Lunchbox “Revibe” (Hoffman board)

Discussion in 'Shock Brother's DIY Amps' started by BobSmith, Feb 14, 2020.

  1. BobSmith

    BobSmith Tele-Meister

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    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...

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  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.
     
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  3. BobSmith

    BobSmith Tele-Meister

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    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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  4. BobSmith

    BobSmith Tele-Meister

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    Turret board for filter caps made.

    I also decided to try the hum-blocking circuit. After much hemming and hawing, the final straw was finding out that the Fender Reverb Reissue uses this. Crap. Oh well. I ordered a bunch of isolating washers (Switchcraft S1029) to use on the 1/4” jacks (2 of those), it also turns out they will work on the RCA jacks as well (4 of those). 2+4 = 6 jacks, 2 washers apiece so 12 total! That’s a lot of washers.

    I ordered som 6a diodes, 10ohm 5watt resistors and 100nf capacitor. More or less what Merlin the Valvewizard suggests in the link from alathIN above. Not sure if it will be worth putting in a switch to lift or ground. Other than being able to compare the 2 schemes, I’m not sure it will give me much.
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  5. King Fan

    King Fan Poster Extraordinaire

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    Cool project. Doug notes this is probably the most complex build/board he lists. His does sound fabulous.

    Is your chassis smaller than Doug's? I see he says that filter caps bigger than his Illinois units probably won't fit even in his chassis, and the external doghouse will work fine.

    The question of how to power-ground an external effects chassis is outside my experience -- but Doug's discussion is pretty convincing. The sleeve conductor of the cable is gonna ground the revibe chassis to the amp chassis...

    EDIT: For those who haven't seen it, here's Doug's version.

    revibe.jpg

    revibe 2.jpg
     
    Last edited: Feb 19, 2020
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  6. BobSmith

    BobSmith Tele-Meister

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    The chassis I’m using is the Hammond 1441-30BK3. It is 17x10x2. Doug’s is deeper than 2” so he can stack more things, but I think it’s too cramped to work on. I think that’s what makes the build complicated. I don’t know the other dimensions. He doesn’t offer it anymore.

    The doghouse, is REALLY tight if using a turret board. I will need a washer spacer to elevate it enough to clear the biggest cap (1” radius). I’m using the shortest standoff I can (1/4”), plus the board thickness (1/8”). I will draw the wires under the board and to the underside thru a couple of grommets. I still haven’t fully decided if I’ll put the dropping resistors on the filter board (like fender does) or on the main board (like Hoffman does). Putting it on the filter board should open up more space for mounting holes on the main board.

    Regarding the ground lift, the more reading I do the more I think it’s a good idea. I bought a SPST mini toggle switch to implement a lift or bypass. There are a few good discussions on “The Gear Page” where Jeff Gehring (designed the Weber version of the revibe) chimes in to offer his experience frequently. Here is a good discussion:
    https://www.thegearpage.net/board/index.php?threads/6g15-fender-reverb-unit-build-questions.1690482/

    I feel pretty convinced this is a good idea to implement. I think I’ll have room to make a little turret board from leftover G10 turret board (shouldn’t take much).

    Now I have to wait until more parts get here...

    I have the parts to do this out of Dale resistors or Carbon Comp, but I think I’ll do it in metal film from Hoffman (blue ones.)
     
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  7. BobSmith

    BobSmith Tele-Meister

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    Some more parts have arrived, and I was able to find some time to dry fit the filter cap board and Hum-blocking ground lift. This is based on the often referenced Merlin Valvewizard strategies for battling gremlins, err ground loops. Right now I plan to implement the optional switch. The board uses 6amp diodes in antiparallel orientation. 10ohm 5watt resistor, and a Panasonic 0.1uf 630v film capacitor.
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  8. moosie

    moosie Doctor of Teleocity Ad Free Member

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    So you're elevating the board above ground. Good move, IMO. You'll need insulated jacks, both flat and shoulder washers. And these require slightly larger chassis holes. I used a step drill bit to modify the chassis I got from Weber.


    Gehring chose to put the cap in the hum-loop-block network close to the input jack, and the diodes and resistor near the power supply. I followed that lead, and it's worked well. You can see my little separate board bolted to the left side of the chassis. And the cap is the big brown blob near the second pot from the right - cobbled out of a silverface amp.

    I didn't have much room here. Weber's "kit" was basically a reverb kit, with a few extra eyelets on the board - in the wrong places, and about a thousand extra components. I had to drill new transformer mounts, to angle the transformer, so it wouldn't conflict with the tank mounted vertically behind the chassis.

    I used a MOD two spring, long decay, which is my preference for combos, too. Boingy.

    Because there was no allowance made anywhere in the Weber stuff for the circuit elevation, and the eyelets were a lot wrong... I decided to design my own layout from scratch, trying to use as much of the existing eyelet placement as possible. As @alathIN mentioned, I mostly followed Merlin's grounding scheme. I kept the feeds and returns all in their own loops - one loop for each secondary filter cap. I did join them all together right at the end, near the power supply. Didn't seem like a one-inch bit of wire would make a difference either way, over there. The unit is really, really quiet. And it quiets whatever it's plugged into as well.

