Gibson Duomedalist Tremolo

[martyblackeye]

Hey fam, quick question for everyone regarding the tremolo photocell (roach).

I re-capped this amp for a friend and everything is looking great except the tremolo. I replaced the photocell with this.

With new roach in place, I get steady trem but the signal is very quiet in comparison to the normal channel. When I put a scope at the phono plug (pin 6 of the pre-amp tubes. The normal channel is screaming, but the trem channel indeed has trem, but is very quiet.

when the roach is unsoldered from that .22uf cap (along the signal path leading to the phono plug and eventually the phase inverter), I get full volume trem channel, but no trem. It appears that this new roach is sucking alot of the volume from the trem channel.

My question is, does this duo-medalist take a specific photocell? I'd say I wired in in-correctly, but when in place I get some decent tremolo, just low volume. It's wired so the dot (optical side) is connected to HT and pin 6.

Any advice is much appreciated. Cheers~

[Helmholtz]

Schematic?

[g1]

Schematic attached (may not match actual unit as per usual Gibson)

[Helmholtz]

Any improvement if you cover the chassis/roach to prevent any ambient light getting to the roach?

[martyblackeye]

Didn't try covering the old roach to see if it functioned better. It is completely wrapped up though. Might try lifting the ground to see if trem works and still get full volume. With new roach grounded I get maybe 2/3 watts, whereas the normal channel is pushing 45w.

[Helmholtz]

What is the dark resistance of the new roach LDR?

How is the volume with the light source of the roach disconnected?

[loudthud]

The schematic in post #3 shows two Depth adjustments. I think the VR10 pot is on the front panel, the VR11 pot is an internal trim pot to set an operating point that is in the right range for the light bulb. The low Volume means the light is too bright. You may need to add resistance in series with VR11, start with 470 Ohms, then go to 1K.

Note: A Fender roach with a neon bulb probably won't work without some modifications. Find a Blackface schematic and copy that.

[martyblackeye]

Okay, I'm getting some more information. It looks like I can either modify the existing circuit to play nice with the new LDR or I take apart the old photocell and replace the bulb. If anyone is curious I found this information here. If anyone has experience replacing the bulb, give me a shout. Otherwise I'll post my progress...

[Tom Phillips]

Here is some information I collected by making measurements on the bulb that Gibson used in their incandescent tremolo bug.


The attached photos show the parts inside a Gibson assembly number "LDR-500" from a ~1965 Gibson GA-77RET Vanguard


The bulb on the right is original and the wires had corroded off close to the base. I was able to salvage enough of the stubs to attach the red lead wires you see in the photo. It’s barely visible in the photo that the bulb had the number “24” printed on it and I don’t know if additional markings have been worn off. Actual current draw with 24V applied to the bulb was 18mA and the filament glowed bright orange rather than white hot. Since I salvaged the old bulb I didn't need to source a replacement so my efforts stopped there and some research will be required to find an equivalent currently available bulb.

The LDR on the left was originally inside a glass capsule. The dark resistance is ~10MΩ long term and ~1MΩ short term. Under light the resistance is ~1kΩ. I attached the green wires to salvage the part.

You cannot substitute a neon bulb in the trem circuit used in Gibson amps unless you modify the circuit. The use of an incandescent bulb with the proper "pre-heat" adjustment via the 500Ω pot in the driver tube cathode circuit gives a nice smooth tremolo rather than the choppy sound of the Fender neon bulb circuit. I have never been able to determine the part number and specifications for the original incandescent bulb. Since the one original bulb only produced the orange glow with 24 volts applied, it seems to me that the voltage rating must have been higher than 24V. I didn't experiment further at the time because I had fixed the problem and I didn't want to add any further stress and take the chance of damaging the only bulb I had. For the same reason, I haven't taken apart the working light bugs in any other Gibson amps. The wire connections are really fragile and the bugs are bonded into metal tubes. It would be really risky to mess with a working example.

Cheers,
Tom

[loudthud]

Looking through an old Allied catalog from 2008, I see a couple of bulbs that could be the right one, except for the leads. Its one of those listings with a couple of rows of pictures across the top, then a long listing with a column for illustration number. Nearly all have some kind of base. It would be helpful to know the diameter of the bulb in the picture.

[Tom Phillips]

... It would be helpful to know the diameter of the bulb in the picture.

Oh Boy. I did not think to measure the size when I did the work referenced in post #9. I'd estimate that it was between 7/32" and 1/4".

[loudthud]

There is something known as a 24MB, 24V 70mA It has the flat top and a bayonet base like a Fender pilot lamp. Diameter is 8.3mm or .33 inch. Same lamp sold by multiple manufacturers. There is also a 28MB, 28V 40mA* and a 60MB 60V 50mA*.

* Numbers don't make sense but I double checked them and that's what it says.

[Mars Amp Repair]

I found these indicator lamps that were used in old style multi line Bell phones. I needed one for an old Heathkit Audio Generator. It looks physically just like the bulb you have in the Gibson bug. They come in different voltages/currents.
An example from Mouser PN: 606-CM24X. I found them below by searching 'Telephone Slide'.
You might be able to removed the side connection plates & have it fit in the bug cylinder.
The one I needed for the Heathkit generator was a low current (can't recall the current) bulb used as a ballast for the oscillator.
Just a thought....glen

[martyblackeye]

The telephone slide bulbs may work, my only concern is that the highest they're rated is 120v, whereas the schematic is asking for 180v and in reality that power supply reads closer to 200v. I'll check if the LDR is still functioning. This reproduction LDR lamp looks like someone is producing the incandescent style tremolo roaches that replace the ones in the Gibson circuits. very cool~

I'll lyk how it goes

[Tom Phillips]

The telephone slide bulbs may work, my only concern is that the highest they're rated is 120v, whereas the schematic is asking for 180v and in reality that power supply reads closer to 200v....

There are other factors at play in the circuit that influence the actual voltage that is applied to the bulb. Note that the bulb is in series with one of the triode sections of V5 and the 500 Ohm tremolo depth adjustment pot. The 180V supply voltage is divided across the bulb, the tube and the pot. As the circuit operates, the effective resistance of the tube changes causing the bulb to change brightness. The tube resistance never goes low enough to over voltage the bulb. For example, the voltage at the top of the bulb would be 180V and the voltage at the bottom of the bulb may vary between 170V and 155V. Under those conditions, the voltage across the bulb remains between 10V and 25V. That's why the relatively low voltage bulb can survive undamaged.