RI fender reverb unit >> eyelet board re-wire

[jbennett]

Hello. I'm a new member. This forum was recommended and I figured I'd sign up since I'm taking on my first amp building project.

I've got a Fender Reverb Unit and I want to rewire it like something from S. Raymond Ave. I've built two of the three boards and ordered many new components. I'll document the build here and maybe some experienced builders can keep me on the right track.

My plan now is to keep the stock Power Transformer/Output Transformer/Choke, tube sockets, power chord, pilot light, fuse holder, RCA Jacks, and footswitch jack. The rest goes in a ziplock bag.

I plan on drilling the chassis for a "dog-house" for the capacitors.

Changes from the original layout will be minimal. Since I'm sticking with the original PT, I'm going with a full wave rectifier, and since I don't really need it, I'm going to skip the AC outlet on the chassis.

Here is my modified layout.... (many thanks to member Tubeswell)



Here are my boards so far...


They are cut from 1/16" Formex.

Thanks in advance for any help. I'll poke around the forum and try to be a productive member myself.

[jbennett]

Got a new meter in time to start assembling...

[Bruce / Mission Amps]

For what it's worth... I found building a standalone reverb unit like this to be one of the easiest projects I ever did.
They even sound pretty good with the tiny 9" tanks, so you can build a little lunch box version of it.

[jbennett]

Thanks Bruce. You're not the only guy to tell me that. I figured it would be a good "gateway" amp. I have a feeling it won't be long before I build another. (assuming this all works out!)

[tubeswell]

Hiya jbennett

Welcome to the forum!

[Greenmachine]

For what it's worth... I found building a standalone reverb unit like this to be one of the easiest projects I ever did.
They even sound pretty good with the tiny 9" tanks, so you can build a little lunch box version of it.

Hey Bruce, can you tell us a little more about swapping tanks like that - sounds intriguing.

Did you have to adjust any values in the circuit?

Also, how might you describe the difference between that shorter tank and the usual 3 spring tank you find in the 6G15?

[jbennett]

Isn't it usually a 2-spring tank in the 6g15s?

I have a three spring in mine now, but might go back to the two spring. You loose the ability to lock the springs down fully to get that dead, slap-back sort of reverb by playing while locked. The pan pushes forward but only muffles the two springs closest to the bottom of the tank. (they are arranged in a pyramid).

[Greenmachine]

Thanks for clearing that up. I was under the impression it was a three-spring tank. I was planning on building one of these without suspending the tank like the original, just placing it in the bottom of the cabinet.

Hoffman amps recommends the three-spring tank for the build - I assumed that's what the original uses.

Other than that lock-down effect, what differences do you note between the two vs. three spring tanks?

[jbennett]

Honestly... I think the sound of the tanks varies wildly from unit to unit. I've heard really springy, deep two spring units that were amazing and some that were dull and less deep. I had an accutronics two spring unit that I replaced for that reason. Just not so deep. Replaced it with the MOD two spring and it was much nicer. Really it comes down to luck of the draw. I've also heard short 9 inch three and two sping units that sound just as big and deep as full length units.

The MOD 4AB3C1B is the one I have. I was sold on it by this video demo...

Listen to this...

[Greenmachine]

Cool! Thanks for the vid and feedback.

[loudthud]

A word of warning:

There was a thread about a year ago where an experienced builder constructed a reverb unit. The problem came up when the unit was connected to an amp he built. There was a hum introduced that could not be fixed. It was not determined how objectionable the hum was, or whether it could be lived with.

When both units have a safety ground from the power mains, a ground loop is created when a cable is used to connect the reverb unit to the amp. As far as I remember, the hum was never fixed. Leo Fender never had this problem because his reverb units from the 50's didn't have the power mains safety ground. I ask a guy I know about this (he's made a hunderd or so reverb units) and his suggestion is to lift the safety ground from the reverb unit. This is very dangerous and should not be done. It may violate safety regulations in some countries.

The only way to eliminate the ground loop will involve isolating all circuit ground connections from the chassis. That means the phone jacks and RCA phono jacks used to connect the reverb springs must not be connected to the chassis. The chassis must be connected to the power mains safety ground. A network of diodes, a resistor and capacitor is connected between the circuit ground and the chassis. This is done on some current production Marshall amps. There is a thread that discusses this in the Theory and Design forum.

[jbennett]

Thanks for taking the time to respond LT.

