Ampeg Gemini G15 No Tremolo

[Perkinsman]

I'm working on this cool mid 60's Ampeg with a 15" speaker which sounds pretty good but the tremolo isn't working. Is the brass wrapped axial wired component an optoisolater? If so how can I test to see whether it's working and more importantly what is the replacement if it's not?

[52 Bill]

You can see that one end of the tube is bayonet based light bulb and there probably is a photo cell on the other end. If you look at the schematic for the amp, you might be able to tell what kind of bulb is in there and how it is wired. A lot of Ampegs used neon lamps in the trem circuit, but I suppose that they could have used an incandescent lamp in some of the amps as well.

If the bulb is filament based, you should be able to read a resistance across the filament terminals. If it is a neon lamp, there will not be a resistance across the terminals.

[Enzo]

Schematic?

Sticking out one end of your cylinder, don't you recognize the base of a light bulb? Brass, and has the little peg sticking out for the bayonette base holder. It might even be home made, I personally do not recognize it as a commercial unit, though it might be.

I found a Joe Piazza drawing, always hard to trust, and it shows a neon and photocell opto like a Fender roach. NE-51 maybe?

It is a neon lamp, and a photocell, each being independent electrically. But we are getting ahead of ourselves, we have decided to worry about the opto before we have any idea what the issue might be. A loss of trem can be due to anything from the trem tube not oscillating to an open resistor to a bad lamp in the bug, or a bad photocell in the bug or wiring to the signal path or a problem with the depth pot. The trem tube could be bad, could be not lighting due to missing heater current, to a bad stomp switch to turn on the trem, or faulty/leaky caps around the tube.

So we find out what is happening. Is there a large trem signal at the plate of the tube? Is the tube heater on? Is it getting B+? From the plate is a 390k resistor down to the lamp, other side of lamp to ground, so if we have trem at the tube, do we also still have it at the lamp? Turn off the room lighting, and see if you can see any evidence the bulb is flashing inside that cylinder.

[vintagekiki]

https://elektrotanya.com/ampeg_gemini_g15_amplifier_sch.gif/download.html

[Jon Snell]

It is a neon and an LDR. Check the neon has voltage across it when the
tremolo is engaged.

[g1]

Here is a factory schematic for the 7591 version of the G-15 Gemini II.

[Enzo]

Thanks.

[drewl]

Most of the time the caps in the oscillator become leaky.

As Enzo said, check for a large AC signal going to the lamp.
It should vary in speed and voltage with the trek controls.

[g1]

Most of the time the caps in the oscillator become leaky.

And the cathode bypass cap is often overlooked as being a critical part of the oscillator.

[Perkinsman]

[QUOTE=g1;n912960]
And the cathode bypass cap is often overlooked as being a critical part of the oscillator.

Is that the 25uf cap on the V7 cathode?

...and thanks for posting the schematic, I couldn't find one that was the same as mine.

[vintagekiki]

Check that the neon bulb (TM1) lights up in the rhythm of the oscillator frequency.
If the neon bulb lights in the rhythm of the oscillator frequency, check whether the resistance of the photo-resistor (point x y) changes in the rhythm the neon bulb.

Before replacing any passive components of the tremolo oscillator, replace the existing tremolo oscillator V7 tube with a new one (not like a new one), and measure all operating voltages. You know the rest ...

[g1]

Is that the 25uf cap on the V7 cathode?

Yes.

[tubeswell]

Schematic?

...
I found a Joe Piazza drawing, always hard to trust, and it shows a neon and photocell opto like a Fender roach. NE-51 maybe?

...

Here ya go - says the roach is a 'Model TM1 Factory Replacement Only. Patent Pending' as if that's any help. At an outside hunch, sticking a Fender roach in there may need a slight tweak in other parts nearby - or not.

[Perkinsman]

Schematic?

Sticking out one end of your cylinder, don't you recognize the base of a light bulb? Brass, and has the little peg sticking out for the bayonette base holder. It might even be home made, I personally do not recognize it as a commercial unit, though it might be.

I found a Joe Piazza drawing, always hard to trust, and it shows a neon and photocell opto like a Fender roach. NE-51 maybe?

It is a neon lamp, and a photocell, each being independent electrically. But we are getting ahead of ourselves, we have decided to worry about the opto before we have any idea what the issue might be. A loss of trem can be due to anything from the trem tube not oscillating to an open resistor to a bad lamp in the bug, or a bad photocell in the bug or wiring to the signal path or a problem with the depth pot. The trem tube could be bad, could be not lighting due to missing heater current, to a bad stomp switch to turn on the trem, or faulty/leaky caps around the tube.

So we find out what is happening. Is there a large trem signal at the plate of the tube? Is the tube heater on? Is it getting B+? From the plate is a 390k resistor down to the lamp, other side of lamp to ground, so if we have trem at the tube, do we also still have it at the lamp? Turn off the room lighting, and see if you can see any evidence the bulb is flashing inside that cylinder.

The lamp measured over 1M ohms and 74vdc across the terminals. V7b (new tube), pin 6 (plate) measures 319vdc with trem off or on...it should be 205/135vdc when tremolo is off/on, so this is reading high. With lights off, I can't see any light coming out of the cyclinder the lamp and ldr are packaged in (TM-1 module referred to on sch). Replaced the 25uf cap. I see a 390 on the sch but don't find one off V7 pin6.The 3 .047 caps measured within 10% except for the .05 cap, which is off by 50%. V7 heaters tested at 3.5v.

Does it sound like the module is not working and if so, can I use a standard Fender optocoupler to replace the entire TM-1? The bulb doesn't want to come out easily which makes me believe that this was sealed unit. I found a company fliptops.net that sells a replacement TM-1 but it's $40, lol.

[tubeswell]

Try a Fender roach and see. There will be ways of getting it working.

Or change the circuit to a vactrol trem - much simpler, works wonders, and parts are cheap