Am I crazy? Squeezed Twin Reverb build

Discussion in 'Shock Brother's DIY Amps' started by Diverted, Oct 18, 2019.

  1. Diverted

    Diverted Tele-Meister

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    This could be the craziest thing I've tried yet and that's saying something.
    I am trying to figure out how to build a very stripped down Twin AB763 into a Princeton-sized chassis and cabinet, running a single 12" on top of it. I'm looking to get roasted, or advised or egged on (people love trainwrecks). Here's why I'm doing it:

    A friend who plays very loud and very clean asked me as a favor (he's paying for parts and drinks) to build a very clean, very loud amp in as tiny an enclosure as possible. He has a Fender Supersonic (I think) 60W, and he says it breaks up too early for him. Same with a Super Reverb AB763 he used to own ... he really, really loves the Twin AB763 (says it's his favorite all-time amp) and asked if it was possible to squeeze it all in to a compact package, with a single speaker. I told him it wouldn't sound like a Twin with just one speaker, but he just wants a single.

    He wants:
    One channel
    No tremolo
    Light speaker and cabinet


    Removing one channel and tremolo brings tube count down to eight. I can just BARELY fit all this stuff in on this 18.25" chassis.

    I don't want to put the choke and output transformers in the traditional spot because that will require either (A) a wider chassis or (B) a significantly taller cabinet for speaker clearance. So if I do it I will have to run the output transformer on the far end, and snake the leads outside of the chassis from one side to the other, so the chassis acts as a shield as the wires pass by the various amplification and reverb stages. The speaker could then be put up closer to the chassis, roughly in the middle.
    I had to rotate all transformers and chokes 90 degrees from traditional, because running the power transformer in the other orientation would not have left enough room to mount the adjacent 6L6 tubes behind it while leaving enough clearance on the other side of the transformer for the 3/4" baffle's depth.


    As for electrolytics, there will be no room on top of the chassis for a dog house so I thinking about running them along the top inside of the chassis, in a line from left to right, between the pots and the circuit board ... pretty much how Hoffman puts them in a lot of his projects.

    I started off with Rob Robinette's no tremolo layout, pairing it with Hoffman's schematic for a single channel, and came up with a layout taking those changes into account (see pic).

    Inside, there should be about two inches between the transformer and the left edge of the circuit board. That's where I will put the bias supply/pot.

    As for cabinet, I was thinking 3/4" yellow pine, with a 3/4" baltic birch baffle. I'd use T-nuts to secure the speaker to the baffle and the baffle to the cabinet.

    The speaker would be an Eminence Legend EM12N, a 200W neodymium that weighs about seven pounds and is supposedly fabulous in this type of application. Very efficient, neutral sounding and clean.

    I've been mulling this over in my head for a while now and am trying to think of all the pitfalls involved. I'm sure there are many things that I haven't considered or won't realize until I'm neck deep in it.

    What do you all think? Do your worst!!!
    IMG_9087.jpg IMG_9087.jpg AB763_Layout_Tremolo_Deleted.png Hoffman AB763 1 Channel.jpg Podsnap.jpg
     
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  2. FenderLover

    FenderLover Friend of Leo's

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    I think you'd be crazy not to try it! A few comments to consider for ideas.

    First, what you propose for the circuit is not much different than the PR circuit, just recycling the trem triode to the LTPI. You could probably use, or lightly modify, the stock PR board. You already know it fits.

    Second, we know a 4x6L6 is not a heck of a lot louder than than a 2x6L6 given the operating conditions. It would pair nicely with one speaker at 8 ohms to boot, and save room. For a little extra grunt use 2x6550 and the same Twin transformers.

    Last, a little local NFB goes a long way for high headroom if you look at some Dumble ideas. Mr. D has modified many BF amps with his brand of crazy, and that's one of them.

    Carry on! Please post pics as you go.
     
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  3. Diverted

    Diverted Tele-Meister

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    Thanks! All good suggestions. Two of them I went over with him sorta, but not all. I'm dealing with a few additional constraints I didn't mention with the post getting so long.
    1. I mentioned 6550s. He said "I love 6L6s. Gotta be four of them."
    2. He really wants an AB763 circuit, stock apart from speaker setup. Went over circuits and he wasn't too into it.

    This amp will be 4/8 ohm switchable as the output transformer has two taps.
    I'm pretty excited to get to it. Do you think the output transformer lead dress runs on top of the chassis will be an issue? Should I shield them additionally in a flexible metal tube (like a toilet supply line), or wrap them in wire and ground that to chassis?

    Thanks for the input! I haven't looked much into Dumble mods, actually. I'll do some reading over the weekend. When I do get started on it I'll definitely post pix along the way.
     
    Last edited: Oct 18, 2019
  4. LetItGrowTone

    LetItGrowTone TDPRI Member

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    Put transformers and filter capacitors in the bottom of the cab.
     
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  5. Diverted

    Diverted Tele-Meister

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    That’s something I had not thought of....


