A Question For Marshall Jcm 800 Combo Owners?

Discussion in 'The Workbench' started by MarshallDog, Feb 11, 2019.

  1. MarshallDog

    MarshallDog Well-Known Member

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    Just wondering if most people (or any) have converted theirs over to EL34's? Why did you do it and what was the resulting tone like? I have the 6550's in mine and love it but am wondering what the difference is in tone. I've listened to some Youtube videos but you really cant tell much if any difference based on a video or at least I can't. When/if you converted it did you simply piggy tail a resistor to the stock one in line with the bias pot or did you change other resistors?

    Thanks,

    MD
     
  2. pedecamp

    pedecamp Well-Known Member

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    Whats in your other 2204's?

    I converted my 2204 to EL34 shortly after I got it cuz I had a collection of EL34's I wanted to use in it plus 6550's are almost twice the price if I wanted to roll tubes. I don't really remember what the 6550's sounded like at this point.
     
  3. MarshallDog

    MarshallDog Well-Known Member

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    I have EL34s in my JMPs BUT the JMPs tone is different from the JCMs even with the same tubes in my experience. Iā€™m guess the main difference would be less head room at higher levels and a touch more compression with EL34s?
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    If you push the power amp into distortion, the EL34s probably sound better.
    But if you keep the level down without overdriving the PA, the 6550s are not a lot different.
    One bias power supply resistor is changed for less negative voltage, the 2 bias split resistors are changed to 220K when changing to EL34.
    6550 is about -52 and EL34 is more in the range of -42 bias voltage, but depending on which tubes because there is a very large variation in the tubes you receive.
    Not all tubes will bias the same, but that goes for all amps all tubes in general. (so be prepared to change the bias power supply resistor to correct that)
    The bias split resistors set the sensitivity, 150K makes it less sensitive for 6550 and 220K makes it more sensitive for EL34s.
     
  5. MarshallDog

    MarshallDog Well-Known Member

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    Good info thanks!

    Why do some people only change the bias supply voltage and not the bias split resistors, why does it work just changing the bias supply resistor?

    Ive read many tech notes where some say its fine to just change the bias supply resistor and others say to change all of them?
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    If you keep the stock 150K bias split resistors it will still work OK. Just not as much overdrive to the power amp.
    220K will help push the EL34s a little harder and then it will saturate more.
     
  7. MarshallDog

    MarshallDog Well-Known Member

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    Thanks AMS, that explains it very cleary!!
     

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