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Old 28th March 2014, 09:45 AM   #1
Green77 is offline Green77  Sweden
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Default Bias el34 pp

Hi all!
Just finished a pp el34 in ul mode.
Have around 440v at plate. I have set
bias to 40mA per tube.
Will that give a plate dissipation of 17,6 W?
Do i think right?
What would you set for mA with 440 platevolt?

Thanks

Regards//Daniel
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Old 28th March 2014, 10:30 AM   #2
DF96 is offline DF96  England
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Is 40mA the anode current or cathode current?
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Old 28th March 2014, 11:05 AM   #3
Green77 is offline Green77  Sweden
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Cathode current. Sorry, miss to say that..
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Old 28th March 2014, 11:44 AM   #4
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Quote:
Originally Posted by Green77 View Post
What would you set for mA with 440 platevolt?

Thanks

Regards//Daniel
I would set it to the level where minimum distortion is achieved, but I quess you have no way to see this.

Then some 50...55 mA cathode current would be good compromise.

What output trafo you have ?
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Old 28th March 2014, 12:21 PM   #5
DF96 is offline DF96  England
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Yes, 50-60mA cathode current should be fine for music. If you are planning to do sine wave tests than a higher starting current may be needed to cope with the bias shift with signal - Mullard used somewhere around 70mA.
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Old 28th March 2014, 12:59 PM   #6
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Quote:
Originally Posted by DF96 View Post
Yes, 50-60mA cathode current should be fine for music. If you are planning to do sine wave tests than a higher starting current may be needed to cope with the bias shift with signal - Mullard used somewhere around 70mA.
70mA would be for two tubes, 35mA each.
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Old 28th March 2014, 01:08 PM   #7
Green77 is offline Green77  Sweden
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Ok, thanks. I set each tube to 50 ma-55ma?
Cos i have 40mA per tube today.
I use fixed bias.

Regards//Daniel
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Old 28th March 2014, 01:15 PM   #8
DF96 is offline DF96  England
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Quote:
Originally Posted by cogsncogs
70mA would be for two tubes, 35mA each.
No, 70mA each. That puts the valve right on the limit of dissipation. It assumes cathode resistor bias. See Mullard 5-20.

For fixed bias there is no bias shift due to signal so 50-55mA will be fine.
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Old 28th March 2014, 02:03 PM   #9
Green77 is offline Green77  Sweden
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Thanks //Daniel
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Old 28th March 2014, 04:54 PM   #10
kruesi is offline kruesi  United States
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35 mA per tube sounds better to me, at first glance. Seems like 70 mA per tube (140 total) would put you closer to class A than to AB1. You'll want to check datasheets and experience first, just my two cent guess here. No sense dissipating all that heat if you don't need to. The object is simply to reduce the crossover distortion. Once that's accomplised, additional bias just runs things hotter and consumes more power.
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