Pentode or triode mode EL84 design suggestions - diyAudio
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Old 10th August 2015, 06:31 PM   #1
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Default Pentode or triode mode EL84 design suggestions

Hey there, sorry for the question that's probably been asked a million times before.

I have some output transformers for a push-pull EL84 amp, but can not be used in ultralinear mode, which most DIY designs seem to make use of.

I also have a power transformer, that when rectified (with silicon diodes) gives about 300VDC.

Are there any nice designs that could suit these types of transformers? Maybe not even using EL84 tubes.
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Old 10th August 2015, 07:00 PM   #2
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If you have high efficient speakers and don't require loud listening levels wire them as triodes. BUT most likely the plate to plate load impedance is for pentode operation (unless the secondary has multiple taps which would give you more options) so design around pentode mode, increase open loop gain so you can use 10-20db global feedback to keep distortion and output Z low.
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Old 12th August 2015, 06:41 PM   #3
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Depending on your speaker efficiency you will need to choose between pentode mode (about 15wpc) or triode strapped (about 6wpc)

For triode mode, John Broskie's EL84 design is very simple and elegant, and from what I read sounds very good. I plan to build one of those myself soon.

If you need the power of pentodes, SY's Red Light District looks like a good candidate. Or you could build a Baby Huey in straight pentode instead of UL.

Last edited by phalanger; 12th August 2015 at 06:50 PM.
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Old 12th August 2015, 08:24 PM   #4
Octavia is offline Octavia  United States
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I know this is DIY and I'm all for it when it comes to speakers but frankly I'm not "gifted" when it comes to building electronics. Having said that I LOVE my modified Dyna SCA35 and think that EL84 amps are terrific. For these Baby Huey's, Red Lights, and EL Triode designs, can anyone point me in the direction or already built and just plug it in models?
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Old 12th August 2015, 11:43 PM   #5
rmyauck is offline rmyauck  Canada
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Check out George's Anderson's simple PP amp using EL84's as he has an affordable PCB which allows all three output transformer configurations. He may still have kits of parts to populate the board available for it. Schematics are in the the threads for it on the forum on this site along with much info.

Assembly Manual | Tubelab

As for full kits the Dynaco ST-35 from PCB maybe go to Triode Electronics.
It has room for a volume control to be added and the EFB mod is a great one for Output tube life and sonics by Dave Gillespie found on the Tronola Electronics site.

Dave's Lab

Last edited by rmyauck; 12th August 2015 at 11:52 PM.
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Old 13th August 2015, 12:46 AM   #6
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To date, nobody has reported back about a full pentode mode "El Cheapo". There's plenty of reason for it to work, just fine. Regulate g2 B+, to maximize open loop linearity. For EL84, as opposed to 6V6 family, "finals", the cathode bias resistor's value changes.

The "ideal" B+ rail is approx. 355 V. Your power trafo needs boosting. Connect the filament winding(s) in series with the primary and get heater power elsewhere.
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Old 13th August 2015, 12:56 AM   #7
Octavia is offline Octavia  United States
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Thanks for the recommendations.
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