I was afraid of the ground loop. Forum member Tueswell suggested the addition of the two resistors on the 6k6 socket across pin7/9 and 9/1 (I think, I forget, it's in my schematic) as a way to silence the hum. If I ran into horrible hum issues I would probably bite the bullet and buy a proper 63 PT and do a half wave rectifier and ungrounded cable.

[loudthud]

Here's a link to the thread I was refering to: Stand Alone Reverb - ground loop busted

Probably others that you should read. I searched "Reverb ground loop" and got 84 hits. Not all were about reverb units.

A two wire line cord will give you problems too. Like the occasional shock when touching the chassis. Here's a link to the thread on the way to break the ground loop: What is the pupose of this?

[jbennett]

Thanks for the links. Hopefully I won't encounter the hum, but if I do it seems like there are a few good methods to try and quiet things.

I don't want to have any occasional shocks... not a fan of that.

[jbennett]

Got started.

Also, went to Leeds Radio on my lunch break. WOW! Anyone in NYC must check that place out sometime.
What a treasure. Picked up some straggling components.

[jbennett]

[Gtr_tech]

Reverb pans come in all sorts of varieties....some with grounded inputs and/or outputs (to the pan). It can make a difference depending on your drive/return circuitry.

[jbennett]

I'm using the MOD replacement for the 4AB3C1B pan.

[jbennett]

Some new photos.

Rectifier:






Main board:






Electrolytics:

[Kazooman]

Looks good thus far. Those two Aerovox caps do appear a bit suspect. Any reason not to replace them?

[jbennett]

Well, they were "NOS" and should be okay. I'll test them out with a power supply soon and see where they fall. I have new .1 caps to drop in if they are no good.

[jbennett]

Okay... well I walked to home depot on my lunch break. Bought a pair of snips and a sheet of zinc. measured, clipped, and folded it (using my desk drawer at the office) in 10 minutes.





Cost less than any available "doghouse" and I get to keep the snips!

[jbennett]

So. I finished my project.



I ended up replacing the aerovox caps with some Mallorys from watts tube audio. But I re-wrapped them in a retro design I cooked up since new mallorys look so boring.



Here are all the photos of my rebuild process. Thanks for looking and for your help along the way.

Only one issue left to deal with. The Dwell knob seems to be faulty, or fried. When I turn the dwell to zero I get a sustained BUZZ. Otherwise it sounds really great without any unwanted noise.

[Kazooman]

Wow!! That looks great!

I am really interested to hear how you like the results of your reverb. My guess is that you are very happy with the sound from your build. I have an original Fender reverb from the '60s and I recently built as close a clone as possible. They sound different. I can cross the leads to the reverb pans and that does not change the difference in the sound. Swapping tubes has no effect. There is still a difference in the sound of the original and the clone played through the same amp (1961 Fender Bandmaster - blonde on oxblood cloth with 12" speaker and tone-ring). I have grown up with the sound of the original for 40+ years and that certainly introduces a major league bias in my listening.

I would love to be able to create the original sound to share with everyone, but there is some sort of "mojo" about the old parts.

For what it's worth (probably not a lot!!!) my 40+ year old gear sounds the same to me today as it did way back in the good old days when I was a budding rock and roll star. The "exact" clone does not have the same sound.

Also for what it's worth, I am not chasing some elusive sound from a legendary player on a specific track. I am making an A versus B comparison and with NO changes in the components in the circuit there is a major difference in the sound.

[loudthud]

Did you just swap tanks between the two units or did you allow one unit to drive the tank and recovery circuit in the other unit? You would have to drive both units in parallel and select the output to feed the amp. That would narrow it down as to whether the mojo is on the drive side or the recovery side.

[Kazooman]

Did you just swap tanks between the two units or did you allow one unit to drive the tank and recovery circuit in the other unit? You would have to drive both units in parallel and select the output to feed the amp. That would narrow it down as to whether the mojo is on the drive side or the recovery side.

I never tried driving the pan with one unit and feeding that signal in to the recovery side of the other. That would be an interesting experiment. You are correct that I would need to drive both units so that there would be dry signal to blend with the signal from the tank. Perhaps I should run both outputs into separate amps and play in stereo

[jbennett]

Kazoo. I'm really happy with the conversion. The reissue was great but it got very tinny and harsh in a bad way with the tone above 4. All the controls are now more usuable and subtle.