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Single-Channel AB763 Deluxe Reverb in a Princeton Reverb Chassis

Discussion in 'Shock Brother's DIY Amps' started by ElliotKnapp, Oct 22, 2020.

  1. ElliotKnapp

    ElliotKnapp TDPRI Member

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    Hi friends, I'm 10 amplifier builds into my lockdown learning curve--I've learned so much, thanks to this and other forums, books, helpful sites like Rob Robinette's, and of course, the experience of doing it over and over! I've been modding the circuits I've built since the beginning, but now I'm starting to get even more creative in terms of repurposing chassis for different circuits.

    So, one of my recent builds was an AB763 Deluxe Reverb head--it sounds great, I had no troubles on startup outside of a bit of tremolo tick (easy fix with lead dress and a jumper capacitor) and 2(!!) ****ty new JJ tubes that crapped out on burn-in. I implemented a "lead" channel on the Normal channel per Rob's suggestions. The amp sounds great and complements my Princeton Reverb...the thing is, it's just too damn big. Almost all of my cabs are 1x12" (that's what I like), so I don't have anything wide enough to place the DR cab on top of, meaning it's on the floor, which is kind of driving me nuts....and it's got me thinking about building a single-channel AB763 DR that lives inside a Princeton Reverb chassis/head. I do like the sound of the "lead" Normal channel, but it's not a deal-breaker vs. a head that actually fits on top of my cabs. I know there's a Hoffman "single-channel DR" out there, but I'm more interested in faithfully (+mods) reproducing the AB763, sans Channel 1.

    Differences I know I'll need to address:
    • Remove V1 and all board components, pots, etc.
    • Is it sufficient to just remove the 220k "channel mixing" resistor that the normal channel hits before feeding the Phase Inverter grid?
    • Add another preamp tube hole (pretty confident I can fit one in the wider space between V2 and V3 and shift the Reverb Driver "up one screw hole" to make room.
    • Route the power transformer hole to fit a DR transformer
    • Various holes to add the choke, cap pan, cover the cap can hole, etc.
    • Anything else I missed?
    Here's my biggest question: does removing V1 and all of its board components necessitate any change to the filter cap that powers it? In Rob's layout and the Fender schematic, the far left cap (D) provides 280v to 3 nodes, powering plates for V1, V2, and V4. Will removing one of these tubes/nodes change anything? I'm learning more electronics fundamentals but have a ways to go on stuff like this.
     
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  2. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    Honestly man, just build a PR. You won't regret it. The only thing that needs to be fixed can easily be done by adding a grid stopper to the PI.
     
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  3. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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  4. ElliotKnapp

    ElliotKnapp TDPRI Member

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    I have built a Princeton Reverb (two actually, both with PI grid stoppers), and you're right, I don't regret it :). But like I said in my post, I think the subtle differences between the two complement each other and am looking for a new challenge.
     
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  5. sds1

    sds1 Tele-Afflicted

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    You are removing a tube, V1 specifically, this means less current draw down the entire power supply. All B+ voltages will go up. The load affects your B+ filtering as well, it's part of the equation, and feel free to find a calculator or SPICE simulator to show how filtering will change with 1 less tube, but we just know from the countless examples out there that one less 12A_7 is not going to affect the filtering in any meaningful way. So no changes necessary.

    It's all pretty negligible but that's what I see going on.
     
  6. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    @sds1 beat me to it. No problem with removing V1 circuitry.
     
  7. schmee

    schmee Doctor of Teleocity Silver Supporter

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    I had a layout but cant find it. IIRC:
    -Filters are fine as is, May as well use the 20/20/20/20 PR cap can.
    -The normal channel 220k to the PI can be removed , leave the little disc cap to the PI and 220k resistor. However, I can't remember if that value was changed in the end or not, I'll keep looking. There was a bunch of discussion about this on the Weber forum many years ago when I was doing it and that's what was decided in the end I think. I wasn't doing Reverb though.
    -re: Chassis holes: one thing to think about is just use the bias tremelo like the PR, but go to a DR style PI, does that get you down to 4 pre tube holes?
     
  8. RollingBender

    RollingBender Tele-Afflicted

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    I did one like that. AB763 with Princeton tremolo and 1-tube reverb. Same tube compliment as a stock Princeton but gets you into the AB763 tone.
     
  9. elpico

    elpico Tele-Afflicted

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    Removing the channel mixing section will double the gain of the preamp. If that's fine, then it's fine. If you prefer the same gain as stock then you need to keep the channel mixing circuit intact and return the 220k resistor to ground. You can bump the resistor up to 250k or 300k if you want to be real finicky about it.
     
    Last edited: Oct 22, 2020
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  10. ElliotKnapp

    ElliotKnapp TDPRI Member

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    Thanks sds1 and all others, for confirming this! It's good to know the why too!

