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Thread: JTM30 bias and other tubes?

  1. #1
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    JTM30 bias and other tubes?

    I just acquired a JTM30 in, like it seems, the original condition. Unmodded and with original tubes. (5881)
    It works and sounds good to my ears. I have not yet done any measuring inside.
    However, I have two questions:

    1 – I may stick with the 5881’s (perhaps throw in a new pair), but what I read here is that the bias (-39.1V) is fairly “hot”. I might add another 4 to 8 V zener to set it a little cooler. What is the approximate current through that zener-stack? (I have some 400 mW zeners at hand)

    2 – I have a few NOS Philips EL34. Is it possible to put these in, and if yes, what do I need to do (set bias? Other?).

    When I decided what tubes I use I will most likely keep it that way, so I think I do not really need a bias-pot.
    TIA!

    Schematics: Marshall Schema's

  2. #2
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    I know that eventually you will be asked to measure the current through the tube eventually, so if you know how... and what aschematic shows and what is true may be quite different. The bias voltages are relatively meaningless without knowing the current, plate voltages, etc, so we'd just be guessing. And given the (in)consistency of current production tubes... Don't worry about what others say is too hot or too cold. If the amp works and sounds good (or better) AND doesn't self-destruct, it's fine.

    That said, I'm assuming those are Sovtek 5881WXTs in the amp. If you replace them with the exact same kind (Sovtek 5881WXT), there's a chance you'll never have to adjust the bias again. Those things are damn-near close to "the indestructible tube" as we ever got. Don't confuse them with
    NOS 5881s or the new Tung-Sol reissues or the 5881/6L6WXT. Check the innards of the tubes, and if they match, you're set.

    As far as EL34s go, I'd take every precaution I could before plugging THOSE in... which to me would be a bias pot AND making sure my power transformer won't burn up from too much heater current draw... also may require some rewiring of the tube socket, grid and screen resistor values, etc.

    Justin
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  3. #3
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    Also: seems you have two identical threads in the same forum? Just grab one and stick with it. I recommend the other one, because Enzo is answering that one...

    Justin
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    " I often feel the need to qualify my lack of tech AND experience equally and avoid any notional considerations so I don't come off like some ignorant half baked amp guru type asshole." - Chuck H -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    Quote Originally Posted by Justin Thomas View Post
    Also: seems you have two identical threads in the same forum? Just grab one and stick with it. I recommend the other one, because Enzo is answering that one...

    Justin
    I know, my mistake. Sorry. The mods can lock one of them.

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    Thx, specifically for the EL34 warning.
    My 5881's I have at home are indeed those "indestructable" Sovteks.

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    OK, I did some measurements in the JTM30, with "known good" Sovtek 5881:

    R between CT and A1, resp. A2: 76.9 and 78.2 Ohm
    V anode-cathode: 420 V
    V between CT and A1, resp. A2: 3.18 and 3.14 V

    Yielding:
    anode current resp. 41.4 and 40.2 mA
    bias power resp. 17.4 and 16.9 W

    Which is approx. 56% of Pmax (30W).
    Vbias at the zeners was -40,4 V

    The amp sounds great (IMHO) and does not cut out or so. Inside it looks like new and it is unmodded.
    The only thing is dat at very low volume (master at 1), there's a trace of distortion audible. Kind of cross-over-like sound.
    At higher volumes (bedroom-volume and up) it's not noticeable anymore.
    Perhaps this is normal for a JTM30? (which you do not buy for sparkling cleans anyway...)

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    Update: the "distortion" I heard was due to a speaker that made a mechanical sound. Not coil-rubbing in the magnet but something else, perhaps the wires.
    I replaced it with a matching Eminence Legend speaker and the amp sounds awesome to me; gigged with it already and worked flawlessly.
    I'm a happy camper ;-)
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    I'm late to the party, but I can tell you from experience the power transformer in the JTM30 will not accommodate the increased heater current requirements of an EL34. We have used 6L6GC (conservatively biased with bias pot mod) successfully with two different JTM30s, and they both have been running for several years that way. But I think 6L6GC would be the absolute limit of that PT.

    I have searched in vain for a drop in upgraded PT for the JTM30 and never found one. At some point, there are only some many "upgrades" that make sense with an amp at this price point.
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    Quote Originally Posted by Tone Meister View Post
    I'm late to the party, but I can tell you from experience the power transformer in the JTM30 will not accommodate the increased heater current requirements of an EL34.
    Thx. for the info. I did two rehearsals and one gig so far with the Sovtek 5881 biased at approx. 40 mA/17 W each. The tone is good for me, the idle dissipation is not too high and I plan to leave it that way.
    Maybe sometime in the future I'll mod the boost channel (a tad too fizzy for me).

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