6550s In A ‘78 2204?

Discussion in 'Let's Talk Vintage' started by Taylor Ramey, Sep 15, 2018.

  1. Taylor Ramey

    Taylor Ramey New Member

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    i just picked up a 1978 JMP 2204 50 watt from a buddy of mine. Upon taking the back panel off, I noticed it’s got a set of 6550s in the power section.
    I know some older Marshalls shipped to the US with 6550s, but I always thought that was in the 80s. However, the tube diagram on one of the transformers shows EL34/6550
    Anyone care to chime in with some info?
     
  2. Taylor Ramey

    Taylor Ramey New Member

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  3. Taylor Ramey

    Taylor Ramey New Member

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  4. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Nope, it was in the mid 1970's to maybe early 1980's or something like that. Its looks to be set up for 6550.
     
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  5. Taylor Ramey

    Taylor Ramey New Member

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    Could it have been modded to 6550s or does it look stock?
    Forgive my ignorance, I’m a newbie when it comes to these kinds of things.
     
  6. danfrank

    danfrank Well-Known Member

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    Looks stock for 6550s. Look at the two 150K bias resistors. If it was configured for EL34s it would have 220K resistors instead.
    If it was made for the USA market it came with 6550s.
     
  7. soundboy57

    soundboy57 Well-Known Member

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    Marshall put 6550's in the US export amps from approx. '74 to '86. Glad to be corrected on the exact years.

    The were more robust than the available EL34 at the time, and they were having warranty issues.

    I have heard that the US distributor actually swapped them in, however, the solder joints
    on any that I have had looked factory.

    Back in the day, seems everyone did the bias resistor mod and put EL34 back in (I did), however these
    days the 6550's sound damn good. Louder/clearer, but a bit more sterile, IMHO.
    Especially compared to the original Mullard EL34 :)
     
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  8. ampmadscientist

    ampmadscientist Well-Known Member

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    This amp came stock with 6550s in USA, the 150K bias split resistors confirm that.
     
  9. gldtp99

    gldtp99 Active Member

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    Yes, a factory 6550 amp.

    It has the 120k Bias Supply resistor (vs the 220k in an EL34 amp)

    And the 47k Bias Range resistor in series with the Bias Trim pot (vs the 56k in the EL34 amp).

    And the previously mentioned 150k Bias Splitter resistors (vs the 220k in the EL34 amp).

    I had to restore my '81 JMP 2204 to the 6550 specs because someone had done the crude "piggyback" bias resistor mods to run EL34's.

    In doing so they heat damaged the surface of the PCB----- not bad enough to effect function but enough that it won't just clean off.

    I wanted to run a pair of Sylvania 6550's I had on hand:

    [​IMG]

    [​IMG]
     
  10. Geeze

    Geeze Well-Known Member

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    I have a '78 2203 that came with the original [maybe] GE 6550's. I liked the sound of them but two wouldn't bias anywhere near what I consider normal. Contemplated changing to '34s but stuck in a set of 6550 Ruby's instead - very happy with the sound into Greenbacks and Creambacks.

    Russ
     
  11. Ned B

    Ned B Member

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    6550s started appearing in US shipped Marshalls as early as 1973.
     
  12. solarburnDSL50

    solarburnDSL50 Well-Known Member

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    I love 6550's in a classic Marshall circuit.

    Congrats on a killer amp.
     
  13. Taylor Ramey

    Taylor Ramey New Member

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    Thanks for all the help guys...
    Does the fact that it’s all oriinal(minus the tubes) and not modded make it worth more?
     
  14. Taylor Ramey

    Taylor Ramey New Member

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    Also, I have the matching 4X12(straight) that came with it...
    Signed by Jim Marshall on the inside and loaded with G12M “Blackbacks” (T1221s).
    I’ve cleaned the tolex up on both and now the tolex looks good as new.
    The head only has a couple dings... no snags.
    The cab has a couple dings as well a snag on the back cover of the cab. The Marshall logo on the cab is cracked and the speaker cloth has a stain... is there a way to clean it up?
     
    Last edited: Sep 17, 2018 at 12:46 PM
  15. Taylor Ramey

    Taylor Ramey New Member

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    This is a black and white picture from my buddy’s Instagram... I’ll post a color one later... the stain is visible, it sits at the very bottom of the cab’s speaker cloth.
     
  16. Taylor Ramey

    Taylor Ramey New Member

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