Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: W5FH on September 01, 2022, 09:57:29 am
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I have some questions regarding output transformers:
I have been studying the Edcor push-pull audio output transformers with some interest due to their screen taps at 40%. I recently did a build using the Amp Supply 8025-H output transformer using a pair of 5881's which worked out very well. I was contemplating trying the Edcor CXPP50-5K and was curious if the 40% screen taps were desirable with the pair of 5881's? I have seen very few guitar amplifier ouput transformers with these screen taps, is there a benefit to be gained? If so, would I use screen dropping resistors as normal or would the screen drops use/values be affected by the taps? I have studied the ultra-linear in Hi-Fi that uses this technique but usually there are no dropping resistors. The Edcor is interleave wound and strikes me as a quality item. Being that it has only an 8 ohm secondary with no other taps, will taking my negative feedback for "presence" circuit off of the same tap as speaker be of any concern?
Thanks all!
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UL OTs are usually used in clean sounding amps. My Magnatone M10A and Sunn Sceptre are both UL and don't use screen resistors. Some later Fender Super Reverbs and Twin Reverbs use UL OTs and some but not all use 470Ω screen resistors. You can find all these schematics in Hoffman's Tube Amp Schematics Library (link at the bottom of this page).
will taking my negative feedback for "presence" circuit off of the same tap as speaker be of any concern?
No problem.
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what he said ^
i've used Edcor for about 3/4 my builds, UL in about 1/2 those, really like them, the guitarists like the clean OD sound.
They work great in HI-FI settings also, built a couple mono-blocks that even got the cork-sniffers to smile.. a little :laugh:
the quality is good, never gets passed warm when driven hard
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Thank you for the replies and info!
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> if the 40% screen taps were
If you were hurting for the last hundredths percent THD you would try all values.
But mostly there is pentode, triode, and in-between (UL).
Classic 5881 replaces 6L6 which is a lot like 7027 which does have a UL suggested condition.
https://frank.pocnet.net/sheets/049/7/7027A.pdf
But note they suggest 8k load. And power is well down (24W) from the pentode suggestion (up to 76W!). And THD not very different.
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... 40% screen taps ... I have seen very few guitar amplifier ouput transformers with these screen taps, is there a benefit to be gained?
See attached article for explanation of ultra linear output - esp the graph diagrams on pages 11-12
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> See attached article
The More Info reference:
https://worldradiohistory.com/Radiotronics_AU.htm
https://worldradiohistory.com/Archive-Company-Publications/Radiotronics/50s/1955_05_AWV_Radiotronics_20_05.pdf
https://worldradiohistory.com/Archive-Company-Publications/Radiotronics/50s/1955_06_AWV_Radiotronics_20_06.pdf
https://worldradiohistory.com/Archive-Company-Publications/Radiotronics/50s/1955_07_AWV_Radiotronics_20_07.pdf
(about 15MB per issue)
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Thank all of you for the very good info -
I am researching the best impedance of OT primary using 40% taps for the TungSol old stock 5881 with around 420 VDC B+. For my last build I used a 50 watt 3.9K OT (APD 8025H). I was considering using the Edcor 50 watt 43% tap 5K impedance OT. Should I move up to a higher impedance?