    I know it'll be laid out different than your project, but parts is parts, and in case it helps... I've attached my layout here. PDF, too, so you can download a readable copy.

    FYI, mine worked the on the first power up... but then took three weeks of tuning to get the depth of tremolo sounding right. I had a lot of help here on the forum, and a 'scope, signal generator (phone app), and signal probe. And Leo's patent documents, which describe the function of each component. That was very helpful at that stage.

    Good luck!

    Screen Shot 2020-02-25 at 03.24.07 AM.png


    Moosie's Revibe Layout.png
     

    Attached Files:

    Last edited: Feb 25, 2020
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  9. alathIN

    alathIN Tele-Holic

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    I'm about where Moosie was 3 weeks from paydirt.
    It works but doesn't sound quite as sweet as others i have heard - so needs tweaking.

    I have some trim pots and can borrow an oscilloscope from my brother (but almost no idea how to use it or what the data means). No better time to learn I guess.

    I really suggest looking closely at Moosies layout. It was a lot easier to understand and much simplified compared to the one on the Weber site. I have not checked closely to see how different the Weber and Hoffman variants are - but - and this is coming from a guy with tons of respect for Doug Hoffman and S.Luckey too for that matter - I don't like the "disconnect the safety earth" solution they used. The Merlin ground lift plan makes a lot more sense.

    My ReVibe has a decently low noise floor at this point but not to Moosie's "quiets every electronic device in a 5 mile radius" standard. It is certainly quiet enough but there may be some easy room for improvement because I ran out of good quality shielded wire and had to cobble some junk and odd bits together (I plan to replace all that).

    Also I have a preamp in the same chassis so I had some longer wire runs - won't be able to fix that.
     
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  10. BobSmith

    BobSmith Tele-Meister

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    Awesome suggestions. I’m going to study @moosie layout. Pretty amazing work considering how much needs to fit in a small space. My chassis and layout is a bit bigger so hopefully a bit more easier to work on.

    Although I see the Weber/Gehring reverb unit (not the revibe) placed the cap for RFI/EMI filtering (not actually needed for ground loop issues) near the input, I’m not convinced the location of connection is material to its function. I will be running a bus bar along the front of my board. I may be selective in the order I attach things to the bus.

    I briefly looked at the Weber/gehring Revibe on the website, but I think it’s not updated or wrong. The circuit doesn’t show the blocking network at all, and the layout only seems to show a 16ohm resistor to ground and not the other components he himself (Gehring) discusses elsewhere on the web.

    I’m going into to start working on the chassis soon (always the challenge for me...)
     
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  11. moosie

    moosie Doctor of Teleocity Ad Free Member

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    I had a slow start on this project, tearing my hair out trying to reconcile all the incomplete and contradictory bits of information from Weber, Gehring, and Hoffman. Also trying to understand Merlin's grounding treatise. I found Gehring's build document at archive.org. That was very helpful early on.

    My build thread is here, with the tuning discussion starting on page 4. It resulted in a few component changes, all of which are referenced in the notes on my layout.

    As the build progressed, a side discussion of the theory behind harmonic trem began here: https://www.tdpri.com/threads/exploring-fenders-harmonic-tremolo.791406/
     
  12. King Fan

    King Fan Poster Extraordinaire

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    Wow, cool, @moosie . Seriously, you're now one of the country's best sources of info on the ReVibe. I appreciate people who write up their research, their builds, and their mistakes, revisions, and results in a clear well-illustrated way, and then provide links. Nice!
     
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  13. moosie

    moosie Doctor of Teleocity Ad Free Member

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    That's frightening. There may be some truth to that, but I think only because so few people build them.
     
  14. BobSmith

    BobSmith Tele-Meister

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    Thank you Sherman (@moosie) for setting the Wayback machine to Gehring’s build. Great resource there!

    Part of the Gehring’s chronicles seems to confirm what I presumed about the capacitor of the hum-blocking network: That it plays an adjunctive rather than central function (RFI, EMI removal) and the location is probably not critical. He did not add it at first and I think he arbitrarily picked a location out of simplicity (in a cramped chassis) rather than a location based on electrical theory or design.

    Here is his quote that makes me think this:
    “I also have a 0.047uF capacitor that is electrically in parallel with these, but it is mounted from the sleeve lug on the input jack to chassis steel by the input, to decouple any RF trash that may show up on the isolated ground buss. I added it later, and it doesn't show up in many of the pictures”

    I’m not sure why he picked 16ohm and 0.047uf, (I went with Merlin suggestions 10ohm and 0.1uf respectively) but it will be easy for me to change out later. I also have the room to place a mini toggle switch to bypass the lift circuit to ground, mostly for curiosity...

