentals.

But I'm sure he'll retort, "your" stupid ;-)

...Jim Thompson

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John Larkin 17 years ago

Or he could ask for help, post the schematic, learn something, and say "thanks."

That seems to be the pattern.

John

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JosephKK 17 years ago

First, why are you hot rodding a small signal triode? You are applying 12 V to a 6.3 V rated heater, 450 V to a 300 V(max) rated plate. Why is the plate load resistor so high? Rp for that tube is only

85,000 Ohms; the plate load resistor should not be more than 100,000 Ohms and not less than 50,000 Ohms. What is the input impedance of the mixer? Begin to see the picture?

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