reverb unit within guitar body?



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reverb unit within guitar body?

Postby repoman » Sat Sep 19, 2020 1:05 pm

There is a guitar model built by Wandre in the early 60s called Polyphon, these guitars had an internal spring reverb effect that mystifies me. At first I thought the guitar was one of those kooky esoteric 60s contraptions where a separate reverb effect box was accompanied by the guitar and the guitar had switching which communicated to the box, but I recently found gut pics and there is actually a spring reverb unit within the guitar.

Image

Image

You can see the black tube is a reverb tank.



Here is a video of someone playing one, it sounds pretty good!

I can't see a battery? When were JFETs introduced? How would this system be powered?

Anyone have any ideas on how this system worked? It looks like the large staggered pole pieces in the center of the guitar have to do with the reverb and the pickups are under the plastic in bridge and neck positions.

Image
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Re: reverb unit within guitar body?

Postby eatyourguitar » Sat Sep 19, 2020 9:44 pm

I looked into this but I guess all I can figure out is that there is a custom inductor or transformer inside. the spring driver does not exist so therefor passive. therefor the recovery is also passive like the pickups. I am not %100 sure. it does look like it is possible that there is a stereo jack but I have no idea what or why. this is 1964. a transistor would be expensive compared to a cheap vacuum tube. it does not look like there is any transistor inside. pulling on the wire in the photo is pretty cringe. one more clue is that magnetic pickups do nothing when they are far away from the strings but a resonator plate works ok. having those 6 metal dots exposed means that they will not be muted by the plastic. having them separate is probably to prevent it from sounding muddy on chords.
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Re: reverb unit within guitar body?

Postby qersty » Sun Sep 20, 2020 7:08 am

Maybe there is a pickup on the spring? Strats sound verby sometimes cause of the vibrato im sure the body resonance would be enough to slop the spring into reverberation
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Re: reverb unit within guitar body?

Postby repoman » Sun Sep 20, 2020 9:57 am

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Re: reverb unit within guitar body?

Postby eatyourguitar » Sun Sep 20, 2020 10:01 am

Yes exactly. The transducer directly connected to the spring is the pickup. The spring is driven by vibration from all directions. The exposed metal dots are directly connected to that big metal frame. All passive. The spring output is wired to a volume pot in the photo. If that was an active driver then there would be no pickup or the reverb would vibrate the body and bleed into the main pickups. That is an alternate theory.
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Re: reverb unit within guitar body?

Postby repoman » Sun Sep 20, 2020 10:11 am

Any idea what that big thing which seems to have a potentiometer on the top is? That seems to be the pot he turns to increase the amount of effect/dwell.
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Re: reverb unit within guitar body?

Postby crochambeau » Sun Sep 20, 2020 10:27 am

A host of posts has made my contribution unnecessary, but I typed it out so you can all read it anyway, if you want to. haha

---------------

Yeah, it would appear as if the reverb tank is itself employed as a transducer for guitar output. You can hear push button selections actuating reverb at points.

To emulate this you would probably want as much isolation as you can manage between the solid/metal neck and the body of the guitar, as bodies are vibration sinks (want to conserve as much energy to move the springs as possible) and minimize handling noise. Hollow body is optimal.

..and yeah, they could have routed that wire in a much easier to service way, my guess would be on questionable service history or a well meaning tech tip toeing around a relic and botching assembly order.
D.o.S. wrote:This thing is like the Blue Box on the amount of acid that lands you in the ER pretending to play it straight while you try and ignore the fact that the walls are dripping oil.


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Re: reverb unit within guitar body?

Postby repoman » Sun Sep 20, 2020 10:31 am

Image
Tons of insulation in the body.

I can't see any wires attached to the odd set large pole pieces in the middle, looks like two white wires would be going to the pickups.
Last edited by repoman on Sun Sep 20, 2020 10:35 am, edited 1 time in total.
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Re: reverb unit within guitar body?

Postby eatyourguitar » Sun Sep 20, 2020 10:35 am

repoman wrote:Any idea what that big thing which seems to have a potentiometer on the top is? That seems to be the pot he turns to increase the amount of effect/dwell.

it is a variable inductor.
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Re: reverb unit within guitar body?

Postby crochambeau » Sun Sep 20, 2020 10:36 am

repoman wrote:Any idea what that big thing which seems to have a potentiometer on the top is? That seems to be the pot he turns to increase the amount of effect/dwell.


Possibly a simple mix or spring volume. That is an enormous can though, I would not be surprised to find an impedance transformer working in tandem with that control to better match typical values/loads of passive pups.
D.o.S. wrote:This thing is like the Blue Box on the amount of acid that lands you in the ER pretending to play it straight while you try and ignore the fact that the walls are dripping oil.


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Re: reverb unit within guitar body?

Postby repoman » Sun Sep 20, 2020 11:16 am

So do you think it is something like a piezo pickup attached to the end of the reverb spring (maybe that bronze plug looking thing surrounded by foam in the end of the tube), the spring is driven by resonance of the body/strings working their way into the spring and then that is mixed into the signal with the potentiometer under the inductor? What purpose would the large staggered pole pieces and variable inductor serve? (Sorry if these are bad questions, I have a very rudimentary understanding of electronics)
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Re: reverb unit within guitar body?

Postby eatyourguitar » Sun Sep 20, 2020 11:24 am

If it is a transformer then it would be to jack up the voltage or to drive the spring possibly. It could also be a tapped inductor like a pultec eq. The brass thing looks %100 like a piezo pickup. Brass is a superb conductor of sound because it has a lot of mass per volume. As a result, piezo crystals get mounted to paper thin brass disks. You can see where that disk was machined on a lathe so that would contradict the brass sheet metal theory. It could be a brass j hook. Then the brass rests against a foam ring. That all makes sense to me. The spring is coupled to two other springs coupled to the tube coupled to the neck.
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Re: reverb unit within guitar body?

Postby eatyourguitar » Sun Sep 20, 2020 11:30 am

Maybe a piezo into a 50K pot into a 500 Ohm coil driving a 5K output coil. That should work.
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Re: reverb unit within guitar body?

Postby repoman » Sun Sep 20, 2020 11:50 am

Cool! Thanks for the input, ill have to try making something like this for a future aluminum neck guitar build. Maybe I can put the spring inside the neck itself.
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Re: reverb unit within guitar body?

Postby eatyourguitar » Sun Sep 20, 2020 4:47 pm

There are 3 types of transducers. One of them is rotational for feedback immunity on a loud stage. This is what you have in an amp. The other two are the same but it depends what direction you are inducing and transducing your energy relative to the spring.
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