Ampeg Gemin 1 G12 Tremelo Problem

[plexi50]

I have a 1966 Ampeg Gemini I G12 with a tremelo problem. Ive gotten pretty good over the last few years finding and fixing trem problems. But this one has me on a goose chase. I have oscillation at the .047 trem caps but no intensity. If you notice the intensity pot is tied in with a small box that looks like some sort of resistor opto thing. Not sure what to call it.  Speed pot functions fine from slow to blazing fast. I am hearing this using my Hoffman signal tracker. Any wild idea? Pic below. Also this amp must be an earlier G12 model as it is using 7868 power tubes instead of 7591 power tubes.

[sluckey]

some sort of resistor opto thing

Good chance that's the problem. Measure resistance across points X and Y (INT pot outer lugs) with the amp on and trem engaged and intensity set to max. Does the resistance change in beat with the trem? If not, replace the opto coupler. A Fender style roach should work. Or you can buy an expensive one from Flip Tops.

Do you find that the INT pot adjusts the volume of the amp? If so, that's another indication that the opto is bad.

[plexi50]

Intensity pot has no effect on volume at all.  Checking resistance readings now.

[plexi50]

13.50K measured from outer intensity pot terminals with amp on and trem engaged and intensity set to max.

[sluckey]

I'd say the opto is bad.

[plexi50]

I'd say the opto is bad.

I was wondering if i could use a fender roach i have here. The voltage feeding the ampeg roach (so to speak) has only 60VDC feeding it though

[plexi50]

Fliptops Vintage Gemini 1 trem opto.

https://www.fliptops.net/catalog/p-100087/tm1m1-r-tremolo-module-for-vintage-ampegs

[HotBluePlates]

... A Fender style roach should work. Or you can buy an expensive one from Flip Tops. ...

[sluckey]

The voltage feeding the ampeg roach (so to speak) has only 60VDC feeding it though

Surely it's more than that! A NE-2 neon bulb wont flash with only 60V. Schematic shows much more than 60V.

http://el34world.com/charts/Schematics/files/ampeg/Ampeg_G12_.pdf

Just stick the Fender roach in it. Or, if you have a NE-2 connect it and see if it will flash.

[plexi50]

Iam thinking from my part values on V1A & V1B and other preamp values i have the wrong schematic. I wouldn't think that having 7868 power tubes instead of the 7591 power tubes as shown on the schematic would make this a different preamp from the G12 Gemini I. I'm beat and knocked out a 383 Titano and 147 Maggie today. The voltage shown on the G12 schematic is plenty Opto gas. I have to gas up myself and go through this later. Do you know of a Ampeg Gemini 1 G12 showing 7868 power tubes? The one on the back panel of this amp is half gone. What a shame. Babbling out loud at this time.

[sluckey]

Don't matter.

[plexi50]

Don't matter.

I AM GOING TO UNSOLDER THE TREM VOLTAGE FEED OF 60 VDC TO THE OPTO AND SEE IF THE VOLTAGE JUMPS UP TO 190 VDC ON THE BOARD.
Sorry about the caps/

[DummyLoad]

i tried the fender trem roach in mine and was not satisfied - it sucks off too much signal. left it out. have not tried the fliptops LDR.


--pete

[plexi50]

Some part is sucking my voltage down in the tremelo circuit. Tubes are good. Cathode caps are all good. It's pretty straight forward yet i am having a hard time with the voltages. I lifted the 60 VDC feed to the opto and it went up to 160VDC. Even so the voltage should be a little higher on the 12AX7 preamp tube. Tinker time again/

[terminalgs]

using these voltages on V7B plate:   "ON"=130V "OFF"=205V.  The off position opens the cathode resistor to ground, so the triode will no longer draw current so voltage at the plate junction goes up to 205V, and the neon will light up and stay lit..   The neon's steady-state of the vibrato-off position is to be "lighted".  hence, the LDR's vibrato-off steady-state is to be small-resistance (as apposed to dark = high resistance).  The intensity pot is a 5M pot in parallel with the LDR, so when with vibrato-off, the volume of the channel should always be "loud". 


If we guess the LDR's dark/light resistances to be 5M/5K  then "off" i.e. "light" 5M||5K=4.9K  and "on" i.e. "dark" 5M||5M=2.5M.  If you turn intensity pot all the was to "off", then there is no resistance.

• If your 5M pot has failed such that it is a zero ohm pot, then the result would be no vibrato effect on the signal.
• If your 5M pot is good, but the neon is dead, then the result would be a very low signal when intensity is turned up, and loud when intensity is turned down. 
• If the LDR and the neon are 'good' and the 5M pot is 'good', and sweeping the intensity pot back and forth has no effect on volume, then the tube isn't oscillating (for example the footswitch is open).

If you have a fender roach, cut the wrapper off with a knife and install the neon.  watch to see if it lights up, and it is flashes with vibrato is 'on'.   you can always put it back to together with some wire shrink wrap.

[jjasilli]

If I understand, all voltages feeding the trem circuit are good.  Hence the problem is within the trem circuit.  FWIW, the Dan Torres approach to a trem issue is:it's easier & faster to rebuild the entire trem circuit than to diagnose the exact issue. 

[terminalgs]

If I understand, all voltages feeding the trem circuit are good.  Hence the problem is within the trem circuit.  FWIW, the Dan Torres approach to a trem issue is:it's easier & faster to rebuild the entire trem circuit than to diagnose the exact issue.

If you peel off the roach's covering (or install a bare NE-2H), the light will tell all.  is it oscillating correctly,  does it turn on, does it turn off.

[PRR]

> voltages on V7B plate:   "ON"=130V "OFF"=205V

This is a high impedance point (hundreds of K) and the specified "20K/V" meter was probably 1Meg on a 500V scale. With a modern 10Meg meter we expect to see voltages a bit higher, like 10%.

The way this thing is rigged, if you lack a handy Neon, put an LED where the neon goes. It should light dimly. If not, reverse the LED. When dim, engage the trem. The LED should blink. If not, you have circuit problems. If the LED blinks, you probably have roach trouble.

[plexi50]

Great info. Thanks to all of you! I installed the new Fliptops Ampeg G12 opto and it is working and sounds very nice.