Ampeg Gemini G15 No Tremolo

[Enzo]

Geez you are just ACHING to change that opto...

Your trem tube plate voltage doesn't change when off or on, so that says to me that the trem is not oscillating.

If you are checking heaters, easiest is to look and see if they are lt. But if you do voltage tests, what matters is the voltage across the heater, NOT to ground. SO you may have 3.5v on each end of the heater, but what matters is the (presumably) 7v ACROSS the heater.

The value of the three feedback caps isn't important. If they are off, it will only change the speed some. Leaky caps can be a problem. You reported plate voltage, but the schematic also gives cathode voltages, how are yours?

[J M Fahey]

Just to confirm/dispel theory, connect a plain neon bulb across the "drive" end and turn room lights off:
DOES it light?
DOES it pulse?

You may use the neon bulb inside an illuminated rocker switch or neon pilot light, even if it has a limiting resistor inside, we donīt need full brightness, just to confirm circuit functionality.

And next time you order something else, include 10 neon bulbs, just to have some available, nice to have sometimes.

[Helmholtz]

319V at V7b plate means the tube isn't properly conducting. Please verify 3.3k between cathode (pin 8) and ground as well as 470k between plate (pin 6 ) and node F. If resistances are good, replace V7.

A good neon will be on all the time.

[vintagekiki]

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)

There is no purpose to perform any measurements if the tubes in the amplifier are old, worn or defective.

[Perkinsman]

Geez you are just ACHING to change that opto...

Your trem tube plate voltage doesn't change when off or on, so that says to me that the trem is not oscillating.

If you are checking heaters, easiest is to look and see if they are lt. But if you do voltage tests, what matters is the voltage across the heater, NOT to ground. SO you may have 3.5v on each end of the heater, but what matters is the (presumably) 7v ACROSS the heater.

The value of the three feedback caps isn't important. If they are off, it will only change the speed some. Leaky caps can be a problem. You reported plate voltage, but the schematic also gives cathode voltages, how are yours?

I thought this response went out yesterday but I must not have been logged in when I hit the Post Reply button...so here goes again.

I did replace V7 with a new tube but no real difference a few volts maybe. The cathodes measured at pin3-.75vdc and pin8 -16vdc. The heaters are working, 6.3v, pins 4or5 to pin 9.

319V at V7b plate means the tube isn't properly conducting. Please verify 3.3k between cathode (pin 8) and ground as well as 470k between plate (pin 6 ) and node F.

The resistance between pin 8 & ground started at about 1M. The longer I held the meters on those points, the more it decreased. The resistance between pin 6 & node F (see pic as I'm holding my pointer where I thing node F is) was 270k.

[Enzo]

COMpare the schematic 3.3k from pin 8 to ground, and your 1 meg. Clearly the cathode path is open, so either your 3.3k resistor is open, or the on/off switch is not closing. Or the wiring is broke.

[Helmholtz]

If the voltage across the neon is above 150V, it's probably bad.
Neons typically ignite at around 100V or lower and then voltage drops by around 20V to the maintaining/burning level.
As neons behave somewhat like a zener, there won't be much voltage pulsing with tremolo.
Rather the brightness is modulated by the current showing in a pulsing voltage drop across the current limiting 390k resistor.

[Perkinsman]

If the voltage across the neon is above 150V, it's probably bad.
Neons typically ignite at around 100V or lower and then voltage drops by around 20V to the maintaining/burning level.
As neons behave somewhat like a zener, there won't be much voltage pulsing with tremolo.
Rather the brightness is modulated by the current showing in a pulsing voltage drop across the current limiting 390k resistor.

Update: I spent some time with the Ampeg tremolo issue to day and these are a few more observations. The vdc across the neon lamp terminals was 74vdc, so that sounds ok by your info. I didn't turn the tremolo on to see whether the voltage varied.

I found that I had mistakenly reported the pin 8 cathode to ground across the 3.3k resistor as 1M but it actually measures 3.4k, which sounds much better.

There was a 10M resistor in place of a 1M across the tremolo unit leads, & the 390k & 470k's s are not present at all. Could this be why the voltage is 319v?

Another observation was that when I turn the intensity down, the volume went up and visa versa....something obviously wrong.

[Enzo]

If the bulb stays on the attenuation remains steady, so the intensity control acts like a volume.