    Then use a high voltage (aviation connector?) for various filter stages, heater supply and power. Hmm...
    I wonder if that would cause issues with reverb tank placement?

    Interesting. Thanks! Will consider.
     
  6. tubelectron

    tubelectron Tele-Holic

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    Why not ? Good Idea ! ;)

    It will be a great, deep and clean sounding amp... Something like an old Boogie MKI or MKIIA Hun-Rev.

    To whidthstand the power I would choose an EVM12L or the like (neodym version exists at Eminence).

    I already did this on a 25W DIY amp, and had no issue. The wires were solely grouped in an spiral wrap sleeve, maintained here and there on the chassis outside by claws.
    [​IMG]
    -tbln
     
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  7. Diverted

    Diverted Tele-Meister

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    Thanks. I saw D-Lab did it with a Princeton converted to 6L6s; he had to move the output to the other side to give the speaker clearance. I didn’t know if the additional current in this amp would make it more sensitive/unstable. Thanks for the encouragement.
     
  8. 1300 E Valencia

    1300 E Valencia Friend of Leo's

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    Randall Smith of Mesa fame did something like this with the first Boogies. I believe he started with Princetons with larger Fender transformers.
     
  9. Diverted

    Diverted Tele-Meister

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    Well, that’s good to hear. Hope this one boogies!
     
  10. Diverted

    Diverted Tele-Meister

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    I was thinking about an Eminence EM12N. 200w neo.
     
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  11. muchxs

    muchxs Doctor of Teleocity

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    Who is the musician and who is the technician?

    Lotta musicians bust my balls with stuff that is barely feasible.

    Here is the recipe:

    Go with an Allen Amps TP25 and a TO26. Dave says the TO26 may saturate early. I've never been able to get one to saturate.

    30 honest watts through an efficient speaker sounds like 100 half- assed watts. Don't ask me how I know.

    You don't need a choke.

    If you keep the power level reasonable you can use a 12" Eminence / Alessandro GA-SC64. While I appreciate the weight advantage of neodymium speakers, I like the tone advantage of the GA-SC64.


    Hint, hint.

    :cool:

    Betcha a dollar I've done more pumped- up Princetons than D-Labs. Mind you, I like his prolific re- purposed creations.

    IMO his Princeton Reverb is rather inelegant.
     
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  12. Diverted

    Diverted Tele-Meister

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    I can’t speak to its inelegance or not but it was an interesting video to watch as it answered me question RE: if I could move that OPT over. It was cool to see as it told me such a configuration could work on a Princeton-sized chassis.

    As for the half-assed watts, not questioning but just curious about what you mean? I’m not trying to invent here, just trying to recreate a proven AB763 circuit in a different speaker/cab configuration. Friend I’m doing this for has been a good friend to me, and I want to get as close as I can to what he wants. So if it’s possible I’m going to stick to that Twin AB763 plan. Thanks!
     
  13. muchxs

    muchxs Doctor of Teleocity

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    "The easy way is hard enough!!"

    Stock with a truncated AB763 circuit, LTP PI, TMB tone stack, etc.

    Just remember the next amp "down" that shares the AB763 circuit is the Super Reverb.

    Iron stout enough to support 4x 6L6 is gonna make this thing feel like it's filled with concrete.
     
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  14. Diverted

    Diverted Tele-Meister

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    You’re not lying. Got transformers this week, 19 pounds between output and power. I figure the amp is going to be around 45 pounds
     
  15. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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  16. printer2

    printer2 Poster Extraordinaire

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    A twin sounds the way that it does partially because it has two speakers. Mind you, he could always have a second cab. The added weight of the choke will not break the amp, it does effect the dynamics of the amp, leave it in.
     
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  17. tubelectron

    tubelectron Tele-Holic

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    Yes indeed, I think that it is an excellent choice : large tone, powerful and lightweight... :cool:

    I plan to buy a Eminence EM12N to replace the EVM12L that I have in one of my amp projects combo, a 100-watters, in achievement. The EVM12L would then be transfered in an assorted additional cab (to be built).

    [​IMG]
    [​IMG]

    -tbln
     
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  18. tubelectron

    tubelectron Tele-Holic

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    I personally find your items disposition on your chassis very good and well weight-balanced. I would do the same, by the way. The room is counted because your chassis is small, but it's completely doable, no issue.

    If you do a very compact, direct wiring (this always preferable), you can manage all right to place the input filter caps in the chassis, near the power transformer (2x 100µ or 2x220µ/350V) and leave one for the rest of the filtering on a fixing collar near the OPT, typically a JJ 40+20+20+20µ/500V which have quite small dimensions :

    [​IMG]

    But it's me, OK ? :D

    -tbln
     
  19. USian Pie

    USian Pie Tele-Meister

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    Isn’t this kind of how the first Mesa Boogie started? I seem to remember is was about trying to fit as much amp in a Princeton cabinet as they could.
     
  20. EsquireBoy

    EsquireBoy Tele-Meister

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