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Tubes - Sylvania 12ax7 vs 5751 3 mica blackplates

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4kids

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I am new to tubes and am wondering, "Are NOS Sylvania 12ax7 triple mica black plate tubes identical in every way to the famed Sylvania Gold Brand 5751 triple mica black plates?" One source claims that if the 12ax7's have the same gain, they are 5751's even though the boxes and labelling on the tubes say 12ax7. Can anyone confirm or comment on this? Is it possible that Sylvania "mislabelled" tubes? I am curious if there are differences in the construction, quality, value (cost) and most importantly sound.

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Re: Tubes - Sylvania 12ax7 vs 5751 3 mica blackplates - tvr2500m 18:56:29 10/06/04 (1)
Cross marking/branding is remotely possible - but very remotely. A Sylvania black plate 12AX7 is an entirely different tube than the Sylvania blackplate 5751. Bambi covered it well. It can be difficult to tell the difference between some REALLY early GEs and Sylvania black plate, triple mica 5751s. The internals look very similar, the etched tube type marking looks the same. We're talking about an early period where it seems that GE didn't also etch those tell-tale dot patterns on the tube along with the type. Both are nice sounding tubes, I think, with the Sylvania being the better of the two, but I still wouldn't expect to pay as much for the GE.
I have some GB-branded Sylvania triple mica black plate 5751s. None have gold pins. I believe the gold pins were reserved for the later gray plate types, both triple and double mica flavors. Not all GBs of any type had gold pins.
The Sylvania 5751 black plate three mica tube is excellent. So is the triple mica gray plate Sylvania 5751, too. I'm not sure which I like better. There're two gray plate variants I've seen. One type with two crimps one side of the plates, and one on the other; and another type with one crimp on either side of the plates. They're both excellent, but the one of myth and legend is the fomer type.
It pays, I think, to not worry so much about the branding - GB, military, other. Just worry about the construction and the performance of the tube. If you're clever enough to spot an unmarked triple mica Sylvania 5751 from its construction you can sometimes pick up essentially the same tube as the ones wearing more vaunted branding for much less money. Definitely a tube to seek.
- SJ
- Tubes - Sylvania 12ax7 vs 5751 3 mica blackplates - 4kids 20:25:55 10/06/04 (0)
  SJ,
  Thanks for the info. It seems that the concensus is that a 12ax7 cannot be a mislabelled 5751. I paid $155US for a "matched" pair of advertised Sylvania triple mica blackplate 5751's and received 2 tubes that are clearly labelled 12AX7's with no other labelling (even the original boxes said 12ax7); they are triple mica blackplates. The source claims these are 5751's because of their gain and that Sylvania never made a 12AX7 that was a triple mica. My question is, "How can they have the same gain as a 5751?" I can't confirm this as I have no way of checking.
  So being that this 12ax7 is a triple mica blackplate, and supposedly has the same gain as the 5751's, do you think that it would "sound" the same?
  Is their some visual aid to identify the tubes by looking at its construction? ie. the plates, etc.
  Finally, what is the street value of a "matched" pair of Sylvania 12ax7's with triple mica blackplates?
  4kids
Not likely these two types were confused - Bambi B 06:25:45 10/06/04 (0)
4kids,
I have 20 - Sylvania triple mica black plate, square getter, gold print, steel pin tubes that were all from the same military bulk source, so even with that many, they are all one model and I am not an expert in the subtleties of different versions. Note how many little features I have to list to define exactly which ones I mean- there are a pile of little variations.
If you read "Joe's Tube Lore" in the FAQ section of this site, the commentator is definitely an enthuastist for the 5751 over the 12AX7 and he explains some of the many small differences.
My impression is that the several Sylvania 5751s could be called "GB" -Gold Brand- tubes could be a consumer labelled version and the military have designations for that system- but are of identical construction.
I also have a few older Sylvania 12AX7s with black plates and square getters, but these are two mica. I have never seen a Sylvania triple mica 12AX7 myself. But, more to the point of your question, I have never heard of a Sylvania 12AX7 that is really a 5751 or vice versa- a major manufacturer and supplier of tubes to the military would not dare be casual about such an important issue. As the gain of a 5751 is only 70% of a 12AX7 this would be quickly noticed and collectors would all be warned: "Hey dude, watch out for those gnarly 12AX7s that are really 5751's- a downer and/or potential bummer if you get one in your phono stage- man".. , < Simulated warning- may not have happened!
The early Sylvania 5751 is extraordinarily nice- a subtle, refined, focussed sound that it quite apart from most 12AX7s I've used. It seems both dynamic, detailed and "fast" yet also well balanced and smooth. I use 4 of the old 5751s in my microphone preamp which is very revealing of it's tubes (6-12AX7, 2- 12AT7) and they are fantastic. It's a pity they have become a bit expensive.
I've spent some time with the old two mica 12AX7s too and these are a different thing- very deep and lush, but a bit heavy handed compared to the 5751 and a looser image, less open or transparent. Still very nice in the right place. These can sometimes be had for little money. A couple of years ago I bought 8 of these NOS for $6 each as Baldwin organ labelled tubes. Two were somewhat microphonic, but what a nice tube to do 1949 Bing Crosby!
Cheers,
Bambi B

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