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Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

JMPrinceton Reverb

Discussion in 'Shock Brother's DIY Amps' started by Snfoilhat, Oct 18, 2017.

  1. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
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    311
    Apr 8, 2016
    Oakland, CA
    Me too me too! There is a raft of brown- and black-era Princeton reworkings on this forum lately and I'd like to share this idea that took shape in another builder's thread and is close enough now to materializing. Quite a few component values that I need to hear and measure before settling on.

    Roughly 15-20W (haven't decided on 8K:8R-15W or 6.6K:8R-22W primary OT, haven't decided on lower or higher B+).

    PR topology (but with PRII's LTP PI and SS rectification), JMP 2103/2104/2203/2204 (inspired) preamp with some fixed values, simple tweed Princeton controls (my favorite by far) a little later than usual to control the high frequencies coming from the wet and keep the gains manageable before signal enters the wet channel (I hope), cathode biased power section because I just don't care strongly about the acknowledged improvements fixed bias offers.

    Hoping for enough voicing options to be able to enjoy both moderate preamp and/or power tube distortion, a gritty clean with prominent mids (at at least some volume settings), and good sounding reverb in at least some settings, without resorting to multiple channels or numerous controls.

    No cathode-follower. It's not gonna be a mini Marshall. Will start life, at least, paired with an Jensen C10N from the 70s.

    Draft schematic and mockup of the faceplate attached. Putting parts (and spares, options) in a cart. Cheers!

    PR_PRII_20W_mashup_late_EQ_04.png
     
    Last edited: Oct 18, 2017

  2. Dacious

    Dacious Friend of Leo's

    Mar 16, 2003
    Godzone
    I don't know if I'd bother with master volume. It doesn't add anything in the context of a low wattage amp that's not a channel switcher. Rest sounds good.
     
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  3. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
    36
    311
    Apr 8, 2016
    Oakland, CA
    JMP_Princeton_Reverb_faceplates_alternate.png
    Less cluttered front panel. Cutting the rectangular holes for 'bright' style slide switches is going to suck, but it's what my heart is pulling for. Up/up should be fairly Fendery, down/down the most Marshall setting.
     
    Last edited: Oct 19, 2017

  4. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    I'd keep the master volume. Your fat cap might need to be larger. Going from .001uF to .002 might not be enough. I'd use a .02uF.
     
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  5. AlabamaOutlaw

    AlabamaOutlaw Tele-Meister

    437
    Feb 20, 2011
    Alabama
    I'm defenitly into that second faceplate. Cool build, you can do alot with these amps. I'm subbed and gasing for my own even more So now, I hope you're happy!
     

  6. Phrygian77

    Phrygian77 Tele-Holic

    649
    Apr 30, 2016
    Crawfordville, FL
    Looking at your schematic, I think there's a potential for too hot of a signal going into the reverb driver. I think I would add a dwell control, at least as an internal trim pot. You could also ditch the blackface style reverb circuit altogether and do something like this:

    http://el34world.com/Forum/index.php?topic=7957.0
     
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  7. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
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  8. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
    36
    311
    Apr 8, 2016
    Oakland, CA
    IMG_1622.JPG
    Second of what will be maybe two dozen light coats of shellac. Princeton Reverb-style chassis in the mail from a supplier in Arkansas that gets well-deserved love on this forum. This will be my third chassis from that shop.

    When it arrives I'll test fit my transformers--to save money on this build I'm going to try an old, unmarked PT from a 2x6V6 Magnavox radio I squirreled away and a 8K:8R PT from some Wollensak tape machine that ran 2x12AB5, not too different from 6V6s I think.

    Then drill/file/nibble new holes in the control panel for the slide switches, master volume, and power switch.

    After the metalwork is done, glue the face plates and do some light sanding to flatten the surface and satin/matte the finish.
     

  9. Phrygian77

    Phrygian77 Tele-Holic

    649
    Apr 30, 2016
    Crawfordville, FL
    How did you do those?
     

  10. printer2

    printer2 Poster Extraordinaire

    May 24, 2010
    Canada
    I like the idea of the reverb to go after the volume and tone controls so that you don't have to adjust the reverb any time you change the volume.
     

