Making a Twin into a true stereo amp

Jukers

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So I've done some searching and haven't found the answer I'm looking for. Also I'm still learning about amps and building/repair. I am a novice so go easy on me.

I had a brain storm and was hoping you could pull me from or push me off this ledge. Is it possible to make a Fender Twin (4 x 6l6, 6 x 12a_7) into a true stereo amp? Essentially a 1 channel Pro Reverb (2 x 6l6, 4 x 12ax7) and a 1 channel Pro (2 x 6l6, 2 x 12a_7) using a single 12” speaker for each.

My conversion thoughts so far:
You would need a separate output transformer for each side
The power transformer is big enough to run the 6.3v heaters
The rectification would work for all the tubes

Questions I am unsure of:
Will the grid stopper resistors isolate the B+ so that there is no signal bleed between the two sides?
Can I use a single B+ to run two separate amps?


I think it would be rather cool to be able to have that perfect Fender tone and run Stereo, Wet/Dry, or Dual Mono (whatever you want to call it) out of the same amp. I know it'll be heavy and that's ok with me.

Also, if they made tube stereos for home audio, there must be a way. Did they have a different style of power transformers?
 
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Jukers

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I guess it would be more like a 1 channel "Bassman" Reverb (2 x 6l6, 4 x 12ax7) and a 1 channel "Bassman" (2 x 6l6, 2 x 12a_7) using a single 12” speaker for each, due to the diode rectifier vs a tube rectifier.
 

Dacious

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You could. It'd probably be easier to start from scratch. The Roland Jazz Chorus 120 is a true stereo amp, it has two 60 watt power sections that each drives a speaker. When you play in chorus mode it sends the dry signal to one and the time-modulated to the other.

I honestly don't know if it'd pay off. You'd need the input split into stereo channels at the first tube section you come to. You might hear a difference in the room with speaker separation but if it's all coming our of one amp probably not.

As usual - look around the world and see if anyone is attempting this. Probably not, which tells you it's not worth the hassle and complications.
 

Willie Johnson

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I've thought that it would be kinda cool to stuff my two Princetons into a 2x12" combo cab together (maybe get an old solid state Frontman or Champion as a donor amp)...but two combos are easier to carry and more versatile. In this instance, it would probably be easier and more practical to sell the Twin and get two of the new 1x12" custom Silverface Pros instead.
 

Jukers

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Well, how many people do you see playing Twins live on stage? They are typically too LOUD. They are only a few lbs heavier than a Super and I have no problem carrying those around. I figure that IF a Twin could be a true stereo amp, maybe 2 x (1x12 Bassmen) in a Twin carcass would be more useful, therefore bringing life back to typically unused amps.
 

corliss1

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Doable? Sure, anything is doable, but it would be a MASSIVE undertaking where you're pretty much only keeping the PT, chassis, and cabinet. Most all the circuit needs to go or at least be physically rearranged in order to do what you want. I second the recommendation that it would be better to start from scratch.
 

EsquireBoy

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You could. It'd probably be easier to start from scratch. The Roland Jazz Chorus 120 is a true stereo amp, it has two 60 watt power sections that each drives a speaker. When you play in chorus mode it sends the dry signal to one and the time-modulated to the other.

I honestly don't know if it'd pay off. You'd need the input split into stereo channels at the first tube section you come to. You might hear a difference in the room with speaker separation but if it's all coming our of one amp probably not.

As usual - look around the world and see if anyone is attempting this. Probably not, which tells you it's not worth the hassle and complications.
Usually the amps that bother to be true stereo have some kind of stereo effect: chorus for the JC, vibrato for the Magnatone Panoramic stereo.
I agree with @Dacious : a stereo twin would probably not make a real difference unless the speakers are put away from each other.

That being said, it could be of some interest if you would put the tremolo on the side that has no reverb.
Tremolo and reverb on the same side would attempt a wet-dry thing, but with the speakers so close to each other, I doubt it would really be interesting.
But tremolo on one side and reverb on the other one, would potentially end up with some kind of chorusing effect, which could be very cool.
 

radiocaster

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Interesting idea, but
1. I could be wrong, but I'm thinking you would need two phase inverters.
2. I know Jazz Chorus amps are popular, but I think the stereo separation with 2 speakers close to each other sucks.

