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Thread: Mod Twin Reverb's Reverb Circuit for Rhodes

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    Mod Twin Reverb's Reverb Circuit for Rhodes

    Hi all! Long story short, I am currently using two Twin Reverbs for Rhodes. I've been using an re-201 for Reverb but I can't deal with the noise of the thing anymore. I'm thinking of getting the Fulltone Tube Tape Echo, but then I wont have that sweet re-201 reverb sound. I don't use the Twin Reverb's reverb because it just sounds way too shrill with the Rhodes, very over driven unpleasant sound. Is there something I could mod in the reverb circuit to be more receptive of the Rhodes output gain to clean up the resulting sound? Just a different tube rather than the 12at7? Or something a bit more involved? Thanks a lot for your time!

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    Exactly which model of twin reverb?
    With the classic type, the V3 cathode bypass cap is simple to lift one leg of; that will noticeably reduce the reverb drive level and also make the driver much harder to overdrive.

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    Thanks pdf! Exactly the type of info I'm looking for. They are two silver faces, ones a 71, the other a 72 I beleve.

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    Quote Originally Posted by pdf64 View Post
    Exactly which model of twin reverb?
    With the classic type, the V3 cathode bypass cap is simple to lift one leg of; that will noticeably reduce the reverb drive level and also make the driver much harder to overdrive.
    While I've never tried this, taking the Reverb Output signal from it's 12AT7 circuit via the low impedance transformer, feeding a stand-alone equalizer, such as a 3 or 4-band parametric Equalizer, which also has a variable Low Pass and High Pass Filter, you could shape the drive signal that feeds the tank. Now, the stock tank in a Twin Reverb, being a 4AB3C1B tank has a 10 ohm input impedance, and a 2k output impedance. You could buy a different tank that has a much higher input impedance, such as a 4EB3C1B, having the same characteristics in springs and decay, only with a 600 ohm input impedance, which could easily be driven by the external EQ. Then, you can dial in the tonal characteristics of the reverb tank.

    Or, making use of what's probably readily available to you, plug the output from the Twin Reverb's Tank signal to another guitar amp, and disconnect it's speaker, and plug the output into the input of the Twin Reverb's Reverb tank. You'd want to do this at low level, since you have enough output capability of this extra guitar amp to do damage, and you don't want to blow the Tanks tiny transformer. Then, you can tweak the Bass, Midrange and Treble controls to see what you can get. I'd start with the Reverb control of the Twin at mid-position, and then begin tweaking the Volume/Master controls of the 'Reverb Tank amp" to arrive at a sweeter sound than you're getting with the stock Twin Reverb's character.

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    Thanks Nevets! I like the extra amp idea, would be a fun experiment for sure. A different tank makes sense too.

    Quote Originally Posted by nevetslab View Post
    While I've never tried this, taking the Reverb Output signal from it's 12AT7 circuit via the low impedance transformer, feeding a stand-alone equalizer, such as a 3 or 4-band parametric Equalizer...
    Is what you're describing coming straight from the RCA out that goes to the reverb tank input? So just putting the EQ in between the output and the tank? Thanks!

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    Have you tried the new Boss RE-20 pedal? A bit pricey, but not compared to what the used RE-201's are going for!

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    Driving the tank from a power amp seems kind of venturous without a dummy load and suitable voltage divider. I don't think the transducer can take more that 1..2W.
    OTOH, an equalizer might not be able to deliver enough power (e.g. 0.5W).

    It is easy to modify the Twin reverb circuit for less drive or less highs (do we have the appropriate schematic?). Also a different tank type may be an option.

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    I have not tried the re-20 pedal but i did try the strymon el capistan, its alright, nothing beats the actual re-201 but i am looking to clean up the signal for live shows, and not gig around with a space echo!

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    Quote Originally Posted by Helmholtz View Post
    Driving the tank from a power amp seems kind of venturous without a dummy load and suitable voltage divider. I don't think the transducer can take more that 1..2W.
    OTOH, an equalizer might not be able to deliver enough power (e.g. 0.5W).

    It is easy to modify the Twin reverb circuit for less drive or less highs (do we have the appropriate schematic?). Also a different tank type may be an option.
    I was wondering about running the amp with no load. Perhaps I could make a dummy load... That sounds like a lot of work though, i will start with the tanks!

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    Quote Originally Posted by burymyteeth View Post
    I was wondering about running the amp with no load. Perhaps I could make a dummy load... That sounds like a lot of work though, i will start with the tanks!
    You could just leave the speaker connected on this 'Reverb Tank EQ Amp, just to be safe. You're using this amp's Volume control (and Master, if it has one) to set the drive level to the Reverb tank in the Twin Reverb. My main concern was the added acoustic level it may put out in the process. The stock Fender Reverb tank presents a 10 ohm nominal load impedance to the amp, though I've never swept the tank to see what it's impedance characteristic looks like. The load is for a tube amp, if that happens to be what extra amp you have to try this with. It it's a solid state amp, though, then you don't need to load it with speaker or dummy load. At the very low level you'd be hitting the reverb tank with, I think it would be sufficient as a speaker substitute. Just remember....this extra amp has a lot more power output than what appears at the Fender Twin's RCA Reverb Output jack that feeds the tank. That's why I suggested presetting the Fender Twin's Reverb control around mid (4-5), and bring up this external amp's volume slowly to find the appropriate drive level. We're basically making this amp unity gain, relative to the nominal drive level within the Twin Reverb.

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    This article https://www.amplifiedparts.com/tech-...d-and-compared could help to find a suitable tank.
    Your TR tank should be the Hammond/Accutronics type 4AB3C1B, having a nominal input impedance of 8 Ohm@1kHz (DCR = 1 Ohm) and an output impedance of 2.250 Ohm@1kHz (DCR = 215 Ohm). Any replacement should have matching values, especially regarding input impedance (meaning 8 or 10 Ohm).

    It seems that keyboard players prefer shorter decay time than guitar players. The re-201 has a short reverb tank, so maybe this the direction to go.

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