Need Help - Gibson Amp Tremolo

[rjgtr]

I'm fixing a Gibson GA77RVT. I've got it working except for the tremolo. The LDR assembly is bad and I need a replacement.

Any ideas?

Can I get a Fender LDR to work? If so how. I've tried it, but still no joy.

Any help is appreciated.

[Firestorm]

Are you sure the LDR itself is bad? I would first replace all the caps in the phase loop, plus the bypass cap on the oscillator tube to make sure there is solid oscillation. As to the LDR itself, Gibson used an incandescent bulb, Fender used neon. I don't know what the neon's firing voltage is (maybe 120V even though it's sitting at a hotter place in a Fender), but you could try wiring the bulb circuit on the Gibson to match the Fender and see if that works. Gregg Hopkins of Vintage Amp Restoration makes a replacement for Ampegs that might work, too, but it is housed in a 1/2" metal pipe for shielding and likely wouldn't fit.

[rjgtr]

Thanks for the response. Yes I'm sure the LDR is bad, a wire is completely out of the unit. I'm trying to not re-engineer the tremolo circuit too much. I have tried the Fender and it appears the off resistance of the photo-resistor isn't very large.

As for the Ampeg replacement unit - I have used those for Ampeg repairs, but at $35 a pop and not knowing if it will work, I'd rather find another solution.

I'm going to try some other things, but any ideas are certainly appreciated.

[Phil_S]

Aha, the impossible to get opto-isolator for the Gibson amp. I have researched this part extensively and have concluded that it surely is no longer available. This is a LDR-incandescent opto-isolator. LDR=light dependent resistor, usually a CdS based item. (Radio Shack used to sell a small sampler bag of these but I don't know if they still do.)

Forget about the parts used by Fender and Ampeg -- these require a redesign of the circuit to function.

My research is for a GA20-RVT Minuteman, but I'm going to assume this is the same part. Voltages and design look very similar, but I did not make a side by side comparison.

The probability is that the light bulb is burnt out. Your alternatives include A) attempting to replace the bulb, B) building one of this using an LDR, black heatshrink, and a bulb, or C) redesigning the tremolo to work without the opto-isolator. OK, you can imagine some variations on these choices.

For option A) the challenge is finding a bulb rated at voltage and mA appropriate to the circuit. I think the bulb should be a 12v bulb with a 20mA rating, give or take a bit. I think the 40mA rated bulbs are just wrong for the circuit. You might find one in an automotive catalog. I've been over the CMI (Chicago Miniature) line of incandescent bulbs and have concluded that some *might* work. So, I haven't tried to replace the bulb. Also, it requires you to somehow open the metal cylinder without destroying it.

For option B) you've got all the challenges of A) and then you must try to find the correct LDR by trial and error. I haven't tried this either.

For option C) you can look at something like the Fender VibroChamp AA764 circuit. I think this is a real possibility, and it looks like it wouldn't take much to get it done. Then, you are worry free. Not authentic, but an excellent tremolo. Since I've heard both, I can tell you that doing this is not a mistake and it is surely close enough.

I know this is long... when mine broke, I think I got lucky and located the last spare on the planet. It took a long time, and I was incredibly lucky to cross paths with someone who actually just gave it to me. Beforehand, I did all that research.

I posted for help on a uesnet NG. I'm attaching my documentation from that inquiry, as I managed to get a couple of engineers to look at it. Note that the initital discussion of a 115v bulb just isn't right, but you can see how the discussion progresses.

Wishing you the best of luck on this. I'll be happy to accept email on this topic if you think I can be helpful My address is psymonds_no_spam@comcast.net, just remove the _no_spam. Please remember that I am strictly an amateur and have no formal engineering or electronics background.

[rjgtr]

I just wanted to report back that I found a solution.

I took a chance and bought several 12 volt lamps and the 5 pack of photo resistors. I hooked the lamps up to see which worked best and picked that one. It is RS part #272-1141. I picked the medium sized photocell. I used heat shrink like the Fender LDR. I also had to resolder the footswitch and while I was at it I put a wire tie on the cable inside the footswitch to keep the wire from being pulled free again.

It wasn't my amp - I was repairing it and making it more gig worthy for a customer. Someone had done a cap job and replaced the original 40/40/40 filter with a bunch of axial caps in series. They wire tied them all together and left them hanging. I ordered a new 40/40/40 from Antique and it cleaned up the mess.

It really is an interesting amp. There's no real distortion, even with humbuckers, but this is what Gibson was after. But you get that 50s/60s jazz sound instantly.

I have a 'tweed' GA-5 and 'tweed' Gibsonette and they do get great drive sounds, but also great clean sounds. It's funny that Gibson made the first popular electric guitar and amp, but by the time the 50s and 60s came along couldn't get their amps together. Probably because McCarty didn't like the bright sound so popular with rock 'n' roll and country players.

[Phil_S]

I would agree that Gibson amps of that era were very clean. I am very pleased to hear that you were able to build a solution and appreciate that you published the bulb choice. This is, perhaps, one of the things that really takes a bit of old school problem solving to get it done. Congratulations on a job well done.

[Richie]

keep an eye on it, i've seen some fixes like this where the bulb would not last over a week or so.. But hopefully you've solved the problem.

Also Jamico sells a large grab bag of photocells. with a lot of assortments of sizes and styles. A couple bags of these ought to last a person a few years of tinkering.

[rjgtr]

Richie, I was a little worried about it not lasting, but since the lamps isn't brightly lit, I think I'm Ok. Since it's a grounding tremolo, if it stops working it the rest of the amp will work fine without it. Then I'm sure I'll see it back.