The SuperToad - AB763 front end, distortion, FX loop, 6973 power tubes... not for purists

Discussion in 'Shock Brother's DIY Amps' started by Badside, Jan 10, 2019.

  1. Badside

    Badside Tele-Meister

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    Working on this for a friend. It's part using spare parts to save $$$, and part making a practical amp for gigging.

    The transformer selection (272DX and 1750E) as well as the power tubes (6973) are simply because I have them already. So why not. This forces me to use a 5Y3 rectifier to not kill the tubes, it's okay, I have one that I don't need.

    I wanted to give him a nice pedal platform amp, and a cathode-biased Blackface has a nice syrupy warmth and just enough headroom for small club IMHO (the bones of it are based on one of my own ams).

    So then... I took this a step further. Why not put a "distortion pedal" right in the amp? With a tube, running on proper high voltage.
    So I added the first two stage of a Marshall SL5 for good measure, and gave it its own Tone control (at maximum it's effectively the stock circuit, but at least you can tame it down if necessary). In itself it might not create that much distortion, but it's also hitting the Blackface front end harder. Some tweaking will be required to get it where I want. This is a first draft.

    And since some effects work better post distortion, why not add in a loop? The Blackface architecture makes it possible to just add a buffer after the tone stack and use the 2nd triode as the recovery stage. Experience tells me it's still possible to drive the power amp hard since we're not losing signal mixing 2 channels together. Purists will not agree of course.

    A few quirks you'll notice:
    - Dual inputs: will work perfectly using just the Drive/Combo input, but if you want to leave your Tubescreamer on all the time into the Distortion side, dual input lets you do that! Or leave chorus on on the clean side only.
    - Relays are fed from a regulated 5VDC supply obtained from the 6.3VAC filament line (Hoffman inspired)
    - The rectified 6.3VAC is also used to feed DC to the filaments of the Distortion tube, using a dropping resistor (too short on voltage to risk a 6V regulator)
    - The loop is basically just a MOSFET follower used as a buffer after the tone stack. Exact value of the voltage divider after it will have to be tweaked. From experience, the tube recovery stage following it should have enough gain to drive the PI.

    SuperToad v0.1.gif
     
    Last edited: Jan 11, 2019
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  2. Badside

    Badside Tele-Meister

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    Quick revision and cleaning up
    SuperToad v0.2.gif

    I still have doubts about the Overdrive circuit though, I'm not sure how much I can rely on hitting V2a hard to get where I want. I need this to get me where a Distortion pedal would on its own, but while hopefully avoiding clipping diodes. I may implement a bypassable MOSFET boost before V1a.

    I'm thinking about breadboarding the OD circuit to test it, feeding into my Blackfacey amp. I just need to whip up a small power supply first (I've got both a 269EX and 269GX that can be used for that)
     
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  3. Badside

    Badside Tele-Meister

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    Breadboarded the Overdrive circuit and as I expected, it's pretty much just that: an overdrive. I know the future owner of this will want more distortion out of it and I need to find a way to give it to him.

    Next thing to try is a MOSFET Boost on the input (defeatable)

    Surprisingly, even partly on a breadboard (tube, pots and transformer are chassis mounted) and horrible lead dress, it's not squealing, it's very stable.

    EDIT:
    For anyone curious, this is the crude setup I put together. Don't do this, it'll kill you :)
    But at least it's fused, properly grounded and has a power switch.
    And truth is... it's dead quiet! No buzz, no squeal, no hum!

    20190111_183217.jpg

    Note that I don't believe the breadboard is rated for the kind of voltages I'm putting it through, so I'm being a bit overparanoid and keeping anything with DC on them at least 3-4 rows apart. And I have the B+ line on the bottom buss and the ground on the top buss.
     
    Last edited: Jan 11, 2019
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  4. dunner84

    dunner84 Tele-Meister

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    Not sure if you have the space, but I had z-pre pcb from London Power lying around, and added it as the second channel to my single channel ab763. Works beautifully. It is a 2 tube pre-amp based on the Soldano SLO. I think it sounds killer. Power supply is 100% blackface, and I added a 3 way filtering switch to go stock-extra filter cap-extra filtercap/diode rec.
     
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  5. Badside

    Badside Tele-Meister

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    Spent a lot of time moving things around on the breadboard. Also, I added a MOSFET follower at the end to prevent tone loss since I'm feeding this into another amp... that helped a lot.

    The conclusion is: the only way I can get the amount of overdrive I need, is by hitting the AB763 harder. The overdrive circuit on its own is cool, but too tame for the intended purpose.
    Problem is... I don't like how an AB763 sounds when hit hard, in great part due to the fact that the input stage is a plate-driven tone stack (low impedance). It's too fizzy IMHO, no point in going all that trouble for something that sounds LESS good than a distortion pedal.
    FWIW, I have tried diodes and LEDs, they only really work at the end of the Overdrive circuit (not enough signal swing after just one stage)... but it just sounds like a very complicated Tubescreamer.

    So what's the solution? Add a DC-Coupled Cathode Follower to the AB763 input stage! And now that I have an extra triode to play with, I can make the Overdrive circuit a 3-stage affair.

    Also, decided to stick with 6V6s. This started a low-price spare parts build, hence the 6973 tubes (have them and their OT, don't need them anymore). But since the future owner lives far away, I don't want him to end up in a situation where he needs new tubes and he can't find them anywhere. Any decent music store will have a 6V6 duet in stock, and cathode bias means an easy swap. I've also switched to SS rectifying, although the jury is still out. From experience, when using the 125VAC tap on the 272DX PT, SS rectifying will give me around 375V, that's as low as I'd want to go here.

    So here's where I'm at right now:
    [​IMG]
     
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  6. Badside

    Badside Tele-Meister

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    I breadboarded the first 2 stages of this last design and ran into the "Low" input of my JCM800 2204 clone, which gives roughly the same circuit sans the tube recovery stage after the loop (that one is all MOSFET), and I like it, I like it a lot. Gone is the buzzy sound of pushing an AB763 front end, replace with something a lot more old-school rock (although the overdrive circuit is inspired by the Slash amp, it came out more EVH like, probably cause I was running Greenbacks instead of V30s... I hate V30s). It sounds surprisingly good at low and mid gain settings as well, I won't have to implement a switch to bypass a stage (and live I can just roll down the guitar's volume, it interacts superbly)

    So I'm settling on this as a starting point.

    Working on circuit layout now. I realized that the line that bypasses the overdrive circuit in clean mode could wreak havoc in OD mode (it's always connected to the first stage's grid), but I found I can easily make those leads super short by putting the relays closer to the tubes. I just have to experiment with how close is too close.

    I'm putting the input jack (not jacks, doing away with the dual input for now) on the back, which will also help with noise as the input lead will only be a couple inches long at most. I've been doing this on all my amps that have a Loop, it's very handy live to have everything on the same side.
     
    sds1 likes this.
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