Please help me make my Mesa 50/50 accept EL-34s

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by solitaire, Feb 7, 2006.

  1. solitaire

    solitaire Senior Member

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    I know it's possible, though Mesa is anything but helpfull on this quest of mine. I would like to make it switchable between 6L6 and EL-34 anyway so the currentdraw won't be that high, only running one channel at a time. I know it's doable.

    Cheers!
     
  2. solitaire

    solitaire Senior Member

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    Just adding notification.
     
  3. solitaire

    solitaire Senior Member

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  4. amoebas

    amoebas Guest


    sure? schem?
     
  5. solitaire

    solitaire Senior Member

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    Well I don't have those - not yet anyway. It's just that I saw this site of a guy called Mofinco who converted his into a Simul class amp by changing out a few bits and pieces. Also the newer 2:50 has bias switches to change the bias current for one type of valve/ tube to the other. Basically I just need to run half the amp at a time, so I won't overload the tranny.
     
  6. John Phillips

    John Phillips Member

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    I posted the answer to this about a week ago... unless someone has deleted the thread.
     
  7. TubeAmpNut

    TubeAmpNut Member

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    I personally would not do this with any mesa amp not designed to accept EL34's. There is simply not enough heater current available in the PT. You would be foolish indeed to switch to EL34's in the Simu-class as well. Just because some one did it and says it sounds cool doesn't mean it will last (a classic example of someone who knows just enough to be dangerous).

    -You've been warned.

    But if you don't care about replacing the PT, all you need to do is replace the screen resistors to 1k-5W and make sure the supressor grid is tied to ground. Which means jumping pin 8 and pin 1 on the output tube sockets. You also need to find were they 'fix' the bias and replace the set resistor with one that will work for the tubes you have selected. You can insert a pot instead, but most of the time people do a crappy job and the pot is left flopping around waiting to break off...

    BK
     
  8. John Phillips

    John Phillips Member

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    I never said anything about the Simul-Class or said it sounds cool. However, MESA recommend replacing the 'Class A' pair of 6L6s with EL34s in the earlier Simul-Class amps which came with four 6L6s, so I would imagine that it's quite safe.

    I also find that last comment insulting given that I have been a professional amp tech for twenty years and as far as I know have never caused a single amp failure - quite the opposite, I'm always inclined to put reliability above tone.
     
  9. dazj

    dazj Member

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    I'm not sure about this particular amp, but would like to add that my simul class came with the 2 x EL 34's + 2 x 6L6's (in the middle) from new, I ran it with them no problem for about a year or so - maybe more for prolonged periods, then changed to GT # 8's 6L6's which are still in there now. Sounded better for what I prefer and again no problems at all. It's not the same amp though and this was designed for either.
     
  10. TubeAmpNut

    TubeAmpNut Member

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    I meant no disrespect or insult, I was merely giving my opinion (I wasn't specifically responding to your post, sorry for the mixup).

    My reference to simu-class was specific to the simu-class 2:90 amp (I mis-read an earlier post). I've read some posts in the past were some folks wanted to do this. In this particular amp, the 2:90, switching to EL34 is bad news. Also, Mesa switched to all 6L6's with the short lived 395 simulcass and continued with their 2:90. I recall (in a conversation with Mike Bendinelli at Mesa) some years ago the quality of EL34's in the early nineties motivated this switch (oddly the Mark series amps kept them in the outer sockets).

    BK
     
  11. solitaire

    solitaire Senior Member

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    Cheers guys! The more I poke around about it the more hopless it seems...:(
    Now suppose I'd split the heating current in two so infact 2 EL-34 valves share a current specified for 4 6L6, is that enough flapping to fly or should I just leave it and get a dedicated EL-34 amp?
     
  12. solitaire

    solitaire Senior Member

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    Sorry, that might be a different thread of which I must have forgotten about.
     
  13. amper

    amper Member

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    I still think you will be much better off trading in your Fifty/Fifty for a Stereo 2:Fifty, which is *designed* to accept either EL34 or 6L6 tubes, selectable via a single switch.

    BTW, it is *impossible* to convert a Fifty/Fifty into a Simul-Class amp. "Simul-Class" means one pair of tubes operating in Class A mode and one pair of tubes operating in Class AB mode. That means you would need four tubes per channel to run in Simul-Class.
     
  14. solitaire

    solitaire Senior Member

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    Well, Monfico did replace the 'tranny' with that of a 2:90 to build his Simul Class. He did however mention parts Mesa supplied him with.

    I hear the 50/50 is better built and have a... hmm... an eq that runs favourably with other pre-amps than the Rectifier.

    I could always put in some yellow jackets on the B-channel and get a dedicated EL-34 amp should I require one. I hear even Eric Johnson had his Plexi converted to accept 6L6s, so there's really no danger on the roof.
     

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