Would make more sense to take a Deluxe Reverb, remove the phase inverter and power tube circuits, then run two separate single-ended cathode biased 6L6s and an extension cab. Probably would make more sense (financially) to do a kit/scratch build than modify an existing amp, even though an existing amp would already have a usable preamp.
 

printer2

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Yes it can be done. The normal channel stays as it is from input to output transformer, swap out a smaller OT than the Twin. The other channel goes to the vibrato tube which is converted to a PI. There is enough solder points to build the PI in the tremolo section. This then goes to the second set of 6L6's that are disconnected from the other set. Then this goes to the other new OT. Will have to add a NFB resistor, also need another pot as a bias control. The two OT's will fit if you move the doghouse over. A competent tech could do it in a day. Someone who is a novice might not get the wiring right in terms of noise or oscillations. But in terms of doing it, no big issue.
 

Wally

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Won’t the two channels be out of phase due to the extra gain stage in the reverb ‘amp’? Also, the stereo effect is best when the speakers have a separation of at least six feet. I do not know if the difference between building the proposed amp and running the wet/dry options into the two channels of the TR as it is.
I am a stickler when it comes to ‘stereo’. I consider a stereo guitar amp to be such that both channels are exactly alike. I keep two Fender Super Champs because I own a stereo ES345TDC. I need an amp for each pickup, and I want those amps to be identical.
Ommv.
 

schmee

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I dont know that you would get much stereo effect with the speakers so close together.

I thought about doing similar to this for a time but not stereo, I wanted a separate Tweed Deluxe in the normal channel so I could switch to that for solos and have "Hotel California" tone! And a 2 x 6L6 in the other channel. But for that I needed a separate rectifier, cathode bias etc too... I didn't care about retaining reverb though at all.... so plenty of sockets...
 

Jukers

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Yes it can be done. The normal channel stays as it is from input to output transformer, swap out a smaller OT than the Twin. The other channel goes to the vibrato tube which is converted to a PI. There is enough solder points to build the PI in the tremolo section. This then goes to the second set of 6L6's that are disconnected from the other set. Then this goes to the other new OT. Will have to add a NFB resistor, also need another pot as a bias control. The two OT's will fit if you move the doghouse over. A competent tech could do it in a day. Someone who is a novice might not get the wiring right in terms of noise or oscillations. But in terms of doing it, no big issue.
Thank you so much! I didn't think about the negative feedback resistor. Having two different values of negative feedback might change the sides just enough. Good looking out man!
 

Jukers

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I dont know that you would get much stereo effect with the speakers so close together.

I thought about doing similar to this for a time but not stereo, I wanted a separate Tweed Deluxe in the normal channel so I could switch to that for solos and have "Hotel California" tone! And a 2 x 6L6 in the other channel. But for that I needed a separate rectifier, cathode bias etc too... I didn't care about retaining reverb though at all.... so plenty of sockets...

In some of the venues I play, the PAs have a stereo capability. If I run two mic's, one for each speaker, I could get a good effect at the venue itself. It also enhances the clarity of effects. Maybe I'm looking for a little bit more wet/dry versus stereo.

For you, I had a buddy do something similar. He took a Bassman and made the bass channel into a Marshall tone stack. It's a Rob Robinette mod and it worked beautifully. You could use an AB switch and change tone on a dime. You can do that mod easily on any of the dual input Fenders.
 

dsutton24

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That's a pretty ambitious project. If you're going to mic the amp anyway, why not build two identical amps into one cabinet? Maybe have an option to connect an external speaker to one of the channels?
 

middy

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Just buy another twin so you can set it 10 feet away and get true stereo separation.
 

wabashslim

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So I've done some searching and haven't found the answer I'm looking for. Also I'm still learning about amps and building/repair.
Are you by any chance the same fella who was wanting to remount his Twin chassis vertically, like a Tweed or Hot Rod Deville awhile back?
 




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