6L6 tubes in a JTM45/Germino 45?

Discussion in 'Amps and Cabs' started by MadFrank, Mar 3, 2012.

  1. MadFrank

    MadFrank Supporting Member

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    How's this sound? I know a couple of guy's here have converted to this configuration, so I was wondering if they could describe the change, please?

    What are the numbers for the re-bias settings, and if you would reccomned it, any good valve suggestions?

    Everyone knows I love my 45, but I'm curious enough to try something new onece in a while.

    Cheers! :)
     
  2. big mike

    big mike Moderator - EL34 Emeritas Staff Member

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    I like it better than the valve art kt66 tubes
    I usually use the tung sol 6l6 gcstr for production tubes

    Bias setting would depend on your plate voltage and such
    But overall I prefer the more balanced tone and clarity.
     
  3. Red House

    Red House Silver Supporting Member

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    This.

    I would start with an idle-current of 70% of max tube dissipation, and go from there.
     
  4. MadFrank

    MadFrank Supporting Member

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    Thanks for the help guy's! :aok

    I think I'll give it a go. I've re-biased the tubes on my Redplate a few times, so I have just enough confidence to try it myself. I'll check with Greg, too.
     
  5. Sirloin

    Sirloin Supporting Member

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    I run TAD 6L6 WGC-STRs in my Germino Classic 45. That was the standard tube Greg shipped these amps with a few years back. I like them but that is all I have ever used in the amp so I can't compare to kt66s. I had some winged C 6l6s and really liked those but the TAD are a little cleaner/clearer to my ears and that's what I need.

    Check with Greg on what he suggests as well as bias settings.

    [​IMG]
     
  6. Stu Blue

    Stu Blue Member

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    I don't know why everybody on this forum quotes 70%... real JTM45s ran their tubes at 60% and 60% is what best for valves in AB1 if you want a decent life and tone. 70% is for the tone deaf.

    BTW and original JTM45 fixed non adjustable bias produces 60% figure for KT66 and 6l6 and 5881s... checked with a Compu-bias on mine... just stick 'em in and good to go..... don't know how accurate a clone the Germino is.. the 40 watts claim suggest not very (against the 26-34 watt original).
     
    Last edited: Mar 3, 2012
  7. big mike

    big mike Moderator - EL34 Emeritas Staff Member

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    Just depends onthe tubes
    Nos at 60% should sound great forever
    New stock sounds better near 70% IMO of course

    Nothings absolute


    Gregg's seems more a real world rating as the original was 34ish before breakup
    Rating watts has always been a joke. Which frequency, how long, before or after distortion.....
     
  8. hogy

    hogy Member

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    This ignores the fact that tubes of the same type fall within a wide spread of operating parameters, even back then. Your particular tubes may bias out at 60% dissipation, other tubes, possibly from the same batch, may not.

    The old JTM45s were fixed bias, and that mean one fixed bias resistor (56k if memory serves), with no way of adjusting bias voltage besides changing that resistor to a different value.

    You stick your vintage GEC KT66s in there and they will be biased at the 60% you mention, another set may exceed 100% and develop glow spots on the plates.

    The way to bias these amps is to substitute an adjustable resistor in there and set the bias where it sounds the best, taking care not to exceed a safe range.


    As for the original question, 6L6s sound great in early JTM45s. I'm talking about the ones with the RS Deluxe output transformer. In fact, supposedly Jim Marshall's favorite was the Tung Sol 5881, a 6L6 variant. The switch to KT66s is said to have happened when those American tubes became harder to obtain in the UK.
     
  9. michael patrick

    michael patrick Supporting Member

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    I prefer 6L6s in a JTM45. The KT66s have a nice midrange thing going on, but I prefer the chimier top end and beefier bottom of the 6L6s. I've got a pair of blackplate RCAs in mine right now, and I'm a happy camper...
     
