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!