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Stellar Sylvania Specifications!

Sylvania 12AX7 test

I’ve heard stories told about fantastic Sylvania tubes with exceptionally high gain factor (mu) ratings in the order of 10% to 20% higher than nominal. Now, the practical implications of this would be that a 12AX7 tube would have a mu of 110 to 120 meaning such a tube could be used to hot-rod a guitar amp, that is increase the gain in the preamp section—just the ticket if you’re looking to tweak your guitar amp to make it a little more responsive or cream up the overdrive. This is the kind of thing we guitarists dream of, and being guitarist of curious disposition, I can tell you that it soon became a matter of vital importance to find out if there was any truth in these tales of super tubes.

It wasn’t too long before I managed to acquire several N.O.S. (new old stock) Sylvania 12AX7s of 1960s vintage—arguably the zenith of guitar (and rocket) technology—to test and, well, I think the photo above says it all. My ‘Tube Imp‘ tube tester indicated a mightily impressive mu value of 112. This tube was a particularly lively specimen from the small batch, however all the fifty year old Sylvania 12AX7s tested higher than a brand new, modern manufactured JJ ECC83, a 12AX7 equivalent, which more often than not exhibits a healthy mu value in the high nineties (when tested on the same tube tester).

These vintage tubes are a quick and easy way of boosting the gain of your guitar amp or Effectrode ‘Blackbird’ preamp and ‘Tube Drive’ pedals to create some additional harmonic saturation. And, although technically equal to RCA tubes, Sylvania tubes don’t command the same high prices, probably because they don’t have the same strong historical associations with the recording industry (RCA Victor, etc) or companies such as Fender, whom utilised the RCA 7025 in their amplifiers. There are still bargains to had and Sylvania tubes retail in the region of $50 each, often pristine and still in their original boxes. Not a bad deal at all considering their heritage and there really is nothing quite like the tone of vintage glass.

Right, I really must fly—got to have a quick look on Ebay!


  1. Thom Opal says:

    Since the amplification factor MU is the product of transconductance times the dynamic plate resistance, I’m left wondering about your tester… does it measure both of those factors simultaneously and calculate the MU value? That would be an exceptionally rare tester (and also mighty handy). Also – what voltage are you measuring at?

    As for those high-output long gray-plate Sylvania 12AX7/7025, I personally have never cared for them. They seem to sacrifice clarity for output. I much prefer the articulation of standard-spec 12AX7, and let the circuit do the heavy gain lifting. Older black-plate (17mm and 14mm) Sylvania 12AX7 are more my taste.

    • Phil says:

      Hi Thom,

      The ‘Tube Imp’ does measure transconductance, however I’m not sure if it measures plate resistance simultaneously and utilises both measurements to then calculate the gain. Plate voltage was set at 160V.



  2. Thom Opal says:

    Thanks for your quick reply, Phil. So the Sylvania numbers measured were just transconductance, then? That would only be part of the story, if trying to relate it to the amplification factor (mu). And 160V seems low to me, much lower than black-face Fender preamp, for instance.

    For a true hi-mu experience with a kind of double-12AX7 in one tube, have you ever tried out a 12BZ7? They’re a couple of dB louder (more true gain, equals a drop in noise floor if you adjust the circuit for same volume) than a comparable 12AX7. Alas, they’re more prone to microphony. Still, if you get a good one, they’re very interesting!

    • Phil says:

      In answer to your questions.

      “So the Sylvania numbers measured were just transconductance, then?”, as I said, I’m unsure how the “Tube Imp” measures gain, it may very well calculate the transfer function from plate resistance and transconductance, or utilise some other method.

      “And 160V seems low to me, much lower than black-face Fender preamp, for instance.” The ‘Fender Blackface Deluxe Reverb” preamp section operates at between 170-180VDC, so, yes, this tube was tested at 5-8% lower plate voltage than in this particular tube amp, however within the limits specified in Sylvania’s 12AX7 datasheet.

      “have you ever tried out a 12BZ7?”. I’ve not had the opportunity to test a 12BZ7. Wasn’t that tube designed and intended for use as flyback oscillator in television circuitry though?

      • Thom Opal says:

        Yes – the 12BZ7 was developed for television use. Other than a tendency toward microphony (related to longer plates & higher-mu), they can make for very intriguing audio tubes. They draw twice the current of 12AX7. If a circuit can withstand that (most do) then they can be subbed for 12AX7. I’ve found them useful as power amp phase-inverters, for example.

        • Phil says:

          Hi Thom, My best guess is that the higher current rating of the ’12BZ7′ was achieved by enlarging the cathode’s surface area to increase its emissivity. This might, in part, explain why it exhibits higher microphony compared with a ’12AX7′. Yes, I’m sure the 12BZ7 (and other television tubes such as the mighty ’12FV7′) will make an excellent tube for use in phase-splitter circuits and V2 or driver stages where immunity to vibration pickup isn’t quite so critical. All the best, Phil

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