  10. Red House

    Red House Silver Supporting Member

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    :facepalm:rotflmao
     
  11. Stu Blue

    Stu Blue Member

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    I've owned my JTM45 since 1971 and the biggest variation I've seen between 6l6s types/makes has been two or three tenths of a watt. (13.1-13.4 watts) and that includes RCA, JAN Phillips, and cheap chinese junk.... I sorry but I don't believe your claim about 60% to 100% variation. IMO there's a lot of neurotic nonsense talked about bias on the internet. I've certainly seen adjustable bias shift as a result of rough handling by roadies etc.... maybe that's where all that variation talk comes from... Mesa certainly haven't had much truck with adjustable bias... not that I like their amps, but they work OK.

    EDIT Techs all like adjustable bias, but since that brings them extra work they would wouldn't they?
     
    Last edited: Mar 3, 2012
  12. MadFrank

    MadFrank Supporting Member

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    Thanks for the great info, guys! Greg shipps the 45 with TAD 6L6WGC's, so I have a matched pair with Cryo treatment coming from Watford Valves. If I don't like them, they wont go to waste in my other amps.

    I have just realised that the 45 doesn't have bias test-points! Damn. That's the whole 'vintage accurate' thing for you I suppose! Lol! How do you measure bias without them? With the Redplate, the red prong goes into the red bias point, and the black prong touches the tube's clip retainer. What do I do with the red prong now?
     
  13. Sirloin

    Sirloin Supporting Member

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  14. dewman

    dewman Gold Supporting Member

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    i love EL34s in the classic 45
     
  15. michael.e

    michael.e Supporting Member

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    I have a Bletchley Stinger 33 that is a beautiful sounding JTM45 type amp. It is running 6l6's. Sounds very full and balanced. The cleans are a tad more sparkly than any of my other 45 style amps that I have had in the past that ran el34's. That said, the beautiful Marshally midrange is there. Killer drive tones and absolutely beautiful cleans. I like this setup most.
     
  16. DICKIE C

    DICKIE C Member

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    I'm currently running on old set of Groove Tubes 6l6GEs in my Classic 45. I know Groove Tubes gets crapped on alot here, I put them in as a result of a prior tube failure. It was only supposed to be temporary but they sound just fine.

    The differences I hear are as one would expect; the kt66 is creamier and the 6l6 is a bit brighter on the top. I don't hear a great deal of bottom-end difference if any at all. There is some difference in the mids but it's minimal. I like them.

    I find that I need to try different pre-amp tubes in V1. To me, swapping these tubes makes much more difference in the tone than the power tubes. Speakers also.

    My $.02 :)
     
  17. jgyn

    jgyn Member

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    I have Svetlana 6L6s in my Marshall JTM45 RI.
     
  18. Stu Blue

    Stu Blue Member

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    When you've got a vintage type amp it's worth remembering that everybody used pure nichel strings back then... with the JTM45 you get classier "clean" tones with them... plated steels are a little harsh and clangy with them.

    Actually 6L6s were not designed as audio valves. They were broad "function" power tubes with a kink in their response curve which adds brightness. The KT series are "Kinkless" audio tubes. The 6V6 was designed as an audio tube too, and, if guitarists were as actually as tonally cute as they like to think they are, we'd all be using 4x6V6 amps rather than 2x6L6. (The EL84 and EL34 were designed (by Mullard) as cheaper to make low vacuum audio tubes).

    The "bass circuit" in the JTM45 is fine with brighter valves both pre and power of course..... RCA if you got the cash, TAD 6L6WGC-STR if you ain't.

    (Jimi Hendrix had brighter caps put in his KT66 JTM45/100s...)
     
    Last edited: Mar 5, 2012
  19. MadFrank

    MadFrank Supporting Member

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    Just a quick update, as I though it best to ask Greg himself about re-biasing the 45 as the LoVo tranny might have different peramiters than other JTM45 types. Greg got back to me (always great service!) and has said that the LV 45 bias should be set at around 38Ma - 60% Plate dissipation (410vdc X .038 = 15.58 watts). The regular model needs 34Ma. I've got a Weber bias-rite coming as well, so I can do it properly. I hope!

    I'll update on how the change sounds when the tubes get here. Cheers!
     

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