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Well, after finishing up with the German table radio from hell, I finally got to check into that 800B and 5751 issue we discussed a while back.The short answer is a 5751 works in the preamp stage, with no loss of channel or other serious abnormalities.
However, as I long suspected, I don't think I would recommend a 5751 for this circuit. The sonics were just flat and lifeless. The result was the same regardless of 5751 used, which included Raytheons, grey plate GEs and RCA blackplate. This circuit just needs the full early gain of a 12AX7 to sound right. For obvious reasons, a 5751 was only attempted in the preamp stage.
While we had the tube caddy out, we did a little rolling with the 800B on the bench. For THIS particular 800B specimen (B series receivers have a little more example to example variation than the Cs for some unknown reason. Some Bs are incredible sounding, others not so), the best combination were TFK ribbed plates up front and some late 1950s Holland Amperexes in the phase splitters. A very close second was Holland Amperexes all around, closely followed by Amperexes in the phase splitters and Ei Elites up front. The preference here seemed to be for a livelier front end with a slightly warmer but lush tube in the phase splitters. This is not too far off from my reference 800B, which sports Amperexes all around and is a killer unit. Notable "nots" were Mullard box plates (anywhere), RCA black plates, some mid 1960's Westinghouses, and some early and mid 1960's Sylvanias. Interestingly, TFK smoothplates all around were a little too bright (even on mellow B&Ws) compared to the ribbed plate/Amperex combination. For some reason, we never got around to a smooth plate/Amperex combination, which would likely sound very good and similar to the Ei/Amperex combo. Nor did we try any Tung-Sols or Mullard long plates. For reference, this 800B was recapped with well broken in 716s to the outputs, outputs were strong Westinghouse 7591As, and speakers were B&W DM302s, all connected by the very best zip cord money can buy.
Follow Ups:
SgmLaw,
I've ended up with just the opposite, lol! I use tele ribbed in the phase splitter and tung-sol in the pre but will try the opposite in the future, thanks again! Am selling all my 5751, oh well.
Regards,
Jack
Curious if you discovered any issues with the slightly higher filament current of the 5751 VS 12AX7. Was your comparison "plug n play" or did you compensate for filament current differences in any way?Don't have an 800 schematic avail but if similar to 500C the 5751 MIGHT cause a slightly lower preamp filament voltage and 7591 bias voltage...maybe not. Not sure slight voltage changes would affect sonics one way or the other.
mine was rebuilt by Paul @ BizzyBee and should be pretty fresh in the power supply and I pulled all the tubes except output, phase splitter/driver and preamp tubes; thus sag should not have been an issue either - FWIW no 5751 worked as good as any 12AX7 I tried...HTH
We ran this 800B sans phono tubes and with a freshly rebuilt bias supply, so the load factor was deemed insignificant. A strong bias supply shouldn't sag enough to worry about. A tired original supply running borderline to begin with might be another matter, however.
Great Post with excellent detail.
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