EL84 Push Pull 25-30 watts - diyAudio
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Old 18th September 2010, 04:28 AM   #1
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Default EL84 Push Pull 25-30 watts

Hi all, I'm a newbie here and looking to build a push pull using EL84s about 25watts. Like Morgan Jones Bevios Vally and the Simple 84, but both of thesse run at about 10watts.

Has anyone got a schematic for a 4xEL84 (per output stage) based on either of these designs, and a really good PSU to match?

Won't to spend time and money making the PSU really solid, and keeping the output stages really simple.

Will be driving from a digital source so don't need the complication of a pre-stage in the PSU.

Thanking all in anticipation ;-)
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Old 18th September 2010, 05:59 AM   #2
TheGimp is offline TheGimp  United States
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Data sheet shows AB1 is only good for 11 Watts per pair, so 22Watts per quad..

The only way I can see of getting 25W from a quad of EL84s is to drive them into class AB2.

Data sheet shows 17W per pair in class B, so AB2 should be good for over 22W and less than 34W.

Hmmm. Found another data sheet with AB giving 17W per pair, so 25W for a quad should be doable.

Drop the OPT impedance from 10K to 5K and put the tubes in parallel. Pick a design.

Last edited by TheGimp; 18th September 2010 at 06:02 AM.
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Old 18th September 2010, 06:05 AM   #3
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The same front end should do... just double the PS current capability, double the output tubes and choose a suitable OPT (1/2 the secondary impedance)

Or you could do like Modjeski (Music Reference), run a pair at 750 volts plate, 300V screens.

http://www.ramlabs-musicreference.com/rm10mk2.html
http://www.ramlabs-musicreference.com/rm10design.html

dave
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Old 18th September 2010, 08:27 AM   #4
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You should use 6P14P-EV (EL84-M) instead of EL84. Take, for example, Pioneer SM-83 or Sansui AU-70 schematic, and rebuild it on your own chassis.
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Old 18th September 2010, 08:35 AM   #5
Arnulf is offline Arnulf  Europe
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With high level input I'd go for no-fuss 1x ECC83 (LTP) + 4x EL84 (PP) per channel if I were you for better noise cancellation. 310V or slightly higher B+ should provide about 25W at the output and leave plenty of margin for LPT tail resistor without the need for exotic PSU.
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Old 18th September 2010, 09:56 AM   #6
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Default Thanks, and...

Thanks guys...I can live with 22 watts, 25 was just a design goal!! On the other hadn I found the attached schematic for 4x EL84 driving into 35 watts at 0.5% THD.

The valves are V1,V2 = ECC83, V3,V4,V5,V6 = EL84, V7 = GZ34 power supply 300VAC on the secondary, the only thing not on the parts list is the output transformer. However, as this is very clsoe to the other designs, how much trouble could I get into using the Bevois Vally spec for a output xformer??

Looks like back to Morgan Jones for another read...
Attached Images
File Type: gif 35Wmono 4 x EL84.gif (15.1 KB, 584 views)
Attached Files
File Type: txt 4xEL84 parts list and set up spec.txt (2.0 KB, 140 views)
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Old 18th September 2010, 04:24 PM   #7
athos56 is offline athos56  United States
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Quote:
Originally Posted by malcolmfraser View Post
Thanks guys...I can live with 22 watts, 25 was just a design goal!! On the other hadn I found the attached schematic for 4x EL84 driving into 35 watts at 0.5% THD.

The valves are V1,V2 = ECC83, V3,V4,V5,V6 = EL84, V7 = GZ34 power supply 300VAC on the secondary, the only thing not on the parts list is the output transformer. However, as this is very clsoe to the other designs, how much trouble could I get into using the Bevois Vally spec for a output xformer??

Looks like back to Morgan Jones for another read...
I would cut out everything before P3, the tone controls, but thats just me.

Athos
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Old 19th September 2010, 09:54 AM   #8
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Default 4 x EL84 PSU, and Output XT, did I get this right??

Hi all, working up the PSU design for a 4xEL84 PP...

Based on the attached schematic (and PSUD2) the current design as a GZ34 half wave with 300VAC, which if I am right, is producing about 370 for B1.

If I want to replace this with solid state full rectificaiton, then by PSUD2, I would need about 275VAC from the power xformer.

Did I get this right??

Also, no spec on the output transformer on this schematic. Any suggestions on the specs of what I should be looking for to drive 8R??

Thanks in anticipation ;-)
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Old 19th September 2010, 01:29 PM   #9
Arnulf is offline Arnulf  Europe
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EL84 PP = 8K Raa so take half that value for 2 EL84 per side (4K Raa or thereabouts).
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