Distortion in 807 amplifier - please advise - diyAudio
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Old 29th December 2003, 12:56 PM   #1
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Default Distortion in 807 amplifier - please advise

happy holidays!

i've been fighting with distortion on my prototype 807 amp. here are the details:

807 parameters (wired for tetrode):

B+ = 370V
Bias = -22
Cathode resistor = 500R, bypassed with 100uF
Screen voltage = 300V

i've tried near to a dozen variations of the parallel 12AU7 driver using tube cad and almost all resulted in distortion which is audible with my test monitor audio B2 speakers.

the latest version of the 12AU7 which seems to have the lowest audible distortion is as follows:

220R cathode, unbypassed
-2.3V bias
8K2 Rp
300V B+

where else should i be looking at?

thanks in advance.

PS.

there is no feedback as i don't know how to calculate for one.
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Old 29th December 2003, 01:09 PM   #2
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Let me get a few details straight: you've got fixed bias AND a cathode resistor on the output stage? What's the idle current?

What's the topology of your driver? What are the DC voltages at each of the pins?
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Old 29th December 2003, 01:21 PM   #3
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Quote:
Originally posted by SY
Let me get a few details straight: you've got fixed bias AND a cathode resistor on the output stage? What's the idle current?
it is around 44mA for the 807 (calculated). should i lower the cathode resistor to 400R or so?

Quote:
Originally posted by SY
What's the topology of your driver? What are the DC voltages at each of the pins?
12AU7 wired in parallel. cathode to ground is 2.3V, plate to ground is 100V.
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Old 29th December 2003, 01:28 PM   #4
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Toss the calculation- you're trying to get an amp to work, not a calculation. What is the actual 807 idle current? And why do you have both fixed bias and cathode bias?

The 12AU7 is going to have a hard time swinging a lot of volts the way you're running it. You might want to try a bigger plate resistor (like 25-50K), then go up on the cathode resistor a bit to try to hit something like 150V on the plates.
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Old 29th December 2003, 01:33 PM   #5
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hi sy,

i only have cathode bias via the 500R resistor.

let me go back to the workbench as measure the current.
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Old 29th December 2003, 02:04 PM   #6
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Quote:
Originally posted by SY
Toss the calculation- you're trying to get an amp to work, not a calculation.
as measured, 44.3mA

sorry about the -22V i initially posted which misled you to fixed bias.
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Old 29th December 2003, 02:27 PM   #7
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OK, that's a reasonable idle current. I'd disconnect the output stage, stick a scope on the plates of your driver tubes, and see if they can actually swing the 20-30 volts you need. My suspicion is that the really low value of plate resistor is causing you the problem.
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Old 29th December 2003, 02:35 PM   #8
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oh, i don't have a scope

fwiw, in one incarnation, i have a 24K plate load on the parallel 12AU7 243V B+, 1K cathode resistor and that gave me 123V plate to ground and 5V cathode to ground. i still got distortion.

thank you for your patience.
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Old 29th December 2003, 02:36 PM   #9
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Can you draw up a schematic of the whole thing, from input to output? That would be helpful in the diagnostics.
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Old 29th December 2003, 02:42 PM   #10
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BTW, if you want to try designing and building your own amp, you need a scope and a generator. They're not expensive and they are as important as a drill and screwdrivers. Your problem could be something as simple as oscillation, but without being able to look at the signals, you're working completely blind.
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