New Marshall JVM410H eating up Tung Sol 12AX7s!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Ariel Pozzo, Apr 9, 2008.

  1. Ariel Pozzo

    Ariel Pozzo Member

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    I just got a new Marshall JVM410H (I bought it after reading many comments on this forum...bold move, but I love this amp!)
    The stock preamp tubes are all chinese, but surprisingly, the ones in V1, V2 and V5 are labeled 00067 and look shiny inside, while the ones on V3 and V4 appear to be your regular gray plate Shuguang and are marked 00054.
    I pulled them all out and installed new Tung Sol 12Ax7s. Immediately i noticed a much lower volume. After replacing the tubes one by one, I found V3 was the culprit. Put back the original and all was fine. So, I replaced that again with the Tung Sol that was in V1 and put a Mesa SPAX7 in that slot. Much to my surprise, after a while I noticed the volume drop again. went directly to V3, pulled the second Tung Sol and put the original back...all was fine again.
    I thought, man, two Tung Sols went busted at the same spot...so I tried again...and installed in V3 the other Tung sol I had in V2 (rembeber, this was working fine) and after a while...kaboom again.
    So...my new JVM ate 3 new Tung sols that were installed in V3...my question: Why V3 is eating up the tung sols? what's different here?
    I remember my VOX AC30CC2 had Tung Sols in V1 and V3, but used a normal chinese in V2 cause they said a more rugged tube was needed there...what's up with that?
     
  2. ibanez4life SZ!

    ibanez4life SZ! Member

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    Probably the high gain stage of the amp....odd that the Tung Sols can't take it though, as they are fairly high gain tubes.

    PERSONALLY, I'm not a fan of them, or their reliability though. The first one I had was bad, and then the replacement went bad not too long later.
     
  3. vibroverbus

    vibroverbus Member

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    ariel - did you get the Tungs from a dedicated tube-dealer or just a music shop? any real tube-dealer does burn-in testing to weed out the weak, music shops just open the box and hand them out. hard to say exactly what the deal is on that tube spot since this is a new amp and schematics aren't easy to come by on the web...
     
  4. Blue Strat

    Blue Strat Member

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    Gain has nothing to do with it. If that stage is a "cathode follower" that could be the problem.
     
  5. Ariel Pozzo

    Ariel Pozzo Member

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    Thanks for the replies guys.
    I remember Soldano once advised me not to use the Sovtek 12AX7LPS in all the preamp slots, only in V1 and V2, they said it wouldn't last anywhere else. I wonder if the JVM410H has the same problem. So far I know three amp manufacturers that wouldn't recommend certain 12AX7 tubes for all preamp positions: Vox, Soldano and now Marshall. What gives?
     
  6. Blue Strat

    Blue Strat Member

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    If it pertains to only one preamp socket, it's because that tube is used as a "cathode follower".

    There was a LONG thread on this on the Amps and Cabs forum a few days ago. The Sovtek LPS has this problem and, evidently, so does the Tung Sol Reissue. Both of these tubes are made by the same manufacturer.
     
  7. Ariel Pozzo

    Ariel Pozzo Member

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    V3 in the Marshall JVM410H is a Cathode Follower.
    You are correct, sir. Thanks.
    :)
     
  8. vibroverbus

    vibroverbus Member

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    it means instead of taking the output signal from the 'top' of the tube at the plate (where the high voltage comes in), the output is taken from the 'bottom' of the tube where the voltage comes out. in the other pre-amp stages high voltage generally doesn't get past the plate because of the load resistor before it. in a cathode follower high voltage is connected straight to the plate, and flows through the tube so the plate and cathode always float at high voltage. from Mike's good catch reference to the other thread I see this is a problem with some of these new production tubes that can't handle big voltage between the cathode and the heaters. makes sense.

    it also means the signal coming off has good current gain but no voltage gain (negative actually).

    it also means the signal coming off is not inverted in phase - it's the same as coming in. other "common cathode" stages are phase inverting.
     
  9. fiddler

    fiddler Guest

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    Which position / stage on a DRRI is the "cathode follower?" V4?
     
  10. Blue Strat

    Blue Strat Member

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    "Cathode follower" is a circuit topology only used in some amps.

    DRRIs and most Fenders don't use this. The one Fender that immediately comes to mind that does is the '59 Bassman.
     
  11. Apprentice

    Apprentice Member

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    i dont believe there is one. the first half is for reverb recovery and im not quite sure what the 2nd 1/2 is for, but im pretty sure its not a cathode follower.
     
  12. JMP2203

    JMP2203 Member

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    i have this same problem in the v3 position of an old engl 620 preamp, but im sure this preamp do not have cathode follower, i believe this position if for the effect loop buffer, its this similar to a cathode follower? i already kill 2 rft and 1 tunsgram 12ax7 :-(
     
  13. JamesHealey

    JamesHealey Member

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    Those tungsol tubes don't like cathode follower circuits it usually kills them instantly, but they're also failing in other high voltage/current positions of various amps.
     
  14. solitaire

    solitaire Senior Member

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    I'm having trouble with TS RIs in my Marshall 9100 in the phase inverter position. Scorching the flashgetter of them and smell like... something is overheating. And this is a position designed for AT7s originally and neither me or Myles Rose with whom I've dicussed this have any clue as to why this is.
     
  15. Eagle1

    Eagle1 Member

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    The new Tungsol is a really crappy tube (they don't last long in anything,)fit something else .I use either a JJ or an NOS Mullard in my JVM.
     
  16. hustist1

    hustist1 Member

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    Hi

    looks like this is either my first post, or the first in years, so hello, but anyway ...

    If the Tung-Sol is failing as a cathode follower, it is most likely because the heater-cathode voltage has exceeded specs. The cathode on a cathode follower could easily over 200 volts. This GE spec sheet, for example, specifies a max of 180V heater-cathode http://www.mif.pg.gda.pl/homepages/frank/sheets/093/1/12AX7.pdf

    Interestingly enough, the Reflektor-made tubes ("Tung-Sol," EH, Sovtek, ...) are actually correct, because they sometimes do indeed fail when the spec is exceeded. It is all those other tubes which do NOT fail which are "incorrect."

    This is why you generally do not use a Reflektor-made tube in a cathode follower position. Might work, might not.

    To avoid this you have to elevate the heaters, which you can read about on the Valve Wizard site if you are curious. Since most tubes don't fail, though, nobody bothers.

    Fender used a cathode follower in many of the larger late-era Tweed amps; including the Super, Pro, Bandmaster (all basically the same amp) as well as the Twin and Bassman.

    steven
     
  17. terminal

    terminal Member

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    perhaps the TS RI's don't meet the spec for max current. 12ax7's can only handle 10% of the current of a 12AT7, which is why the 12AT7 is a popular PI tube.

    also, in some cathode follower circuits where the grid is connected to the plate via a resistor, on startup, the grid voltage can rise to a higher voltage than the cathode. the cathode is still cold and the tube hasn't biased itself yet. most tubes can handle this for a few seconds of startup. a tube that is delicate, or barely meet specs might be damaged.

    It sounds like the easiest thing to do is "not to do it anymore!" :) buy JJ's.

    Its a good this thread came along, I have a TS RI 12AX7 in my VOX AC15TB, i just moved it to V2 (I think!) one of those is the TB tube with a cathode follower, I need to go make sure its not in that position!!!
     

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