Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: goldstache on October 09, 2023, 03:44:46 pm
-
Working through a Ampeg G-15
And can’t get the trem operable.
I got around 55vdc at opto part, and it’s wiggling 1v or so, when engaged.
Illumination, confirmed.
Voltages are pretty spec.
Oscillator has 125vac swing after first phase shift, 47vac after 2nd, 7vac at grid. All modulating.
LDR out of circuit Tests 100k dark(amp off)/30k amp energized trem off.
Huh.
Just so I’m not crazy, (I am), I’m under the impression that in this amp, you are modulating series resistance into the tone stack.
Intensity pot tests pretty good.
0-12R
50%-320k
100%-4M
In circuit strapped with LDR in para, I’m getting
0-10R
50%- 76k
100%-100k or there abouts.
So trem engaged, in essence you have a 12R resistor headed to TS. At 50% intensity, it appears as a shifting 76k etc…..
What I can’t measure is the LDR resistance swing when being illuminated. So no info there.
So this trem just varies series resistance and that’s enough to give volume amplitude???
Wild.
Anybody got any leads on a TM-1 work-alike?? That’s the way I’m leaning. I know fliptops has one. Just wondered if anyone had a workaround.
Thanks
https://ampeg.com/support/files/Schematics/G%20Series/G-15%20(Gemini%20II)/G15%20Schematics.pdf (https://ampeg.com/support/files/Schematics/G%20Series/G-15%20(Gemini%20II)/G15%20Schematics.pdf)
-
LDR out of circuit Tests 100k dark(amp off)/30k amp energized trem off.
Looks like you need a new TM-1.
So this trem just varies series resistance and that’s enough to give volume amplitude???
Correct. Works very well when the LDR can swing between low resistance to megohms of resistance.
-
> trem just varies series resistance and that’s enough to give volume amplitude???
The input of the tone network looks like about 300k at 250Hz and 100k at 3500Hz. Put zero or 100k in series, very slight change. Put hundreds of k in series, lot of loss.
If your LDR no longer goes to very high resistance, yeah, it may be lame. They are seeming to have 50 year life (at least unencapsulated). Or you could have a light leak in the roach. Wait for midnight and turn off all the lights.
-
Thanks guys!
I did electrical tape the snot out of it once I tested and didn’t see the LDR doing things I figured it would. (Isolated for testing)
Makes sense.
If I wanted to test out some LDR modules.
Would I just set up the bulb, with a fixed supply voltage, and measure out Resistance of LDR as I bring the supply +/- a volt? To kinda emulate the lfo wiggle I observed on the bulb?
Then there’s the time it takes to react. Oy veh.
Probably grab an after market module.
-
You need a module with a neon bulb. A neon NE-2 takes about 85-90v to fire. You must use a series current limiting resistor (typical 100K). I would buy a Fender tremolo roach as found in Hoffman's store. Probably work just fine.
-
Ampeg-specific parts supplier:https://www.fliptops.net/catalog/cat-100033/tremolo-vibrato (https://www.fliptops.net/catalog/cat-100033/tremolo-vibrato)