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Old 5th December 2020, 09:08 AM   #751
rock12 is offline rock12  United States
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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, 02:17 PM   #752
sgtnoah is offline sgtnoah
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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, 04:20 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, 04:21 PM   #754
itsikhefez is offline itsikhefez  United States
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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 20th January 2021, 11:34 PM   #755
sgtnoah is offline sgtnoah
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itsikhefez, thanks! New Edcor iron should be here in a few weeks. It will be fun finishing this project up!
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Old 29th March 2021, 07:37 PM   #756
rock12 is offline rock12  United States
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Default Heater voltages

Bought some premium tubes for my working PP had been using JJ the Millard el84s red plate quickly which caused me to recheck readings. My heater voltage is 6.5(sort of hot) rectifier heater 5.1 volts( just hot) using a Hammond 272jx I have a plate voltage of 343 measured at l-pri-1. How can I lower my heater voltages? I think the tubes I bought were junk they red plated I my HealthKit sp14 also
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Old 29th March 2021, 07:47 PM   #757
Einric is offline Einric  United States
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Tubelab Simple P-P
Your heater voltages should be fine.
Try putting a 5u4 in the rectifier slot.
I run a 5u4 for everyday use, I have a Matsushita GZ34 when I drive 4r speakers.
I've used JJ, Sylvania, Canadian Marconi & Polam EL84's and have never had red plate issues.
I AM running the optional choke, oil filled motor run cap & Hammond 272JX PT.
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Old 30th March 2021, 03:23 PM   #758
rock12 is offline rock12  United States
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Thanks for the reply I guess I will brave it and try. Nice looking amp enjoy
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Old 1st April 2021, 12:18 AM   #759
PDL is offline PDL  United States
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Your heater voltages are within 3% of spec. Your line will likely vary much than that throughout the course of the day..

When you say hot, do you mean temperature? That's kinda how thermionic emission works.
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Old 4th April 2021, 10:04 PM   #760
iorrus is offline iorrus
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I *think* I just bought all the parts from Digi Key for the SPP except for tube sockets and tubes/transformers.

I've downloaded my order as a csv file which can be uploaded to the digi key site, this might save someone some time ordering in future. However I replaced all the Xicon resistors (sourced from mouser originally in the tubelab parts list with DigiKey equivalents as Mouser required minimum orders of 200), in addition some of the capacitors in the parts list were end of life and replaced with equivalents so it's not exactly the same but as close as I could get it and doesn't require two suppliers. Total cost was €58 so I got free shipping too.

In addition I bought the following from Don Audio in Germany:

Edcor XPWR066-240 Power transformer
Edcor CXPP25-MS-7.6K
2x JJ ECC81 / 12AT7 Gold Tube
4x JJ EL84 / 6BQ5 Power Tube JJ Electronic
1x JJ GZ34S - 5AR4 Tube
6x Tube Socket Noval Ceramic Print 9-Pin Gold

Only thing missing (I think!) is the octal tube socket as it wasn't in stock in Don Audio. Hope this is useful if anyone in Europe/EU is trying to build this.

I'll update with my progress, if anyone has any suggestions please let me know.

Last edited by iorrus; 4th April 2021 at 10:20 PM.
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