EL34 Push Pull in Class A

arnoldc · 14th May 2006, 02:48 AM

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.

arnoldc · 14th May 2006, 03:29 AM

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?

danFrank · 14th May 2006, 07:04 AM

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 µ
Plate Current ................................. 100 mA
Grid No. 2 Current ............................ 15 mA
Load Resistance ............................... 2K Ù
Power Output (approx) ......................... 11 W

Hope this helps... Daniel

arnoldc · 15th May 2006, 12:33 AM

Hi Daniel,

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

Cheers!

ray_moth · 15th May 2006, 07:36 AM

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.

arnoldc · 15th May 2006, 08:13 AM

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!

CijTele52 · 15th May 2006, 08:47 AM

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.

arnoldc · 15th May 2006, 09:04 AM

Hi CijTele52,

At that operating point, is it biased in Class A?

CijTele52 · 15th May 2006, 02:22 PM

I estimate that the EL34s should work in class A up to 20W output before moving to class AB for the rest.

jane · 15th May 2006, 03:21 PM

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.

14th May 2006, 02:48 AM	#1
arnoldc diyAudio Member Join Date: Apr 2003 Location: Makati	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.

15th May 2006, 03:21 PM	#10
jane diyAudio Member Join Date: Jul 2004 Location: Norway	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. __________________ Life is hard - Then you die.

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14th May 2006, 03:29 AM	#2
arnoldc diyAudio Member Join Date: Apr 2003 Location: Makati	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?

14th May 2006, 07:04 AM	#3
danFrank diyAudio Member Join Date: May 2005 Location: California	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 µ Plate Current ................................. 100 mA Grid No. 2 Current ............................ 15 mA Load Resistance ............................... 2K Ù Power Output (approx) ......................... 11 W Hope this helps... Daniel

15th May 2006, 12:33 AM	#4
arnoldc diyAudio Member Join Date: Apr 2003 Location: Makati	Hi Daniel, Thanks for the info. I thought I should be looking for something else. Cheers!

15th May 2006, 07:36 AM	#5
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	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.

15th May 2006, 08:13 AM	#6
arnoldc diyAudio Member Join Date: Apr 2003 Location: Makati	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!

15th May 2006, 08:47 AM	#7
CijTele52 diyAudio Member Join Date: Feb 2005 Location: CA	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.

15th May 2006, 09:04 AM	#8
arnoldc diyAudio Member Join Date: Apr 2003 Location: Makati	Hi CijTele52, At that operating point, is it biased in Class A?

15th May 2006, 02:22 PM	#9
CijTele52 diyAudio Member Join Date: Feb 2005 Location: CA	I estimate that the EL34s should work in class A up to 20W output before moving to class AB for the rest.