Optoisolator for Gibson tremelo

[Tim Gagan]

Hello,
Does anyone know if a Fender tremelo optoisolator will work in the
tremelo of a Gibson GA 20RVT Minuteman? If not, is the proper
optoisolator available somewhere?
Schematic here: Loading Image...
Thanks!
Tim

[Phil Symonds]

Post by Tim Gagan
Hello,
Does anyone know if a Fender tremelo optoisolator will work in the
tremelo of a Gibson GA 20RVT Minuteman? If not, is the proper
optoisolator available somewhere?
Schematic here: http://www.drtube.com/schematics/gibson/ga-20rvt.gif
Thanks!
Tim

Tim,
That part won't work. There is a recent thread started by "Ylo" on 12/4/03
concerning the GA35RVT Lancer that he just bought with a broken tremolo.
You can catch up on a rather detailed discussion there. To summarize,
assuming the bulb is blown, I think you have 2 or three options:
1- try to replace the bulb in the existing opto
2- try to build a new one (not as hard as you think)
3- see if you can work with a Vactrol (maybe it's really get lucky).
I'd suggest you read Ylo's thread and then post your questions here.
BTW, I've got the Minuteman and know this amp rather well.
Phil

[OvrReactor]

Post by Tim Gagan
Hello,
Does anyone know if a Fender tremelo optoisolator will work in the
tremelo of a Gibson GA 20RVT Minuteman? If not, is the proper
optoisolator available somewhere?
Schematic here: http://www.drtube.com/schematics/gibson/ga-20rvt.gif
Thanks!
Tim

The Fender style optoisolator with the neon lamp is NOT a good sub. I've
successfully used Vactrol optoisolators in Gibson amplifiers, though. You WILL
need to do a bit of tweaking, since the incandescent lamp is being replaced by
an LED, and the volatge drop and current draw are different. Just assume that
the LED in the optopisolator drops 2.2 V and draws 20 mA, and adjust the
resistor in series with the (now burnt out) lamp accordingly. Make sure that
the resistor you choose will dissipate enough heat. I think I ended up using a
1 Watt resistor for this mod.

I tried to find my notes for the modification, but they've vanished (for now).
I just thought I'd chime in on this thread and let you know that a Vactrol WILL
work, and the next time I get a Gibson amp in for repair, I'll crunch the
numbers again and post the results. For now, you'll have to use a
calculator....

Best regards,
Tom Mitchell
website 1 - http://members.aol.com/ecc81
website 2 - http://www.over-reactors.com

[Jimmy]

Post by Tim Gagan
Hello,
Does anyone know if a Fender tremelo optoisolator will work in the
tremelo of a Gibson GA 20RVT Minuteman? If not, is the proper
optoisolator available somewhere?
Schematic here: http://www.drtube.com/schematics/gibson/ga-20rvt.gif
Thanks!
Tim

Back in the 70's a friend showed me how to build these from a light bulb and
photo resistor. It was super easy, just stick the parts facing each other in
a piece of rubber tubing. trouble is I cant remember the parts used. I bet
they are out on the net somewhere. Also if the bulb went bad it was realy
easy to fix. You can make them for Fenders too but I believe they use a neon
bulb.

[Aguyathome]

Post by Tim Gagan
Hello,
Does anyone know if a Fender tremelo optoisolator will work in the
tremelo of a Gibson GA 20RVT Minuteman? If not, is the proper
optoisolator available somewhere?
Schematic here: http://www.drtube.com/schematics/gibson/ga-20rvt.gif
Thanks!
Tim

No, the Fender part won't work, but can be modified to do so. I bought the
replacement Fender tremelo "roach" from Vibroworld.com. It has a LDR (light
dependant resistor) that registers about 4M ohms when "dark," or covered up to
seal out light. The bulb that comes with the Fender piece is a neon bulb that
is designed to run at about 380 volts, the Gibson unit, although also using a
resistor that reads about 4Megs when dark, uses an incansdescant bulb that runs
at about 140-150 volts. By cutting open the vibroworld part and heat-shrinking
the LDR and a mini incandescant bulb with the right voltage-handling ability,
you should be able to make a replacement. I just happen to own the same amp,
and have looked all over the planet for a replacement LDR, Sylvania part
#LDR500, and they can't be found anywhere. I bought the Fender piece, measured
the "darkened' resistance, and cut it open. Now I'm just trying to find a small
incandescent bulb that can handle something in the 150-175 or so voltage range.
Maybe if I find something good for 200 volts I can make it work. I'll let you
know.
By the way, I have all kinds of old Fender equipment that I love, but I think
the reverb in the Minuteman blows away anything built in to my old Fender
combos or my stand-alone '63 Reverb Unit. It has an unbelievable clean sound
that makes your hair stand up with a strat plugged in. I actually think it
sounds better with my strat than my Les Paul. Great little amp, but you'll need
to go through it and replace every last chocolate drop capacitor, since they
dry out and will sometimes literally crumble to the touch. Also, the three big
paper-covered filter caps (70mf, 20/20/20mfd and 20mf) will need to be
replaced. Once you go through it the bottom end and everything else will pick
up a great deal. They sound absolutely huge when recorded on my digital
8-track.

Take care,
JD

[Phil Symonds]

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