    Of course, I can't believe I hadn't thought about just using the cap can--with the common JJ version, one of the caps inside is 40uf, which even covers the 2 parallel caps from the doghouse. I will totally do this.

    Since I already have a PR and a 6G3, I'm already pretty good on bias vary trem (and I actually enjoy the opto trem's slightly steeper curve as a separate option), so I'm ok drilling another preamp tube hole. I'm really not too worried about a couple pre tubes being slightly snugly located (although I have considered covering over a hole or two and re-drilling more than one...will reserve judgment until I've got all the parts on hand). Great feedback, thanks all...
     
  11. ElliotKnapp

    ElliotKnapp TDPRI Member

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    I do want to get really clear on this point. @elpico , when you say "removing the channel mixing section," you mean removing both 220k resistors, right (one for the Normal Channel, and one for the Vibrato Channel). So, to keep the vibrato channel the same as on a stock DR (which is definitely what I want), should I:
    a) Omit only the Normal Channel 220k resistor, leaving the Vibrato Channel 220k resistor and .001 cap (what I think @schmee is recommending), OR
    b) Leave both 220k resistors and the .001 cap, and send the "unused" Normal channel end of the Normal Channel 220k resistor to ground? (this is how I'm interpreting what elpico is recommending).

    When you talk about bumping up the 220k (assuming it's the Normal one), what would the reason for doing this be? Are you then simulating the additional load that would have been there from the now-absent V1?
     
    Last edited: Oct 22, 2020
  12. David Barnett

    David Barnett Doctor of Teleocity

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    Well, a single channel DR in a PR chassis would just be a PR with a few changes - different transformers and a long-tail pair phase splitter are the biggies. You can probably find a DR spec power transformer that fits the PR cutout.
     
  13. elpico

    elpico Tele-Afflicted

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    Either way works, but if you want the stock amount of gain as you say then you need to do option b.

    The normal channel triode at the other end of the mix circuit is an AC ground. That means to your AC signal the mix circuit is a voltage divider. It cuts the vibrato channel signal in half (approximately).

    You're right about the other thing. Probably not worth worrying about about, but if you wanted to get real specific about making the gain the same as stock... The plate of the triode you want to delete isn't a perfect AC ground. It presents roughly 40k to AC ground. That means the voltage divider doesn't quite cut the signal in half, on the shunt side you've got roughly another 40K of impedance in addition to the 220k mix resistor. It's not a big difference though, if you take the 220k straight to ground you'll have approximately 8% less gain than stock.
     
  14. EmWeAh

    EmWeAh Tele-Meister

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    If you're willing to get rid off the tremolo have a look at Rob's Blackvibe with reverb.
     
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  15. NTC

    NTC Tele-Meister

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    At the risk of blasphemy: consider making the vib oscillator and driver (follower) with transistors or an IC or 2 and eliminate the need for the extra tube hole.

    I will go away now.
     
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  16. sds1

    sds1 Tele-Afflicted

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    No come back I like your style.
     
  17. tubegeek

    tubegeek Friend of Leo's

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    As long as you use vintage push-back wiring and a full-size CTS speed pot to hook it up, it'll be OK.
     
  18. zook

    zook Friend of Leo's

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    I've said this before. If you use a 12DW7 to drive and recover the reverb it will free up half of a 12AX7 so you can convert it to a long-tail pair phase inverter. That will make a big difference.
     
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  19. ElliotKnapp

    ElliotKnapp TDPRI Member

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    @elpico Awesome, thank you! So glad to hear that I've at least learned enough to know what to look for, even if I have a long way to go on circuit analysis and calculations...voltage dividers are on my list of fundamentals to master, since they're omnipresent in guitar amp design and a simple but somewhat mysterious concept when applied (at least for my brain).

    I also noticed when looking more closely at the AB763 layout that I'll also want to bump up the cathode resistor to around ~1.6k for V2 (which will be my new V1) to maintain the status quo, since it was shared between V1 and V2 at 820Ω before. Thanks again for the wisdom.
     
  20. ElliotKnapp

    ElliotKnapp TDPRI Member

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    A classic Blackvibe (no tremolo, no reverb) is on my list, too...in (guess what) a PR chassis and box! I've actually been hedging on this one since coming up with the DR in a PR idea--I'm wondering if it'll really be distinct enough from a DR, especially if I were to just get a 15" speaker and run the DR through it. Do the transformers, 6L6s, and minor component changes make enough difference to warrant building one?

    For this thread's build, I just really want to get as close as I can to the DR without a Normal channel, since it's such a classic circuit and I do like the reverb and trem (I'm using an Accutronics tank for my DR and a MOD for my PR, so there's a noticeably different reverb flavor that I'm keen on keeping as a sonic distinction).
     
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