Princeton Reverb Build w/ Hammond Model M Transformers

Discussion in 'Shock Brother's DIY Amps' started by Jesco, Feb 15, 2020.

  1. Jesco

    Jesco TDPRI Member

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    Hello again, fine folks!

    I acquired the power transformer and output transformer from a stripped Hammond Model M organ. It is a PP 6V6 amp; originally, it came with a field coil speaker, which I will not use. After some research I decided to use the transformers to build a BF Princeton Reverb. The chassis (from Zack in Arkansas) and small parts (from Mojotone) have arrived, and I'm ready to start. I hope.

    I probably should have asked this earlier, but will there be any problem with this plan?

    Power Transformer: According to PRR on Hoffman Amplifiers Forum, "this particular amp [Model M Hammond] can be re-connected for 415V at 115mA. This suggests 6.6K or 8K loading and 25W-35W output". He seems to be a very knowledgable dude, from the million posts I've seem from him.

    Output transformer: I measured it, and it has a winding ratio of 37.5 to 1. If my calculations are correct, the reflected impedance would be 5,624 ohms with a 4 ohm speaker or 11,248 ohms with an 8 ohm speaker. That seems high for a PP 6V6. Anyone see any problems with 5,624 ohms using a 4 ohm speaker?

    Schematic included:
     

    Attached Files:

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  2. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    There is no "problem" with your plan per se. Hammonators have been built and enjoyed by guitarists for years now.

    To use the BF Princeton Reverb circuit is not a problem. The expectation for the Hammonator to sound, act, an respond like a BF Princeton Reverb when the PT and OT do not share the specs of the BFPR is unrealistic IMO.
    I have a Hammond carcass as well. I decided to stick with traditional deluxe/princeton character just like his suggestion of either the 6.6k or 8k OT.
     
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  3. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    IDK if it fits your budget or desire.
    OT best bang for buck (for new) that I have found is Mouser, hammond 1750pa. It is hammond's version of the marshall 18 watt. 8.4k/4,8,16. It has a larger footprint than similar fender.

    Hammond paints over everything. You'll have to scrape paint off the wires.

    I have no affiliation/connection with any of these companies.
     
  4. Jesco

    Jesco TDPRI Member

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    I am ready for it to not sound exactly like a BFPR, but I suppose I was hoping for something closer to BFDR. The original amp, I believe, was a 12 watt into an 8 ohm field coil speaker. If the amp indeed puts out more than 22 watts, I imagine there will be problems with the bias wiggle tremolo. Maybe the best solution is to get transformers for the BFPR and keep the Hammond transformers for a separated, higher wattage amp project. I’ll have to think on this unless I hear any other suggestions.
     
    Last edited: Feb 17, 2020
  5. Milspec

    Milspec Friend of Leo's Silver Supporter

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    I have seen your plan carried of a few times now by my local amp guy. He tends to wire them more like the Dr. Z Carmen amps than the Fender Princeton. They all end up sounding really good, so I think you will be happy with the results.

    Those old field coil speakers were actually good sounding Rola speakers in most cases. I certainly understand wanting to rid yourself of the field coil design, but I do like the sound of those speakers a lot.
     
  6. schmee

    schmee Poster Extraordinaire Silver Supporter

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    Larger transformers for a PR are not a bad thing at all. More headroom, quiter too, but keeps the PR flavor. Not sure about those trannys though...
     
  7. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    The PT would be fine for a BFDR. Just need the OT in the 6.6k range. The BFPR would be tough to capture with the extra power of the hammond.
     
  8. Jesco

    Jesco TDPRI Member

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    Thanks for all the replies!
    The speaker is gone, and, furthermore, I’d like to have more experience before I start messing with field coil speakers. I would love to hear one in a guitar amp though.

    Thanks for the suggestion on the Mouser/Hammond OT suggestion. I think I’m going to go with the Allen TO20, mainly because I plan to use a 12” speaker in a standard PR cab, and this transformer is designed to fit.
     
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