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RE: I'm leaning towards harvesting the transformers for DIY monoblocks

24.15.196.130

In Reply to: RE: I'm leaning towards harvesting the transformers for DIY monoblocks posted by Eli Duttman on December 26, 2020 at 03:58:24

Eli,

Thanks; all very good suggestions.

If I go the "transformer harvest" route I will probably stay close to the original 99D power amp circuit, in honor of the Scott origin. (You mentioned DC for the first tube(s); Scott did some interesting things with the first two 12AX7s, lighting them with the cathode current of the output tubes, at starved voltages.)

For my revived Scott 99Ds, in whatever incarnation, I plan to use a batch of very nice 1960's Russian 6P3S (without the "-E"), which appears to be a close equivalent of 6L6 GB/5881. The 6P3S-E version is probably overkill in the Scott 99D with merely 365V on the output plates and 280V on the screens. I see folks are reporting that the -E version begins to redplate at >40 watt dissipation. On the hint of member Michael Sampra, several years, back I have been using 6P3S-E in my Eico HF-87 with very good results.

Follow Ups:

RE: I'm leaning towards harvesting the transformers for DIY monoblocks - Eli Duttman 15:15:48 12/26/20 (2)
Quite a few manufacturers used the "Cheap Charlie" method for DC heaters. FWIW, I loathe it. The thought that the signal might modulate the temperature of preamp tube cathodes makes me cringe.

Look closely at New Sensor's offerings. The tube sold as the Sovtek 5881 is 6Π3C-E. The variant is definitely stout, but it's not a 6L6GC equivalent.

Eli D.
- Would the real 6P3S please stand up - Spantou 03:52:05 12/28/20 (1)
  Indeed, the 6P3S-E is no 6L6GC and plate dissipation of > 40 watts is unproven (to me). Although they are more robust than their conservatively rated 20.5 watt dissipation would indicate I had some initial issues with 6P3S-Es red-plating in the Eico HF-87 mentioned above. However, that amp routinely runs it's EL34s at above 30 watt dissipation, so the 6P3S-E is not a drop-in. In this amp, after increasing the cathode bias (don't remember how much) they have been very satisfactory and good-sounding tubes to my ears.
  
  See: https://www.audioasylum.com/audio/tubes/messages/22/226635.html
  
  6P3S tubes are less robust still, but as best as I can tell they are rated for at least 19.5 watt dissipation, and plate to cathode voltages of around 375 V, so I'm going to give them a try in my revived Scott 99Ds, that originally had 6L6GB tubes.
  
  This link is the best source of tube data I have seen for the 6P3S; please let us know if you have a better source:
  - 6P3S Tube data (Open in New Window)
  - RE: Would the real 6P3S please stand up - Eli Duttman 06:11:04 12/28/20 (0)
    Perhaps a clue lies in the TungSol 5881 datasheet . That document shows the conservative design center value for max. plate dissipation = 23 W. 25 W. would be reasonable for the design maximum number.
    
    Eli D.

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