  11. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
    36
    311
    Apr 8, 2016
    Oakland, CA
    It's a full-size drawing in the program GIMP (Photoshop or Illustrator-ish, free) I have been tweaking for a few builds--correcting small errors in the locations of all the holes. Everything is separate layers so I just rename the controls each time. Export to PDF, print on the office supply place's wide printer ($4.44 today, it's always different...) on medium matte paper. Build up coats of shellac until it's 1-2mm thick. Probably not as durable as plexi or screened aluminum, but the price is right.
     

  12. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
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    Apr 8, 2016
    Oakland, CA
    Chassis arrived today. Getting some of the hardware laid out.

    PT/OT size mismatch! Still have to test this PT out. Scavenged from a 4X 12AB5 stereo power amp just this weekend, but it looks like it might be great for this job. Fully loaded it put out a B+ of around 300VDC.
     

    Attached Files:


  13. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
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    Apr 8, 2016
    Oakland, CA
    Mounted the transformers and the large electrolytics (actually old cans used as a placeholder till new parts arrive). Going with a pair of 2-section cans so that I can have better control over the grounding scheme than a single 4-section can. The can in the V7/rectifier tube spot is awfully close to this oversize PT. I pushed the mounts as far over as they would go to get some air space. I don't anticipate this PT running as hot as a stock Princeton Reverb runs, but will have to monitor things going forward.

    As anticipated, using a drill and needle files to cut the rectangular 'bright' switch openings in the chassis sucked, but worked. Mostly fun so far! Cheers
     

    Attached Files:

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  14. Phrygian77

    Phrygian77 Tele-Holic

    649
    Apr 30, 2016
    Crawfordville, FL
    I was going to suggest just drilling 1/2 holes, with the mounting screw holes above and below, depending on how close the other adjacent holes are.
     
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  15. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
    36
    311
    Apr 8, 2016
    Oakland, CA
    Progress. New electrolytic filter caps, pots, switches, jacks, tube sockets, and lamp. Reverb transformer. Going to try the dual-gang 250K-A master volume/grid leak combo.

    Some fiberboard and eyelets arrived, reminding me that i haven't done any kind of layout yet :oops:
    Onward!
     

    Attached Files:


  16. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
    36
    311
    Apr 8, 2016
    Oakland, CA
    IMG_1684.JPG
    Heaters wired. Filling in V5 because it is directly beneath the fuse holder, which I temporarily pulled. In the past I have taken 'build the power supply first' too literally, and had to maneuver around with the shaft of the soldering iron a danger to everything above the socket while working in that area. Control and screen grid stoppers are on a small terminal strip, and I'll attach a long lead for the cathode connection, then reinstall the fuse holder and establish the power supply.
     
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  17. Phrygian77

    Phrygian77 Tele-Holic

    649
    Apr 30, 2016
    Crawfordville, FL
    I'm wondering how your going to get the front Keps nut on for the chassis strap with that big iron in the way.
     

  18. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
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    Apr 8, 2016
    Oakland, CA
    None of my photos gives a good view, but I think the chassis strap hardware will just clear the PT's flange (foot?).

    My very first eyelet board:
    IMG_1688.JPG
    It's 3 inches wide, which I think is wider than Fender's boards used to be. These are what AES sells. The extra width will go to the Marshall-style strain relief holes for flying leads. The large window on the left is located at the Princeton Reverb's cap can, just big enough for the terminals to show, which may prove to be clever or stupid. We'll soon see!

    Edit: forgot to say, I finally tested the power transformer:rolleyes:

    Unloaded it's measuring -290 - 0 - 290 VAC on the HT and -7 - 0 - 7 VAC on the heaters.

    So I think I'm good for a B+ in the lower 300s VDC and 12.6VAC heaters appropriate for these tubes. My pilot light is only 6.3V, same as any Fender pilot light run off the heaters, so I have to utilize only half the voltage. I think I can place the lamp between one leg of the heaters and the heater center tap for half of 12.6V, but am not sure. I believe some of the heaters in the donor power amp were wired in this fashion.
     
    Last edited: Dec 3, 2017

  19. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
    36
    311
    Apr 8, 2016
    Oakland, CA
    IMG_1691.JPG
    Component board and backing board test fitted. Lamp works properly on half the heater voltage supply. Gotta populate that board. Cheers
     
    Last edited: Dec 3, 2017

  20. Snfoilhat

    Snfoilhat Tele-Meister

    Age:
    36
    311
    Apr 8, 2016
    Oakland, CA
    IMG_1693.JPG
    Almost there
     
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