Allen Old Flame mods

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jzucker, Dec 29, 2005.


  1. jzucker

    jzucker Supporting Member

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    I recently picked up a 2004(ish) Old Flame head. It's the version with the bassman style output transformer and higher plate voltages so it's a true 40-50 watter.

    However, it sounds a bit flubby. I was wondering what I could do to sweeten it up. Phase inverter bypass cap? What else?

    I'm thinking maybe a Mercury Magnetics OT or will that make any audible diff?
     
  2. hasserl

    hasserl Member

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    Phase inverter bypass cap? What is that?

    If you want to tighten up the bottom end a little you might try reducing the value of the preamp gain stages cathode bypass caps. If they are the typicla Fenderish 25uf, try going with 4.7uf instead. You might also try smaller coupling cap sizes if the cathode bypass cap changes doesn't tighten it up enough.
     
  3. jzucker

    jzucker Supporting Member

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    That 4.7uf might be a little too bright. The bypass cap I'm talking about goes to the input of the phase inverter. Fender used .001 in the blackface amps and then switched to .01 in later amps which gave them quite a bit of flabbyness. I can't remember what's in the allen.

    I'm thinking more of phase inverter and tone stack values like changing the bass tone cap from .1 to .05.

    I guess these same mods apply to most fender amps as well since the allen is somewhat of a copy. I've heard of folks putting in a dropping resistor to simulate the loading of the trem circuit which is missing from the allen as well...
     
  4. Slick51

    Slick51 Colonel Curmudgeon Silver Supporting Member

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    That .001uf (or later, .01uf) cap is usually called the PI input cap or coupling cap. It isn't a bypass cap in the normal sense.

    I'd probably try a 10uf as the cathode bypass cap in the first gain stage, then listen to it for a while. If that didn't do it, I'd put the 25uf back in, then reduce the coupling cap from that stage to the next one by 30% or so, then listen to it for a while. I'd reverse the coupling cap and then do Hasserl's tone stack cap mod next, and listen. Next I'd put that cap back in, then reduce the PI input cap by 50%, then listen....

    Just take the time and do the changes one by one and really evaluate the results. A combo of these changes will fix it, but your ear has to be satisfied. What you pick up here will help you mod future amps!:cool:

    Tons of good info here:
    http://www.unclespot.com/AmpINFO.html

    just my ramblings; YMMV.

    Slick51
     
  5. jzucker

    jzucker Supporting Member

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    Thanks for all the tips! I'll give some of those a try. And sorry about calling the pi input cap a bypass cap. Duh...I should have known better.
     
  6. jzucker

    jzucker Supporting Member

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    While we're on the subject...Anyone ever taken this circuit and changed it to a cathode follower (ala marshall) but retaining the reverb?
     
  7. -CM-

    -CM- Something Clever Here Silver Supporting Member

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    Contact Dave Allen himself at his website. He's a heck of a nice guy, and I'm sure he knows exactly how to accomplish what you want.
     
  8. jzucker

    jzucker Supporting Member

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    I've talked to David many times. He's a great guy but he's not too interested in talking folks through making his amps sound different than his design. Can you blame him? :D
     
  9. Jim Collins

    Jim Collins Member

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    Really? He has helped me through several mods to all three of my Allen amps. Most of my complaints had to do with too much low end being passed through -- typical blackface Fender sound. I ended up changing several caps in the amps, some of which had a more significant effect than the others.

    One change that helped a lot was altering the values of the caps that are in the input and output to the master volume. You'll see two, .1uF caps that input to the master volume, and two, .1uF caps that come out. I changed each of these to .022uF caps. This was David's suggestion.

    Another change I made that had a significant effect was in the tone stack. The standard slope resistor is 100K. I changed this to 56K. There is a .1uF cap and a .047uF in the tone stack. I changed each of these to .022uF. That made the tone controls more workable. I got this change from Dave Funk's Tube Amp Workbook.

    Another change I made to my 40 TONESavor was to convert it to accept either 6L6 or EL34 power tubes, with only a bias change. I run EL34 tubes in that amp, and it tightened up the bottom end.
     
  10. jzucker

    jzucker Supporting Member

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    All those mods are pretty standard fender amp mods that you see on the tube amp boards. Thanks for reminding me and listing them all in one place though!

    Regarding the tubes, I've rewired my tube sockets to accept EL34s before but you really need a different secondary OT impedance to properly match up with EL34s. Additionally, the 34s draw more current which (depending on the PT rating) could cause the PT to overheat.

    I was thinking of possibly switching to a MM OT designed for a Marshall 50 and running EL34s...Not sure about that though as I have about 10 matched pairs of 6L6 tubes and only a couple of EL34s...
     
  11. Jim Collins

    Jim Collins Member

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    Another difference is the fact that my Old Flame and TONESavor each has the smaller OT. I built the 40 watt TONESavor (2x12) first, with the larger, Bassman-sized OT, but I decided I didn't care for that much iron. It was just too clean and too loud. I preferred the smaller OT, which is what I also have in my Old Flame (2x10).
     
  12. jzucker

    jzucker Supporting Member

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    Thanks for all your help and advice Jim. I thought I wanted the extra iron but maybe if I swap to a different OT I'll go with the smaller sized one.

    One significant thing: I switched my 2x12 cab's speakers out to a pair of Eminence Legend 12s and the amp sounds significantly better. Still a bit flubby but I think the mods you mention should take care of that.

    Thanks again!!!!
     

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