Lunchbox “Revibe” (Hoffman board)

Discussion in 'Shock Brother's DIY Amps' started by BobSmith, Feb 14, 2020 at 12:28 AM.

  1. BobSmith

    BobSmith Tele-Meister

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    San Jose,CA
    After a brief hiatus with a solid state rebuild, I’m back with another tubular project. I got plenty of amps, so wanted to try something new and different. A revibe! For those not familiar, this is a combination of Fender Reverb Unit (6G15) and the “vibrato” (aka tremolo) circuit of the Fender Concert (6G12).

    Soupy, surfy, springy, dripping wet goodness!

    I’m basing this on the Hoffman circuit (using his board) with a couple of minor changes. The chassis will be another Hammond box (worked well for my Blues Junior build), but given the complexity of the circuit, and the size of the reverb tanks (shy of 17”) I needed the 17x10x2 form factor.

    Some basic design choices: a full Mercury Compliment of transformers, metal film resistors (the hard-to-read blue ones), F&T filter caps and Xicon coupling caps. I bought 3 different reverb tanks (TAD, MOD and Belton/accutronics). I plan to try them all. Any one have a favorite?

    I didn’t like the way the Hoffman design tucked the filter caps all over the chassis, so I will be making a turret board under a cap can atop the chassis.

    I’m still debating how to mount the reverb tank. I have 2 choices: I could either mount it horizontal on top of the chassis (tight!) or against the front panel. The reverb tank is setup for horizontal mounting (the only way it seems to be sold), but the internet-based wisdom seems to indicate it’s better to mount it vertically (jacks upward preferred). To keep the output away from transformer magnetic field though I would need to mount it jacks downward which is still “second best”. I’m not sure which way to go and open to the wisdom of TDPRI. Hit me!

    I bought an extra Hammond bottom cover which I planned to cut down and use as a mounting panel/cover for the reverb to the front panel.

    Grounding is also somewhat in the air. I don’t plan to use the galactic floating ground that I think Weber does. Hoffman suggests not connecting the power cable green to the chassis because of ground loop potential from the downstream amp which also has a chassis ground. Hoffman has also updated his schematic to reflect “Tore T” modification which improves noise.

    I also plan to replace a tremolo tube cathode RC pair with an LED, to pulse the tremolo rate.

    I also need a cool name...

    EBEFEA4F-CBC2-4E37-B5E2-0598DDFC2D95.jpeg B14D93CB-AF93-4A78-B5B6-DE90695E6AD8.jpeg 1B755F99-D52B-463E-881E-9573839D4E8D.jpeg D6F48554-44B4-4C54-BC4A-178628DCC3F0.jpeg EA11DE25-6645-4A47-9A98-07B243224E1A.jpeg
     
  2. alathIN

    alathIN Tele-Holic

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    TDPRI user Moosie used a variant of Merlin's star grounding scheme and got great results.
    Doug Hoffman is a very smart amp guy but i'm leery of his "leave off the safety earth" solution.
    https://www.google.com/url?sa=t&sou...FjAAegQIARAB&usg=AOvVaw0Si1Yp7qjRr72et6IMZHFs

    I'm building a modified Weber Revibe now and plan to use a variant of Merlin's ground lift circuit too.

    Looking at Merlin's diagrams, it seems like he is keeping all the circuit grounds associated with each power node separate, including the filter cap ground for each node.
    Moosie didn't go that far - at least according to his layout diagram.
    Following his lead, my plan includes:
    Chassis safety earth separate from everything else.
    Circuit grounds from each power node start out segregated, then join up and connect to Merlin's ground lift circuit.
    All of my filter caps ground to a single bus, which then connects to the ground lift.
    Only after the ground lift do any of these ground to the chassis. This chassis ground lug is separate from the safety earth ground lug.
     
  3. BobSmith

    BobSmith Tele-Meister

    Posts:
    191
    Joined:
    Feb 12, 2018
    Location:
    San Jose,CA
    So after playing so chassis chess, I came up with a layout. It’s tight.

    I have to leave a 3/8” gap front and back for the cover. Part of the winding of the reverb OT will overlay the cover by a very small amount, but it will fit. There is also a 1/16” gap between the cap can and the reverb tank. In the photo below, the TAD version is demonstrated, but the others will fit as well. The picture also shows where turret board mounts and ground lugs will be. All hole locations are transferred to the chassis with the exception of the vibrato LED holder which is on order and in transit. It will go between speed and intensity controls.
    2B374026-E861-46CA-8241-18E3CB491108.jpeg
     
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