Tremolo bug in Epiphone EA-26 RVT

[pchilson]

Hi,

The Epi is the same as the Gibson GA-20 RVT Minuteman.
Link to schematic

I'm trying to find a replacement for this and I'm finding info to suggest the VacTrol VTL5Cx series but don't know which one to order.

Tubesandmore carries the VTL5C1 and the VTL5C3 and I've seen somewhere else to use the VTL5C4. The VTL5C3 has an extra lead over the other two.

Help! Which one do I get or is there another choice?

Thanks

[pchilson]

No one has ever replaced the oscillator in a Gibson amp?

[52 Bill]

I guess I never have replaced one of those.

Is the bulb burned out or is the LDR weak? The schematic shows a 10 volt drop across the bulb. I would think anything with a 12v bulb would be a reasonable replacement.

[pchilson]

Hey Bill,

Actually the leads were pulled free from one side of it. Not knowing what was inside this thing I pushed the leads back in and used a rubbery kind of glue to stabilize them. It apparently made contact cause the tremolo is working again but I'm not confident on its longevity cause of the damage to it.

I would just like to know what a compatible replacement part would be or some other part that can be made to work with mods. The bug in there is a Sylvania LDR-500 and apparently you can no longer get these.

Lots of Gibson/Epi RVT amps floating around out there and I guess not one tech has ever needed to repair/replace the oscillator so it looks like I am the ONE...

[Robert M. Martinelli]

What Bill said....
the schematic calls for a 10V drop across the bulb, this means you'd need to tweak the circuit a little to make a VTL5C3 fit in (the original opto-coupler has a bulb lamp as a light source, the VTL5C3 has an LED). All in all I think you'd better search for an old-fashioned bulb lamp-LDR assembly, or build one by simply taking a small 12V bulb lamp (same rating) and an LDR, put them face to face and use some black heat-shrinkable tubing to keep them together and insulate them from outer light sources. An alternative construction method is to take a wood dowel, make two holes at the two ends so that the bulb and the LDR barely fit in, place the bulb and the LDR in place and use some glue or sealant to keep everything together.
Hope this helps
Best regards
Bob

[52 Bill]

What Bob said...
If you dissect the original, you could become the supplier to the world of trem bugs for vintage Gibson/Epiphone amps.

[pchilson]

Ok, I need some help here.

I have verified that the LDR is operational, to what specs I don't know but when I hook it up to my DMM and go dark to light I see the change in resistance.
I have tried this 12v/25mA incandescent from radioshack and also this 12v/50mA incandescent from radioshack.

Out of the unit I get both lamps to light up hooked to two series connected 9v batteries no problems. The 12v/50mA I just stuck back in the casing and connected it back into the circuit and nada. Pulled it apart and put the 12v/25mA bare into the circuit. Nada again, won't light the lamp.

When I measure across the lamp I should see a roughly 10/12 v drop but I get the same readings with no drop and the lamp isn't lit. What am I missing? What gets the lamp lit?

Schematic

[pchilson]

New development. When I activate the tremolo footswitch I can read an approximate 12v "pulse" in the lamp circuit but the bulb fails to light.

Why doesn't the lamp light? Is the lamp of improper spec? I have no way of knowing what the spec should be as its an obselete part and nothing is noted on the schematic.

Any ideas?

[pchilson]

Any help please?

[pchilson]

The lamp lights off two 9v batteries in series but not of the amp with 146 volts across it. It does not burn out it just doesn't light.

Are the amps wrong for this application?

Sorry, I'm not an EE.

[52 Bill]

What voltage do you get at the cathode of the tube V5B? The schematic shows a typical 2Vdc. Have you tried to adjust R49?

The lamp is in series with the tube, so if the tube doesn't turn on, there is no path to ground for the lamp to light up.

[pchilson]

Hi Bill,

I have not measured voltage of the cathode on V5B (I'll get that later this evening). I have adjusted R49 and it does increase/decrease voltage on the plate of V5B.
I'll report back later on this.

Thanks for the help!

[pchilson]

Pin 8 of V5B I get 2.6 v and I can dial it with R49.

With the tremolo switch off I get the same reading on both sides of the lamp. With the switch on I can see some flucuation in the voltage but now only around 1 to 3 volts.

The lamp never lights.

I get approiate voltage readings in all other places in the amp with the approiate percent increase due to increased wall voltage.

Everything appears right but it the lamp never lights so something has to be wrong.

[52 Bill]

Maybe try a different bulb, rated at 3 volts.

Have you checked or tried a different tube?

[pchilson]

I have used all 3 of the 12AU7 in that spot. I haven't tried a 3 volt lamp but I guess I could. That defies logic though but maybe its the kind of thinking needed.
There should be a logical explanation to this, eg...the lamp is of incorrect spec, correct the spec and correct the problem. Only issue with that is there is no way to know what the correct spec is.

It seems like such a simple design but I don't posess the knowledge to debug it. That is why I've come to where the tube amp circuit experts live but can't seem to get anyone to jump in here save for you Bill which I do appreciate. Seems a waste and illogical to live without this part of the circuit because of one obselete part failure.

Just venting here... Thanks for your input.