Re: CAthode to ground, 3.4k close enough. Does clicking the switch off and on make that go from 3.4k to open?

[vintagekiki]

Another observation was that when I turn the intensity down, the volume went up and visa versa....something obviously wrong.

This is OK because by changing the intensity potentiometer (4M) "bypass" the photo resistor in the opto-coupler TM1.
Your observation indicates that the tremolo oscillator is most likely not working or there is a break opto-coupler TM1 (contacts x y) that is connected in parallel to the intensity potentiometer (4M).

The operating point for neon in TM1 is determined by the divider 390k and 1M.

Question:
- Whether and how much changes the voltage at the anode V7b (pin 6) when the Tremolo foot switch on/off changed position. The voltage change is written on the schematics.
- If the Tremolo foot switch is in the on position, does the voltage at the anode V7b (pin 6) pulse in the frequency of the Tremolo oscillator and does the pulse rate of the anode voltage change with the change of the tremolo speed potentiometer (.5M).
- If the voltage at the anode V7b (pin 6) changes and pulsates (frequency of 6 - 10 Hz), the Tremolo oscillator is correct, the opto-coupler TM1 is faulty.

In the links below you will see how an opto capler can be made (DIY)

https://www.youtube.com/watch?v=fJOHkoPZFQ4

This video series will be in two parts. In this first part we'll explore the features, change power cord, power caps, and troubleshoot the reverb (Echo). In Part 2 we will troubleshoot and fix the tremolo circuit by devising a new photocoupler / optocoupler roach and demonstrate the tone.

https://www.youtube.com/watch?v=NXWYZCPzIq8

In Part 2 of this video series on the vintage Ampeg G-12 Gemini tube combo, we will discuss how to make a new optocoupler or photocoupler, aka "roach", for the tremolo and we'll finally hear this beast in action.

1)
https://www.youtube.com/watch?v=3BKstpMFaas
Pay attention to time 7:03 where you can see how the tremolo oscillator is tested.

[Enzo]

I think it is clear his trem tube is not oscillating.

[Perkinsman]

If the bulb stays on the attenuation remains steady, so the intensity control acts like a volume.

Re: CAthode to ground, 3.4k close enough. Does clicking the switch off and on make that go from 3.4k to open?

No change on the cathode to ground, it remains at 3.4k with the power on, switch on or off. What does that mean?
Also, I placed the meters across the bulb terminals with power on, switch off and read 23mvac, switch on and read 9mvac. What does that mean?

[vintagekiki]

When repairing any device, so and amplifier, there must be a certain systematicity in the work.
It cannot be measured just to measure something without knowing what is measured or the purpose of the measurement.

Some questions have been raised but no answers. No problem. Let's start over.
1. Are the footswitch jacks (tremolo & echo) placed in their sockets.
2. With ohmmeter on the male jack footswitch, check that the cable, wiring and both switches are correct.
3. Whether the replaced V7 tube is new or as new.
4. How much changes the voltage at the anode V7b (pin 6) when the Tremolo footswitch is on and how much when is off. The voltage change is written on the schematics.
5. How much is DC voltage on the 390k and 1M divider for neon (TM1)
If there is no voltage at anode V7b (pin 6) check/replace anode resistor 470k.

Question.
Is the Echo (reverb) on G15 working properly, and is it on/off with the Echo footswitch.

Follow-up when it is determined whether the V7 tube has the correct voltages.

https://ampeg.com/support/files/Discontinued User Manuals/G-15 (Gemini II)/G-15 Manual.pdf

[Perkinsman]

Thanks Kiki for making me doublecheck a few of the basics. As it turns out, this unit has a homemade footswitch for trem/echo. When it was on the bench, I couldn’t check reverb of course (due to the length of the tank wires) but I couldn’t get the trem working no matter how many times clicking the trem switch with no success, which made me look to the components/wiring on the chassis. After a few minor component changes, tubes (& this thread) I was about to give up when last night I re-installed the chassis back into the amp for a dry run. I starting playing & hit the reverb button & BAM, the trem comes on! When I hit the trem button the echo came on. The homemade pedal builder wires the pedal backwards. Shows to go ya.

[vintagekiki]

... As it turns out, this unit has a homemade footswitch for trem/echo ... The homemade pedal builder wires the pedal backwards ... Shows to go ya.

The old rule of life says. Do not take anyone at their word, but check in person, because in a large number of cases the human factor can be the cause of failure.