Converting a Marshall 9100 for EL-34s

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by solitaire, Sep 1, 2008.

  1. solitaire

    solitaire Senior Member

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    I own a Marshall 9100 made for russian short bottle 6L6GCs labeled 5881s at 2x50W. I know this amp came in one version for the above tetrodes and for the ever so much more popular EL-34 pentodes.
    In terms of circuitry of the amp most things look the same, comparing a schematic that appears to be for the EL-34 version.
    Any idea if there's anything that would need changing/ replacing going from these tetrodes to pentodes except the output impedance from what you can tell from the schematics in the link, please fill me in on it!

    http://www.drtube.com/marshall.htm#M9000
     
  2. rockon1

    rockon1 Member

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    I know EL34's draw more heater current than 6L6's so I would imagine the first thing I'd want to find out is if the OT is up to the task. Bob
     
  3. SatelliteAmps

    SatelliteAmps Member

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    For the record, 5881's are not 6L6GC's with different labels. 5881's compare to 6L6GB's which are a bit different in their requirements.

    When converting between the 5881 and the EL34 in any amp is pretty straightforward. First is to find out if the power transformer is up for the task. The next is to make sure the socket is wired correctly. Then a few adjustments are usually needed in the bias and screen supplies. That's it.

    Unfortunately, on the website you linked to, it says that EL34s will not fit in that amp.

    "Beware when replacing the original Sovtek 5881 tubes with EL34 tubes as these amps are only 3 rack units high. The Svetlana EL34's and JJ EL34's don't fit, as they are too high! The Svetlana 6L6GC's do fit, but the JJ 6L6GC's don't."
     
  4. solitaire

    solitaire Senior Member

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    Thanx for your reply, mate!
     
  5. solitaire

    solitaire Senior Member

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    Thanx, mate! I suppose you mean the PT. This being a dual monobloc amp that probably features 100W PTs intended for the EL-34, KT-66 and other european valves in mind I'm rather sure it can handle the heater current required.
     
  6. SatelliteAmps

    SatelliteAmps Member

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    You shouldn't need to worry about changing output impedances when changing these tubes. It was intended for either 5881's or El34's. So it should be the same as any EL34 to 5881 conversion like any other Marshall.
     
  7. rockon1

    rockon1 Member

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    Well with my very limited knowledge if the tranny can handle it then its would just be a matter of the proper bias resistor-unless the bias pot has enough range already. FWIW-Ive used 6L6's and 6V6's in my EL34 based Rivera Chubster. The bias pot even had enough range than the only thing I needed to do was adjust the bias. I seem to recall the possibility of a 6L6 amp having the sockets wired slightly differently? Bob
     
  8. SatelliteAmps

    SatelliteAmps Member

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    The sockets MIGHT be wired differently. Check to see if pin 1 and 8 are both tied together. What are you screen resistor values? Probably need to be bumped up a bit. And lastly, the bias supply will need to be adjusted. Do you know is in the amp as a bias resistor?
     
  9. WILLIEKENDRICK

    WILLIEKENDRICK Supporting Member

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    Sol,
    Man, I'm NOT trying to rain on your parade, but I had a 9100 several years back. It 'could' slice your head off!! You sure you want to convert it to EL 34's??
    Remember this is the main amp used by both guitarist for Judas Priest for a good reason... :)
     
  10. solitaire

    solitaire Senior Member

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    Pin 1 and 8 are definately tied together. Screen resistors are 5.6k ohm, 1/4Ws.

    The bias is controlled by a trim-potentiometer I have localised. There's also a balance pot for the splitter/ divider I know the function of but don't know how to operate.

    Cheers!
     
  11. SatelliteAmps

    SatelliteAmps Member

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    THere should be a resistor in parallel with the bias pot. That might need to be changed so you can get enough of a bias adjustment to be correct. Other than that, the tubes should go in fine.
     
  12. solitaire

    solitaire Senior Member

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    I'll check that out whether or not my machine is to spec with the schematic refered to above regarding that resistor. Hang on.
     
  13. solitaire

    solitaire Senior Member

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  14. solitaire

    solitaire Senior Member

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    Oh, it's allright! You're saying Judas Priest had the 5881 version? That would probably mean they had 5881s in their '70s heads too.

    It's not that I don't like 6L6 type valves - I do, it's just that for a long time I've been trying to work something out using my much cared for Mesa/ Boogie Fifty/Fifty with various 6L6 type valves without finding a tone suitable for rock. The Fifty/Fifty not being able to run european type valves as their heater current would strain the PT the only solution would be finding an amp that would.

    The answer presented itself in this 9100 I bought used (for half that a used Marshall 20-20 sells for, to give you some proportion). I would guess it being the 5881 model was the main price-setter here. Apart from the mounting brackets this 9100 was in very fine condition, indeed and that would not motivate its price.

    This amp model was originally produced with 5881s but later came with either those or the EL34s currently featured in the very similar EL34 50-50 made today. That's to say the conversion shouldn't be too drastic and really isn't all that blasphemic in any way. It's a bit like the JTM45 being made with either the KT66 or the EL34, all a matter of taste and none is more right than the other.
     
  15. solitaire

    solitaire Senior Member

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    Got schematics from the Scandinavian distributor for the 9100 (same as those available at www.drtube.com) and the current EL34 5050. So far the only difference I've spotted is the 5050 has a different PT.
     

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