Blackface 6L6 to EL34 mod conversion--need a resistor value

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Angle Loss, Aug 2, 2008.

  1. Angle Loss

    Angle Loss Supporting Member

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    I am in the middle of modding a Fender Bandmaster to accept EL34's. It is a '71 Bandmaster Reverb that has already been black faced. I have taken the mod from Ken Fischer (Trainwreck) referenced in Aspen Pittman's Tube Amp Book. I've completed the whole mod except one resistor and the biasing of course.

    My problem is that the article says, "Increase the value of the resistor in series with the bias diode to reduce the bias voltage to obtain proper bias. This will vary from amp to amp..."


    So, does anyone have an acceptable value for a black face Bandmaster/Super Reverb, etc. that would save me much time from guessing?
     
  2. trey85stang

    trey85stang Member

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    what value is in there now? if the bias voltage taken off the hi voltage tap, grab a few 2-3w resistors from 300k - 500k or so, you shold also be able to adjust the resistor after the bias pot as well to fine tune if you get a close but not close enough match from the 330k-500k.
     
  3. Angle Loss

    Angle Loss Supporting Member

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    From what I understand (pause to insert dancing banana to emphasize the seriousness of that statement...:banana ), there is a wire coming up from the transformer which goes to a little board right behind the logo. That board only has a cap (not in series), resistor and diode before heading to the bias pot. It sounds like it is referring to that resistor (1K). If there is a value that has worked for someone else, that might put me on a faster track. Thanks!
     
    Last edited: Aug 3, 2008
  4. mark norwine

    mark norwine Member

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    Man, you're gonna make that PT run HOTHOTHOT.

    I've replaced quite a few BF PT's where the owner re-configured for 34's.

    Some will say "so what...it will be fine". I've fixed enough amps to say "I wouldn't do this".

    As for the resistor, do NOT mess with the current-limiting resistor. If you aren't getting enough (or proper) swing range, change the "tail" resistor on the bias pot. Lower value if you want to run hotter, higher value to run cooler

    Have fun.
     
  5. waxnsteel

    waxnsteel Member

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    If you're trying to get a more "Marshall" like sound out of your amp, consider changing speakers/cabinet to something more like the sounds you're looking for. Just changing output tubes won't do that much if done properly.
     
  6. Angle Loss

    Angle Loss Supporting Member

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    Thanks guys, all of you. I'm not looking for a Marshall sound, I just love the space between Fender and Marshall. I'm already using Celestions in my cabs for this amp. My favorite tube is the 6V6, which is already in my original '65 Princeton and TopHat Club Deluxe. I like EL34s much better than 6L6/5881 and wanted the slight change that it brings along with a few other mods I'm doing.

    I noticed that you advise against it, Mark. I generally plan to keep the amp under 4-5 (usually around 3) on the volume knob. I usually use pedals for drive. Would this still be a dangerous situation at these levels? The reading I did showed most reports of no problems, however, I do want to be informed of risk here. Any advice on this matter would be appreciated. If I happened to blow a PT, would it cause any other damage to the amp?

    Please keep the knowledge coming, many thanks!
     
  7. mark norwine

    mark norwine Member

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    Levels & pedals has nothing to do with it. The issue is with the filament draw.....900mA for a 6L6 vs. 1.5A for an EL34. Times 2 tubes, and you're overloading your PT by 1.2Amps. Not good.

    Readings? What readings? Did you measure your filament current? It's "up"...believe me.

    My adivice: "I wouldn't run 34's in a BMR"

    How much more serious can you get than a blown-up power transformer? I mean...seriously! Cook the power transformer & you're concerned about a handful of fifty-cent components that might go along for the ride? I think smoking the PT is about as grim a failure as you can have....
     
  8. Angle Loss

    Angle Loss Supporting Member

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    Thanks for the explanation Mark. By readings I meant forum readings and mainly the books I was using--which I've assumed that Ken Fischer and Aspen Pittman knew their stuff. Perhaps some of the other BF/el34 people had replaced their PT's as well. :hide

    I build pedals, but that stuff is easy compared to amplifiers. I'm not in any way an electronic engineer, just a guitar player with basic electronic theory. I guess I'll be sticking to the 5881s in there. Thanks for your time.
     
  9. WaltC

    WaltC Member

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    slap a pair of the new TungSol re-issue 5881's (or nos ones if you have the money <G>) and bias them up right and see what you think. Not EL-34s by any stretch, I feel like it's a pretty good compromise between 6L6 and EL-34s.

    And what Mark says is gospel. The PT might be fine for a while (six weeks, six years, who knows) but when it goes it goes inside the tranny and you then replace it or get it re-wound (lots of $$$). Not worth it IMHO either.
     
  10. mark norwine

    mark norwine Member

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    If 34's are your heart's desire, there is an option:

    Weber makes a PT that's a very nice unit....it's a drop-in replacement for the BMR but the filament winding has been beefed up a bit and can support the use of 34's without cracking a fever....

    Just a thought,

    mn

    EDIT: The Weber in question is W022768. It's a fine piece of iron; I've used them myself to do this mod. The filament winding on the Weber can handle up to 6 Amps (you need 4.95 Amps) compared to the stock transformer, which is rated at 3.75 Amps.

    It does have a few extra wires you won't be using, so you'll need to deal with those. If you don't know how, this is a job best left to a pro....but, as I said, if you really want 34's (I love 34's!!), this is a viable option.
     
    Last edited: Aug 4, 2008
  11. doctorx

    doctorx Member

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    I rewired my Bassman heads years ago to accept EL34's. Yes there was a difference but not that much. I rewired them back to 6L6's.

    If you are not diming the amp I doubt if you will hear much difference at all in your Bandmaster. The 5881 suggestion is a good idea. Plus there is the heater current issue, although I had no problems with my Bassmans.

    I would take anything Aspen Pittman says with a grain of salt.
     
  12. jay42

    jay42 Member

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    The OP can remove V1, V3, and V5 to see if he likes the sound. Have to use the trem channel....or remove V2 instead of V1 and use Normal. If he doesn't like the sound, no harm done, no extra expense required, stick with 6L6's.
     
  13. jbltwin1

    jbltwin1 Member

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    EL34's are cool in those amps but the current draw problems HAVE to be addressed. The other way around is to provide a little transformer JUST for the heaters. Then you can bias and go. Just make sure that it will handle 5 or so amps by itself and you are good to go.
     
  14. Angle Loss

    Angle Loss Supporting Member

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    Thanks again for the help. I guess for now I will try to tweak it with the 5881s in there. It is sounding pretty good, but I think I can do even better. I plan to leave the vibrato channel alone and work on the normal. I'm putting the negative feedback on the ground switch, and am going to add a presence control through the ext. speaker jack. I think the normal channel will get the tonestack changed to earlier Fender values. If anybody has an interesting mod, I'd like to hear about it.
     
  15. doctorx

    doctorx Member

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  16. Angle Loss

    Angle Loss Supporting Member

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    Thanks Doc, I'll probably do that.
     

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