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
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
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 ?
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.
Ok, thanks. I set each tube to 50 ma-55ma?
Cos i have 40mA per tube today.
I use fixed bias.
Regards//Daniel
Cos i have 40mA per tube today.
I use fixed bias.
Regards//Daniel
No, 70mA each. That puts the valve right on the limit of dissipation. It assumes cathode resistor bias. See Mullard 5-20.cogsncogs said:
For fixed bias there is no bias shift due to signal so 50-55mA will be fine.
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.
In addition to listening, have you compared THD with 35 mA and 55 mA ?
I have, and the difference is essential.
Does the amplifier work at class A or AB1, depends on the used power level.
With 55 mA the class A operation extends to higher power level, but finally AB1 is reached.
I have, and the difference is essential.
Does the amplifier work at class A or AB1, depends on the used power level.
With 55 mA the class A operation extends to higher power level, but finally AB1 is reached.
In an ideal world a valve would have a square-law curve at low currents and then linear at high currents. To get distortionless push-pull you would either bias at the centre of the square-law region or at the centre of the linear region - anywhere else would introduce distortion. Real valves are not that simple, but the general principle still applies: there are two good bias regions, one at quite low current and the other at quite high current. Sitting between them increases distortion, but perhaps not as much as it would for an ideal valve. My guess is that for an EL34 with a 440V HT 35mA is right in the middle so best avoided. Use either 10-20mA with fixed bias for AB (B except for the smallest signal)) or 50-60mA for AB (A except for the largest signals).
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