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7027As for Mesa?

Discussion in 'Amps and Cabs [BG]' started by Rockbobmel, Sep 4, 2004.


  1. Rockbobmel

    Rockbobmel Supporting Member

    Can you use these in a Mesa 400+?. Isn't the voltage real high in these tubes? Maybe too high for the amp.. My V-4 has them and I love-Em. Thanks, Bob
     
  2. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    We probably need some input from Ben Strange on this one, but here's my take. You can use them, but your amp might need to be tweaked a little, specifically it might need a bias adjustment in the output stage. 7027a's typically operate at higher voltages than 6L6's, which means they'll probably change your sound a little (might get a little "browner" would be my guess), but as far as plate voltage you should be okay. There are other considerations, like heater current, but there you're in luck 'cause the 7027's take about the same juice as a 6L6. But they have a slightly different output impedance, meaning your speaker load won't be exactly matched anymore (this may or may not be a problem, depending on the exact circuit and the particular output transformer in use). Also the 7027 circuits typically use a higher current PI, like a 12au7 (12at7 will work, you may get a little more distortion than you're used to though). You can find the data sheets for both tubes here:

    http://www.pmillett.addr.com/tubedata/HB-3/masterindex.html
     
  3. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Yes, just verified that. The bias voltage is 10 volts different between the Mesa and the V4. You can find the V4 schematic on schematicheaven.com, and the Mesa schematic here:

    http://www.tubefreak.com/mesa.htm
     
  4. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    I got nothing on that... I'll have to check Tuesday (4 day weekend! Woohoo!). I sure did bookmark that tube index in a jiffy, though. Thanks, nonsqtr!
     
  5. Wouldn't 12-7027's be kind of expensive, especially if they are good USA NOS. Seems kind of like overkill to me.
     
  6. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    The old Tesla factory is making them currently. The old GEs would be hard to find, and really expensive to boot. The JJs would be the way to go, provided they will work.
     
  7. There's no reason to do it. A 7027 is just a 6L6GC with some extra grid connections. The voltage ratings are just sales hype, even back in the day when they were US made. If you want a better 6L6, you'll have to use JAN 7581's.
     
  8. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    But different tubes do sound different, so perhaps it would be beneficial if they imparted an improvement in tone. Or perhaps not. Who knows?
     
  9. It's the SAME guts with two extra grid connections. There will be NO difference in sound whatsoever.
     
  10. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    From what I understand of it, the 7027a's generally are more powerful than the standard El34 or 6L6, and kind of sound like a cross between the two. They're also reported to impart a more "hi-fi" tone than either the EL34 or 6L6. Whether this is an inherant design difference, or whether the JJ (Tesla) factory is just making them pretty well these days, I'm not sure.
     
  11. No. A 7027 is just a 6L6GC. Back when RCA debuted the tube, they took their existing 6L6GC design and added extra grid connections for ultralinear operation in HiFi's. When they juggled the numbers with ultralinear connections, the plate voltage rating and power went up slightly and they used it as a marketing tool. I don't know where you got the idea that they're anything else, but they're not. The Tesla 7027 is likewise their version of their 6L6GC with the extra connections. Unless you have to have the grid connections, it's waste of money to buy 7027's.
     
  12. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Sounds like someone's been reading Lord Valve. People go round and round with this one. The "original" 7025's actually were different, they had a slightly different internal structure (hence the slightly different output impedance), an extra small cooling fin, a different getter, etc etc. But PBG is right in that these days, the modern manufacturers like JJ just put a different label on a 6L6 (and if you go to the JJ spec, you'll see they're the same, you'll just get redirected to the other tube chart). If you get a "real" 7025A made by GE though, you'll note some substantial differences in construction. Good luck finding one, and it'll probably cost an arm and a leg if you do.

    The Mesa runs a little cooler than the V4, so you're probably okay on the bias, if you want you can use either of those types of tubes (or a 7581 as PBG said, those can be nice sounding tubes too). The "extra" grid connections are just on the base, in other words the connections "inside the tube" are the same. You can use a 7025 anywhere a 6L6 is called for, but maybe not the other way around (depending on the wiring in the amp, which can be easily modified for interchangeability).

    If you do find a GE 7027A, check it out. A dozen of 'em would be an expensive experiment, but they're still a prized tube in the audiophile world (like the GE 6550A).
     
  13. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    As I understand it, isn't JJ striving to make them like the old GEs? In which case, wouldn't they sound different?

    If there's no inherant tone/power difference, why bother making them?
     
  14. Oh please; I take LV with a much-needed grain of salt. It's funny that it's not just him and me, Ned Carlson has a whole page dedicated to 7027's and Ampegs and the whole 7027 mystique in general.

    I'll assume you mean 7027 and you are wrong. I have RCA and GE 7027's and there is NO difference AT ALL between ther internals and their 6L6 counterparts, all of which I also have as well. The larger cooling fins are a myth(most likely left over sales hype) as is the assertion that there is a different getter structure. A GE 7027 is identical to all of GE's other 6L6GC's, short bottle and all, same for RCA with taller bottle construction.

    This much, at least, is correct.

    And just so it's clear, I have all four variants of 7027s, RCA 7027 and 7027A and GE 7027 AND 7027A. They're just 6L6GC's. I HAVE the tube and it's 6L6GC countrepart; there is NO difference in construction internally whatsover.

    ...because it's easy money for little to no extra work. You take a 6L6GC and run two extra grid lines to the base pins and charge 20% more per tube, all extra profit.
     
  15. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Well, sounds like you're quite the expert. I'll defer to your superior wisdom on this one. :D
     
  16. Nice sarcasm. If you'll buy me a digital camera, I'll take pictures and SHOW you. Bet I'd be faking that/'making up more pseudo science', too. :meh:
     
  17. notanaggie

    notanaggie Guest

    Sep 30, 2003
    PBG is correct on this............

    If you look at Sylvania, for instance, and check say the 330V plate voltage condition, every stinkin number is the same, including bias voltage, screen current, load, and power output . Looks like the others agree too, but I didn't check every one.

    The only difference is that BOTH grid 1 (control) and grid 2 (screen) have an extra connection pin.

    I hadn't paid much attention to it, so I had to look it up.

    Now, as to them matching up with the much-vaunted Mesa "STR", well that is another issue. Re-biasing might be a good idea, or at least a check.

    I don't put much faith on that STR deal, but its great marketing.
     
  18. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    Ok, here's the skinny - you can put them in there, but you'll have to do some modding. The pins are in different positions, so you'll have to re-wire the tube socket. Also, some bias and voltage mods may need to be made. It doesn't appear that you'll get any significant improvement by doing this. Stick with what you got.
     
  19. No you won't. 7027's have EXTRA pin connections, but the basic pinout is the same, and there will be NO voltage or bias issues.
     
  20. notanaggie

    notanaggie Guest

    Sep 30, 2003
    Where did that come from?

    Unless the unused pins are used for a tie-point for something else in the amp, there is NO pin problem whatsoever.

    As far as the bias, maybe, or maybe not.

    The famous Boogie STR tubes are probably sort-of correctly biased when new, but after use, they start to drift off. So the fixed bias isn't right all the time. It might get "righter" with time, or it might get "wronger", but for sure it isn't the same over time.

    So what's the difference? Boogie tubes wrongly biased, or 7027s wrongly biased?

    I'd check bias for ANY re-tubing, Mesa just makes it more difficult to correctly set it.