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Old 5th December 2020, 10:08 AM   #751
rock12 is offline rock12  United States
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Join Date: Nov 2010
Location: midland mi
Default New build SPP

Thanks George for another great design completed mine and enjoying it as much as the SSE
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Old 17th January 2021, 03:17 PM   #752
sgtnoah is offline sgtnoah
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Join Date: Mar 2015
Tubelab Simple P-P
Hey all, I'm just putting the finishing touches on a new P-P build. Edcor 25w opts, EL84s, probably going with UL wiring. Need to order a few more parts and contemplating the C101/C201 feedback cap as well as the R104/R204 resistors.

Please help double check my thinking in this department. Have read in previous posts that this amp likes around 6dB feedback (set by the resistor). The capacitor helps with oscillation and square wave performance - and it is a balance between the two. I've recently fought a modified Dynaco MkIII in this department and hopefully learned a couple lessons. If I've learned correctly, the bump on the descending side of the sine wave (and corresponding FFT extremely noisy) is oscillation and requires a change to the feedback cap.

Sooooooo, If I'm on the right page so far, something in the vicinity of 120 pf should get the job done. If I may ask, what values of the feedback caps & resistors are working out for people?

Thanks in advance!

-- Pete
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Old 20th January 2021, 05:20 PM   #753
itsikhefez is offline itsikhefez  United States
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Tubelab Simple P-P
Hi sgtnoah! For my OPT's (Hammond) 120pF was not enough to make the square wave nice... at the same time, over ~400pF caused some high frequency oscillation (which was cured with a small cap across the screen connector from the OPT)

I kept the standard 5.6K (IIRC) resistor.

So.. some experimentation may be needed here
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Old 20th January 2021, 05:21 PM   #754
itsikhefez is offline itsikhefez  United States
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Join Date: Aug 2015
Tubelab Simple P-P
For wiring in Triode, is the 7.6K that is currently installed OK?
Also, do the cathode resistors need to be increased from 270ohm?

My questions are based off of the info in the Mullard datasheet
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Old 21st January 2021, 12:34 AM   #755
sgtnoah is offline sgtnoah
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Tubelab Simple P-P
itsikhefez, thanks! New Edcor iron should be here in a few weeks. It will be fun finishing this project up!
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