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  • Switching power tube type

    When you switch the power tube type of a fixed bias amp, what modifications do you have to do other than switching to adjustable bias and setting the bias? Is that all?
    formerly n00b

  • #2
    Re: Switching power tube type

    you need to be a little more specific - from what to what, in which amp?

    the change in power tubes alone doesn't require a change in bias method, but you do need to consider the wiring of the socket. for instance, a common switch is from 6L6 to EL-34, and that requires rewiring the sockets, since the pin assignments are different for those tubes. It may also require adding a resistor if the current draw is different.

    however, this shouldn't be a difficult job for a good tech
    "music heals"
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    • #3
      Re: Switching power tube type

      I'm thinking of switching from 6L6 to 6CA7 in a Peavey Classic 60/60 stereo power amp.
      formerly n00b

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      • #4
        Re: Switching power tube type

        well, the pinout for a 6CA7 is the same as an EL-34

        sorry, but it's best to consult a tech for this one
        "music heals"
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        • #5
          Re: Switching power tube type

          Curly is completely right, converting 6L6 to EL34/6CA7/KT77 is fairly simple for a tech or someone experienced with tube amp maintenance, I've done it back and forth a number of times. I too would suggest going to a tech if you're not very comfortable working on tube amps, Ive seen too many Fenders (some vintage pieces too) seriously hurting because someone either just popped in EL-34s or messed up the conversion.

          The problem (bias notwithstanding) is that on 6L6-style tubes pin 1 is unused and wired as a terminal, while EL34-style tubes' suppressor grid is pin 1. People who just swap tubes (or make a mistake re-wiring) now have the signal and bias (terminal on 6L6) connected to the suppressor grid (should run to ground) and therefore fry their tubes and sometimes more very quickly. If you still want to do it yourself, I can suggest some books...

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          • #6
            Re: Switching power tube type

            Yet another issue with going from 6L6 to EL34-ish tubes is heater current. As I recall, EL34s take twice as much as 6L6s so there could be some issues if your heater circuit can't provide enough juice.
            Originally posted by LesStrat
            make sure that you own the gear, not vice versa.
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            • #7
              Re: Switching power tube type

              Thanks, those were what I was looking for. Dan from Torres said all I'd have to change is the bias so I'll have to ask about that....
              formerly n00b

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