EL34 Push Pull in Class A - diyAudio
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Old 14th May 2006, 02:48 AM   #1
arnoldc is offline arnoldc  Philippines
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Default EL34 Push Pull in Class A

I want to run my current EL34 PP in AB into pure Class A. But apparently, there is no spec on the datasheet for running the EL34 that way. In comparison, the 807 does have push pull Class A specs.

Am I missing something? Has this been done before?

I'd appreciate your help as usual.
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Old 14th May 2006, 03:29 AM   #2
arnoldc is offline arnoldc  Philippines
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sorry, incomplete detail...

not in triode mode, but pentode.

and the real intention for this is i want to use a current regulator to bias the EL34 (ditch the power resistor and have more or less equal current between tubes). will use the LM317 or DN2540 for this.

maybe if i use 50mA constant current on the cathode, then i can force the tube into class A. With 50mA each EL34 at 400Vp (just a number I picked ) where am I in the operating region?

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Old 14th May 2006, 07:04 AM   #3
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The El34 has 25 watt dissipation. So, at 400v this would be 62.5 ma per tube. At 350v, it would be around 70ma per tube. Check the tube specs to see what the max current the EL34 cathodes can handle.
At 350v at 70ma you would probably be working in pure class "A" . Probably at the 400v plate voltage also. Again, check your tube specs / curves to make sure.


Below are typical values for a single ended class "A" amplifier. You could make this push-pull using two tubes. Plate to plate load would be 3.5-4k.

Class A Amplifier
Plate Voltage ................................. 250v
Grid No. 2 Voltage ............................ 250 V
Grid No. 1 Voltage ............................ -13.5 V
Plate Resistance (approx) ..................... 15000
Transconductance .............................. 11000
Plate Current ................................. 100 mA
Grid No. 2 Current ............................ 15 mA
Load Resistance ............................... 2K
Power Output (approx) ......................... 11 W


Hope this helps... Daniel
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Old 15th May 2006, 12:33 AM   #4
arnoldc is offline arnoldc  Philippines
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Hi Daniel,

Thanks for the info. I thought I should be looking for something else.

Cheers!
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Old 15th May 2006, 07:36 AM   #5
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Keep the B+ low for pentode class A, preferably under 350v. If B+ is too high, you will get too much dissipation with large signals. You might find it interesting to see this article. Look for "The last word on Class A". He explains it quite well.
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Old 15th May 2006, 08:13 AM   #6
arnoldc is offline arnoldc  Philippines
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Hi Ray,

I was able to dig up your old posts at the Asylum. It looks there was no definitive answer there too.

Thank you for the link, been to this site before but I wasn't digging enough.

CHeers!
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Old 15th May 2006, 08:47 AM   #7
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I recently built an EL34 PP in ultra linear mode. The driver was 6J5GT with CCS, cap couple to Electra-Print phase splitter autoformer to a pair of EL34s. Each EL34 was running at 370V plate, -26V cathode bias, 65mA. The OPT was Hammond 1650R. The power is 27W/channel. It sounds great with my Fostex 166E. Giving clear and clean sound with warm mid and full bottom end bass.
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Old 15th May 2006, 09:04 AM   #8
arnoldc is offline arnoldc  Philippines
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Hi CijTele52,

At that operating point, is it biased in Class A?
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Old 15th May 2006, 02:22 PM   #9
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I estimate that the EL34s should work in class A up to 20W output before moving to class AB for the rest.
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Old 15th May 2006, 03:21 PM   #10
jane is offline jane  Norway
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You can do an estimate for max class A power on the primary side by using P=I^2 x R.
Max current swing = 2 x Iq. Since 2 x Iq is peak current you have to divide it by sqrt(2) x sqrt(2) = 2.
P[Amax] = 1/2(2xIq)^2 x Ra-a/4

If Iq=70mA and Ra-a=4k, max class A power will be 9.8W.
There will be some loss in the transformer as well.
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