Can a 70's bassman PT handle the heater current draw of EL34's?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by scottosan, Jan 17, 2006.


  1. scottosan

    scottosan Supporting Member

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    Can a 70's bassman 50 PT handle the heater current draw of EL34's? I can't seem to find any tranny specs for these fenders.


    Thanks,
    Scott
     
  2. Shea

    Shea Member

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    There's no guarantee, but I have a hunch it probably could. A friend and I both have run EL34s in Fenders with no problems. If you try it, you should check the heater voltage to make sure it doesn't drop too low under load, and periodically feel the power transformer to make sure it's not getting any warmer than usual.

    So, it's one of those things that I can't really recommend doing, but practically speaking, I think you'll probably get away with it.

    Shea
     
  3. Geetarpicker

    Geetarpicker Member

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    I've owned lots of blackface and silver face amps. Basically they all tend to run hot, even at idle. To me that is sign that the heater winds are already under plenty of load even with stock tubes. That said I'd install a small fan inside the bassman cab, aimed in such a way to help cool the corner near the PT. I bet if you keep the heat down you will be ok with EL34s. I put a fan in my '69 bassman head. The PT in that amp would get "sticky paint" hot, if it was left on long enough even with a modest bias of 35ma per tube. The fan made a huge difference. No need to cut up the back panel or box to do it. Maybe just a couple holes to mount some L brackets to support the fan. You might also try pulling one preamp tube if you don't need both channels to reduce the filament load.
     
  4. doctorx

    doctorx Member

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    I have two Silverface Bassmans, a 68 and a 69. I modded them to use EL34's and did so for over two years without any problems. I didn't have a 12AX7 in the bass channel, that may have helped.
     
  5. mr coffee

    mr coffee Gold Supporting Member

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    The UL/70 watt models.. I'd use good tubes and watch the bias on a earlier sf one.. I never really saw the need for el34's in a bassman (but I have rebuilt them for 6V6's..RAWK)
     
  6. billdurham

    billdurham Member

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    According to the RCA tube manual...6CA7s draw 1.5 amps of heater current. 6L6s draw .9 amp. Pretty big diff!! I'm not saying it won't work as the other guys have said that it does, but thats over half an amp more. Might want to put a fan on it!

    BD
     
  7. humbert

    humbert Member

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    Sorry to hi-jack a thread, guys, but how do you mod a silverface for EL34s? Is it just a matter of changing the bias of the tube sockets?

    Also, would it be possible to run a Dual Showman with 2 6L6 and 2 EL34?

    Thanks!
     
  8. doctorx

    doctorx Member

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    I got the mod out of one of the early Tube Amp books by Aspen Pittman. I'll try to dig it up and post it. You have to change a couple of connections on the power tube sockets. Then of course you would have to rebias the amp. If you have a silverface you should change the bias section from bias balance to adjustable bias (like in the blackface amps), that's an easy mod.
     
  9. darkbluemurder

    darkbluemurder Member

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    Basically you need to solder the grid stopper resistors (1k5 in Fenders) off of Pin 1 which is not used for 6L6/6V6. With EL 34s Pin 1 is the suppressor grid. This can either be wired to the cathode, Pin 8, i.e. to ground (the suppressor grid and cathode are internally connected in 6L6/6V6 tubes) or to the raw bias supply (this is often referred to as "Traynor mod" although Garnet used it as well. I believe it is described on the website of Randy Lizard Tone Lounge under the Marshall section). Finally you need to rebias.

    Indeed the EL 34 draw a lot more heater current so this may not be the most recommended mod for 6L6 Fenders and is definitely not recommended on 6V6 Fenders.
     
  10. Smitty

    Smitty Member

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    Yes you can mod it to EL34s and you don't have to put in a fan. That won't do anything for heater current. Also, the bias point won't effect the heater winding. Just don't exceed 38 ma and you'll be fine. An excellent mod.
     
  11. billdurham

    billdurham Member

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    Smitty....it was a joke..sorry

    BD
     

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