Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: thermion on November 10, 2011, 12:40:49 pm

Title: ampeg gu-12 trem issue
Post by: thermion on November 10, 2011, 12:40:49 pm

I got one of these in the shop - the trem won't work. I force-grounded the oscillator cathode and can see the plate voltage bobbing around on my analog meter; how much voltage swing should I see? Turning the trem speed knob seems to have little effect on the rate of bob.
I also just replaced the cathode bypass cap and the 3 0.047 phase-shift caps, if this helps.
Any advice?
Thanks in advance.

Title: Re: ampeg gu-12 trem issue
Post by: xm52 on November 10, 2011, 02:52:39 pm

Look at the photo module in the tremolo circuit.

I don't have readings for the GU-12 but I do have info on the module that is in the Ampeg B12XT. The modules are not the same in these two amps but perhaps the info can be helpful.

On the GU-12, check that VR1, the 4M intensity control pot is working.

Measure the voltage across the neon bulb with the intensity set at minimum.  On the B12XT this is 50-60 VDC. Excessive voltage indicates a weak or open bulb. A low voltage indicates a problem with the oscillator circuit.

Disconnect the leads to the photosensitive resistor. Turn the amp on, set the intensity to minimum. Measure the resistance of the photosensitive resistor. 100K-300K is typical on the B12XT. Very high or infinite indicates that the bulb is not lit or the resistor is bad.

Title: Re: ampeg gu-12 trem issue
Post by: thermion on November 10, 2011, 03:07:47 pm

yes the clairex photomod CL1110...hard to find any info on this part.
someone out in audioland says a standard fender "opto-roach" will do the job...any chance of this being true?
fliptops has the TM-1 module for $35 but I'm not sure this is the right part (or the right price).
i'll poke around the intensity and speed pots tonite.
for reference, how large of a voltage swing should I see on the plate of the oscillator?
thanks.

Title: Re: ampeg gu-12 trem issue
Post by: PRR on November 10, 2011, 09:18:42 pm

Oscillator swing is normally BIG.

Most meters will not read the very low-frequency voltage correctly.


The schematic indicates a neon-lamp opto. Can you get a neon lamp? Wire it where the oscillator feeds the opto and see if it blinks right.

Title: Re: ampeg gu-12 trem issue
Post by: terminalgs on November 11, 2011, 07:20:59 am

the trem circuit is nearly the same as the earlier Gemini I G12, and the fender opto-bulb unit works in G12s  ( difference is the G12 has a 1M R parallel with the neon bulb  and the extra afterbeat switch ).    I'm not sure how easy the fender part would be fit your GU12's printed circuit board.

Title: Re: ampeg gu-12 trem issue
Post by: xm52 on November 11, 2011, 07:26:49 am

If you do need to replace the optoisolator, the original does have a neon bulb as PRR said. The Fender ones like the VTL5C3 have an LED. The circuit would need some modifications.

To add to the previous post, if you look at the schematic below for the G15 Gemini II that uses the TM-1, the circuit is very similar to the GU-12 and it looks like it would work. I would verify with fliptops that the TM-1 would work in your amp just to be sure. If you want to keep the circuit as original as possible, the TM-1 looks like it would be the best way to go.

(http://img716.imageshack.us/img716/9854/g15geminiii.gif) (http://imageshack.us/photo/my-images/716/g15geminiii.gif/)

Title: Re: ampeg gu-12 trem issue
Post by: thermion on January 17, 2012, 12:41:55 pm

i wanted to confirm that the tm-1 module from fliptops will work in the gu-12 circuit. unlike the clairex photomod cl1110 that came stock, the tm-1 must have the case grounded otherwise audible clicking with be evident. i took a 100W gun and spot-soldered a ground line and ring terminal to the outside case of the tm-1. the ring grounds out to chassis on the nearby mounting post for the circuit board access panel using a lock washer and existing nut. works great, speed is a little too fast to be musically useful. i will slow this down by tampering with cap values in the phase shift oscillator.
with the reverb up and the trem set slow and full intensity, "he shot me down...bang bang". far out.
thanks hoffman heroes!