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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 Friend of Leo's

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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 Friend of Leo's

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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 Poster Extraordinaire 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.
     
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  6. schmee

    schmee Doctor of Teleocity 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 Friend of Leo's

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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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  9. Jesco

    Jesco TDPRI Member

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    I finally finished this project.

    A Princeton Reverb using a power transformer from an old Hammond M Model Organ and an Allen TO-26 Output transformer. I popped a Celestion Gold in it, but not sure if that is going to stay (I plugged my 5E3 clone into the Gold and I think that combo is going to have to be permanent).

    IMG-3406.jpg IMG-3397.JPG 60809830580--5E678780-ACF6-4E53-8605-55C5384044F9.JPG
     
  10. Jesco

    Jesco TDPRI Member

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    I am having a few problems.

    1) The volume pot is weird. I don't get any volume until the pot is on about 2 and a half. I don't remember if the pot is linear or logarithmic. Could this be caused by a linear pot? I feel like I'm getting full volume at 10. I don't acheive tube breakup until it is on 7.

    2) I'm not sure I love the sound on this, which I attribute to the oversized PT that I was warned about. It sounds good, but not "crap my pants" amazing. Everything is working. The reverb is good, the tremolo is great. It just feels a tad thin. I have been A/Bing it with my 5E3 and it just doesn't seem to have the same amount of body to it. Maybe that is just because it is very clean. I haven't put any pedals in front of it yet. It sounded better with Tong-sol 6v6s in it but I replaced them with JJs because of the high voltages.
     
    Last edited: May 7, 2020
  11. Jesco

    Jesco TDPRI Member

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    Voltages in case anyone is interested:

    6.3 = 6.32
    5= 5.1

    B+1 452
    B+2 442
    B+3 290
    B+4 365

    V1
    p1 196vdc
    p2 0vdc
    p3 1.47vdc
    p6 194.7vdc
    p7 0vdc
    p8 1.41vdc

    V2
    p1 438vdc
    p2 0vdc
    p3 8vdc
    p6 438vdc
    p7 0vdc
    p8 8vdc

    V3
    p1 192.2vdc
    p2 0vdc
    p3 1.41vdc
    p6 193vdc
    p7 0vdc
    p8 1.52

    V4
    p1 270vdc
    p2 0vdc
    p3 2.5vdc
    p6 221vdc
    p7 23.3vdc
    p8 64.1vdc

    V5
    p3 447vdc
    p4 440vdc
    p5 -38.4vdc
    p8 0vdc
    Ia = 20.5ma (JJ 6v6)

    V6 447vdc
    p4 440vdc
    p5 -38.3vdc
    p8 0vdc
    Ia = 20.3ma (JJ 6v6)
     
  12. BigDaddy23

    BigDaddy23 Tele-Holic

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    Looks nice!

    On the body, have you tried giving the tone controls a real boost? That will add a bit more to the signal with this sort of tone stack. The 5E3 single tone knob is a really different beast. Also what rectifier are you using?
     
  13. Jesco

    Jesco TDPRI Member

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    I have messed with the tone controls, full bass, full treble (which is too much treble). I will need to really sit with it for a while to figure out exactly what is missing. It could just be my tiny workshop room. I have 2'x4' acoustic panels up but the amp needs to be up pretty loud and the room is very small (6x10).

    I have a NOS GE 5u4GB rectifier.
     
  14. BigDaddy23

    BigDaddy23 Tele-Holic

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    Goodo - I find mine over driving at about 4-5/10 on the dial but that doesn't mean much comparatively - Also found the tone controls made a big difference to the gain structure into the power amp hence my suggestion there. A more open space will definitely make it sound different!

    FWIW - My specs are very similar to yours voltage wise (inc bias current).......same rectifier too.
     
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