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1969 Fender Bandmaster Reverb Output Transformer Question.

Discussion in 'Shock Brother's DIY Amps' started by Timbertea, Sep 16, 2010.

  1. Sep 16, 2010 #1
    Timbertea

    Timbertea Tele-Holic

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    I have a 1969 Fender Bandmaster Reverb that has been converted into a small combo; obviously have no concerns with keeping this vintage. While its a pretty pleasant amp (other than you can hear the optical sensor for the tremolo) in most respects, I want it to break up later, have better tone when it does, and just that little bit more oomph.

    I noticed the OT was kind of small for the wattage this is pushing, so I figure that is a good place to start for a mod. I have no idea of the primary impedance, just that the output impedance is 4ohms.

    There are tons of OT's out there. I've heard of people putting in an OT from a bassman to get more headroom, and better breakup. I see tons of replacements offered from Mercury Magnets, Heyboer, Mojo (do they build their own?), and others, but I'm wondering which output transformer is going to give me the results I want? Up until I hit the $300 mark the cost isn't an issue, which leaves a lot of room to play.
     
  2. Sep 16, 2010 #2
    Wally

    Wally Telefied Ad Free Member

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    Using the BAssman OT has been done for decades to increase the wattage, punch and headroom in these amps. IF that is what you are looking for, go for it. Imo, one would want to have an impedance match with the speaker/speakers..whether 4 or 8 ohms---I don't know what speaker arrangement you are using. IF I were in your position and didn't have a restriction on the budget, I would go with the MM tranny.
    I personally like what the 'smaller' OT's in Bandmaster, Pros, Pro REverbs, Vibroverbs, Vibrolux REverbs do. I like the saturation when pushed. This, of course, is subjective....and the speaker has a lot to do with the final results. Lot of variables to deal with.....
    Have fun with it.
     
  3. Sep 17, 2010 #3
    Timbertea

    Timbertea Tele-Holic

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    If it broke up the way my old vibrolux did I would be thrilled, but its a little too muddy at the volumes I need it at out. Its a ton of fun in the house because of that sound though. That is why I am thinking a better OT will help the issue, I just need to gain 2-3 notches of workable mostly clean on the volume knob.
     
  4. Sep 17, 2010 #4
    Wally

    Wally Telefied Ad Free Member

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    If I had the pleasant past experience with a simular amp...and the Bandmaster REverb and the Vibrolux REverb are very simular, I would start considering tubes and biasing before I changed out the OT....just a thought. IF I had that amp and watned the most headroom out of it with modern production tubes, I would go with the Winged C 6L6's and bias them to the cooler side of things.
     
  5. Sep 20, 2010 #5
    Timbertea

    Timbertea Tele-Holic

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    The best I've been able to do with tubes is GE7581a's in the power section. I tried the Winged-C's already, that was what I had in before the 7581's. The 7581a's are a big improvement over the Winged-C's in terms of headroom, and tighter bass. If I roll off the pots a bit, it gets me close, but the breakup still isn't quite what I want.

    I haven't tried different pre tubes yet, but if its ends up being I'm stuck using 7581a's in it, those get pricey to replace. I'm not too blessed at the moment with a mess of pre-tubes to roll. The OT starts looking better as a solution if I can go back to slightly cheaper tubes.

    I may take it up to the speaker store where they have a wall of speakers, and see if I can do better there too. That is also easier than wiring in an OT. I think I'll still get back to this point though.
     
  6. Sep 21, 2010 #6
    Wally

    Wally Telefied Ad Free Member

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    And...the questions are.....has that amp been modded to be a true bias circuit? Are you rebiasing tubes and/or trying different bias points/plate dissipation to see what each tube is doing and can do? Withotu that ability to bias cooler or hotter and taking the time to figure out the dissipation factor, one is kind of shooting in the dark and hoping to find the set of tubes that yields what one wants. Even the next set of 7581's may well operate differently than do these that you are liking right now.
    I agree...it has been a while since I wanted to afford a regular expense of 7581 replacements.
     
  7. Sep 21, 2010 #7
    tubeswell

    tubeswell Friend of Leo's

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    Re: the Primary Z, those Fender 2 x 6L6 amps are all usually 4k or 4k2, but anything from 2k5 to 6k6 will work, with anything between 3k2 and 5k being 'optimal'
     
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