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Distortion in 807 amplifier - please advise

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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.
 
SY said:
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?

SY said:
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.
 
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.
 
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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12AU7 Load

arnoldc said:

Hello ,
The 12AU7 anode load is too low , also there is no feedback so I'd expect the high output impedence will react heavily with your speakers . The 12AU7 may not allow enough gain , especially with an unbypassed cathode resistor : suggest an EC8010 , 6N1P or E80CC instead , wrapping feedback from the output transformer secondary to the input stage cathode

316a
 
Hey ArnoldC,

Try hooking the input to the 807 via the interstage capacitor, bypassing the 12AU7. Of course, remove it from the 12AU7 plate connection first. You should get a good enough output to hear if the output stage is distorting or not. Tetrodes have a high enough gain to drive with a small voltage. At least you can, without a scope, listen to see if that is the bad stage.

Otherwise, I think that the 12AU7 stage is the culprit. You are paralleling them, so the values for resistances for optimum performance should be halved. So, to get a decent response from the 12AU7, use about 5 mA per triode, or ten for paralleling. If you need a bias of 4 volts (optimum for 125 V at the plate), that makes the cathode resistor 4V/10mA or 400 ohm (V/I=R. Ohm's Law). The plate resistor should then be 125V (about half of the 250, or 245) divided by 10mA, or 12K5. This should give you optimum performance for the 12AU7 in parallel (I used the plate curves from the tube manual to arrive at these calculated values) .

One other thing... use a capacitor on the input. There is perhaps a load down of the grid circuit being imposed by your source. This will change bias and create distortion. Also perhaps a 220K grid resistor instead of 100K.

My :2c:

Gabe
 
Arnold
From looking at your output stage I feel that your screen biasing looks imcomplete. There should be a capacitor between screen and cathode as i recall. But conventional you could utilize a voltage divider to bias the screen to. Whats your screen voltage? from the big 50k resistor, screen current should be really low this would not load the tetrode right. They need to be fed with suffice current with respect to plate voltage and grid voltage. You have to view the pdf for 807. My i suggest looking for a super big pdf around 6-8mb. All you need to know about 807. I think should be available at retrovox website. Great site about screen voltages and care for them and understanding them
http://www.webace.com.au/~electron/tubes/index.html
As mention the tetrode as high gain and high output impedance as well.

changed grid to cathode. Was not thinking when typing that but thats what i meant bummer:smash:
 
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