EL84 PP using 5K plate to plate primate OPT - diyAudio
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Old 9th March 2010, 07:02 AM   #1
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Default EL84 PP using 5K plate to plate primate OPT

Hello,
I recently bought an old Pioneer Receiver/amplifier for salvage parts. It runs on a PP topology uging the EL84 tube. When I got the amp in the mail and pulled the cover I found it had aftermarket OPTs made by Chicago STD. Apparently it had been serviced sometime ago and these where installed in place of the original Pioneer branded OPT's. These seem to be very nice potted OPTs much to my surprise, and probably much better than the original. My plans were to strip the amp of the power trafo and OPT's and build a simplified EL84 PP amp. As I began to peruse the various forums looking for the ideal amp I noticed most EL84 PP designs are using a 8K p-p primary winding on the OPT. I was wondering if anybody knew of a good design that would use a 5K p=p primary, like the 2 I now have stripped out of this Pioneer.

Also, the OPT states the mA rating is 75mA "each side"??? Am I to consider this a total of 150mA for the OPT (75mA per 1/2 of the OPT)?

If this is the case I may just decide to build a KT77 based amp.

Thanks,
Jeff
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Old 9th March 2010, 07:20 AM   #2
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Quote:
Originally Posted by jmillerdoc View Post
most EL84 PP designs are using a 8K p-p primary winding on the OPT
or higher...

Quote:
Also, the OPT states the mA rating is 75mA "each side"??? Am I to consider this a total of 150mA for the OPT (75mA per 1/2 of the OPT)?
If the current capability is indeed that high you could do a parallel PP EL84 amp

dave
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Old 9th March 2010, 08:17 AM   #3
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by jmillerdoc View Post
Hello,
I was wondering if anybody knew of a good design that would use a 5K p=p primary, like the 2 I now have stripped out of this Pioneer.
My Mengyue Mini operates around 5k since the OTs are intended for 8ohms, but I have more like 5 ohm speakers. You lose some of the available power, but the distortion is lower and more 'triode-ish' since you get more even harmonics from pentodes at low load impedances, and less odd, and the even harmonics cancel out of course. http://www.audioxpress.com/magsdirx/...encowe2996.pdf

Quote:
Also, the OPT states the mA rating is 75mA "each side"???
That's the maximum average current it can handle though each half of the winding. (i.e., to each valve)
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Old 9th March 2010, 07:30 PM   #4
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... since you get more even harmonics from pentodes at low load impedances, and less odd, and the even harmonics cancel out of course.
Many years ago I asked David Hafler why the nominal primary impedances of his output transformers were lower than "book"... and he gave me an answer that essentially mirrors what you have said....

That pentodes operating into lower impedance loads produce greater amounts of 2ncd HD but much lower amounts of 3rd HD. And the second largely cancels... so your left with a win-win.

Dyna built many tens of thousands of SCA-35's and ST-35's (EL-84 PP) with transformers whose nominal primary impedance was 7100 ohms CT.


MSL
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Old 9th March 2010, 07:34 PM   #5
TheGimp is online now TheGimp  United States
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You can also wire your 8 Ohm speakers to the 4 Ohm terminals to give a reflected 10K p-p impedance if you have such taps. Since the transformers are over-rated for the 8-10W you will deliver, it should pose no problems.
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Old 9th March 2010, 09:14 PM   #6
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You can also wire your 8 Ohm speakers to the 4 Ohm terminals to give a reflected 10K p-p impedance if you have such taps. Since the transformers are over-rated for the 8-10W you will deliver, it should pose no problems.
But remember that the effective load impedance that your tube/s will see is the reflected impedance in parallel with the primary inductance.

Unless your transformer has enough inductance to properly support a 10K primary impedance... your tube won't be seeing a 10K effective impedance at all frequencies.

You also want to take into account how reactive of a loadline this would create... i.e., the phase angle of the reultant load impedance.

Generally you cannot just simply "ratio" transformers and expect performance impunity. Transformers really are not universal devices... but are designed to work best at their nominal design centers.

MSL
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