STR ratings for Mesa tubes?

Discussion in 'Amps and Cabs [BG]' started by seanlava, May 13, 2005.

  1. seanlava


    Apr 14, 2005
    This one is for the tube-aholics out there: Do the STR numbers on Mesa 6L6 tubes have any relation to the tubes' performance characteristics? If so, what does a high STR rating sound like, vs. a lower STR rating?
  2. superfly


    Aug 4, 2004
    When I bought my used Buster 200 I talked to a guy at the Mesa Hollywood store about tubes. He said the numbers just are in aid in matching tubes. But he never really noticed any sound difference.
  3. Mesa stole the term "STR" from old US made Sylvania tubes comissioned by CBS Fender (as the GE 6550A was for the 400PS) for their ultralinear amps (Twin, Bassman 100, Bassman 135). The REAL only true "STR" tube is a Sylvania 6L6GCSTR, which had a higher voltage and mechanical shock rating than a standard 6L6GC. Philips later bought Sylvania and made the same tube but called it a 7591 and sold a buttload of them to the military. Why the backstory and what does this have to do with Mesa?

    Mesa originally used those very tubes in their first amps, guitar AND bass. When the supply of US and Western European tubes dried up in the early 90's, Mesa began importing Chinese junk tubes and kept using the STR numbers to mislead customers into thinking they were still getting far superior tubes. They now use mostly Russian and Slovokian tubes and the "STR" numbers to tell the country of origin apart.

    Their "ratings" numbers are a crock of a WHOLE different sort.
  4. seanlava


    Apr 14, 2005
    I also have a Buster Bass head. Any recommendations for power tubes? (I favor a gritty, grinding tone along the lines of Billy Sheehan.) I currently have the amp stocked with mesa str 430's.
  5. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The original "STR" was a Sylvania "Special" designation, and stood for something on the order of "Special Tube Request" (maybe "rating", I don't recall).

    Back when tubes were still common everywhere, the deal was that if you made tube equipment, you bought tubes that were selected to fit your application.

    So if you bought 6L6 tubes, you could get yours selected for a particular bias range, and just plug in any tube in teh box, not fuss.

    If you bought preamp tubes, you could get them selected for gain, noise, or whatever. For instance there is a standard spec for the 12AX7 low noise version, it's given in any RCA tube manual.

    If you were smart, you made your optimum tube spec right down the middle of what the manufacturer made, so that it was easy and cheap to get them.

    So, then, if the consumer bought replacement tubes down at the local drugstore (yes you could do that), they were whatever was left over after all the prime ones were pulled out and sold to the manufacturer.

    Sylvania had a bunch of STRs. Each had a number, and they went up to at least 40 or 50 different ones, for different tubes. (information from the Sylvania tube factory manager, back in the 80s, just before it closed).

    The specs for an STR number for a particular manufacturer was usually confidential. Some became more standard. And other manufacturers had them, they just didn't call them that name.

    AFAIK, the one Boogie touts is a bias/gain spec. It just means they are s'posed to be pre-matched, so any Boogie "STR" 6L6 matches with any other.

    These days, half the folks making tubes don't really understand their product, so its anyone's guess as to how "real" a spec like that is in practise.

    But if Boogie sells them that way, they should BE that way, however, no matter who makes them. Boogie is usually good about that sort of thing.

    If someone else has a supposed "equivalent", that is a "well, maybe".


    Sure you don't mean the 5881? Its a cross to 6L6GC as far as the curves, and has same pinout.

    The 7591 has slightly different specs, and a different pinout, as well as a smaller size. Its a red-hot little item, the few amps I have seen use it (Sherwood hifi) ran really hot.
  6. Finger slipped: 7581, not 7591. I should have caught that. (There needs to be a D'OH smilie) I've got a few of them as well as whole bunch of Sylvania STR's that I got from an old music store. I've got a few 7591's, too, but no Ampeg Jet to put them in. :D Thanks for keepin' me honest.

    BTW, AFAIK, "STR" meant "Special Test Requirement."
  7. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Yep.....that should be correct.

    I hadn't run into the 7581A before, but you are correct about it.