    With the parts from Hoffman (switchcraft jacks and whatever he is using for RCA jacks) standard Switchcraft S1029 shoulder isolation washers will work for both kinds of jack which is cool. I can sandwich 2, one from each side, for pretty easy isolation.
     
  15. moosie

    moosie Doctor of Teleocity Ad Free Member

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    Is your chassis material quite thick? Because on mine, I could not have used a pair of shoulder washers (S1029). The shoulder of one washer completely covers the inner edge of the hole. Two of them would bump into each other, and not lay flat.

    I see several merchants saying the same thing, that the S1028 and S1029 are to be used in conjunction, to fully insulate a metal bushing from the chassis.


    Yup, I remember reading that about the .047 uf. Right, it's about RF, not ground loop. Which I guess is why I thought maybe best over towards the preamp, where the issues would be a problem, but I didn't fully understand why, and just followed Gehring.

    The values... don't seem to matter much. Of that cap, or of the resistor. We're talking OHMS here. I imagine the goal is to elevate the circuit just enough that we herd electrons one way or the other. Too much elevation and we have a potentially signficant voltage potential, similar to what the diodes are there to prevent. Like if you used a 250k resistor... But 6 ohms either way? I have a feeling you can use whatever is convenient.
     
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  16. alathIN

    alathIN Tele-Holic

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    Mine is 16ga aluminum and the Switchcraft shoulder washers seemed just the right depth.

    I think the Marshall-style input jacks are also isolated from the chassis. The ones I have laying around here also come with the shunt option to ground them out when nothing is plugged in.

    Speak of the Devil.

    I posted the grounding scheme review over on EL34world and those sharp-eyed cats noticed that I used a 15K ohms resistor in my ground lift rather than 15 ohms like I thought I was. I guess that "k" makes a difference?
    Not sure what that would do or not do, but it seems like an order of magnitude is worth correcting.
    As far as I can tell it seems to work despite this...
     
  17. moosie

    moosie Doctor of Teleocity Ad Free Member

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    The diodes go open at 0.9V, right? I think it's safe to say the 15k resistor created a voltage differential much greater than this. So, you had no elevation, and no hum-loop control.

    The resistor must be sized to keep the voltage drop between zero and the diode threshold.


    What's not clear to me is what happens if we didn't have those diodes... And in scenario #1, the resistor breaks in half (goes open). And scenario #2, you mistakenly use a very, very large resistor.

    Will the amp work in either case? The first case seems like a no-brainer 'NO', but the second? Severely reduced current, and very high voltage throughout the circuit? Start a fire?

    And of course, at the moment, one of your circuit grounds could shift, and touch the chassis wall, and all that would happen is increased noise potential because you bypassed the HLB circuit. But if you have a large voltage differential.... hot chassis.

    I'm not sure if any of that is accurate. Anyone care to enlighten?
     
  18. alathIN

    alathIN Tele-Holic

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    [​IMG]

    Now here we are back in the correct order of magnitude.

    The only resistor I had close to the correct value was pulled from something else and had short leads - but still long enough to wrap around my "splice-on" leads for a good mechanical connection. I think this is probably OK but just for good measure I will probably replace it with a new one.

    Posting a sound clip in the other thread.
    I am not 100% sure I have harmonic vibrato here.
    https://www.tdpri.com/threads/another-harmonic-vibrato-revibe-thing.1008189/
     
    Last edited: Feb 27, 2020
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  19. BobSmith

    BobSmith Tele-Meister

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    The manual from Fender ‘63 Reverb Unit Reissue explains this beautifully:

    “R23 places 15 ohms of resistance between audio ground and chassis/earth ground. This minimizes hum by eliminating ground loops internally and externally when connection with other units. R22 and R23 are Flame Proof/Fusible resistors. If excessive current flows through theses resistors, they will not burn, they will simply open.
    CR5 & 6 (across R23) provide an important safety feature. IF the guitar amp chassis becomes electrified, current will flow through the coax cable to the power supply ground of the Fender Reverb unit. The current will seek earth ground through R23. When R23 opens, the earth ground connection is broken. This will electrify the reverb unit’s ground and thus the guitar (ouch!). CR5 & 6 provide an alternate path to earth ground if R23 opens.”

    Also a big current draw/bleed off will blow the fuse. Which is why I think higher amp diodes are a good idea. Fender uses 6amp diodes as well. They do not use the rfi filter capacitor.

    I also like the “ouch” part they threw in there.
     
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  20. BobSmith

    BobSmith Tele-Meister

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    I finalized the layout on the chassis (covered in blue tape) and center punched all the holes. I placed a Sharpie dot on all the spots before bringing it to the drill press. The only holes missing are the 4 for the hum blocking network board, the 4 for the top cage cover (tapped M3 into sheet metal) and 6 for the bottom cover (rivnut M3).

    Photos are of top, front and back. If you can see my pencil markings you get an idea for the layout.

    This is the same process I used for my handwired Blues Junior.


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