Ampeg GV-22 Vibrato Module

[mikeskory]

We're working on a customer owned Ampeg GV-22. We've replaired the power supply, replaced many caps, fixed reverb (more bad caps) and now into the vibrato. I got a VM1 / VM1A Vibrato Module from flip tops. My question is how should I mount it. I put 6 wire leads on it but due to it's size it'll have to go on the print side. Have anyone done it this way? Thank you, Mike

[mikeskory]

This idea won't work. Good thing I did not solder it. The new part is plainly labeled "Audio" side and "Tremolo" side. I will have to solder it in from the component side.. If the tremolo works, I'll have to just mash it in.

[g1]

How was the original mounted? I would think the replacement mounts in the same place, but with black box side to ampeg circuit board, affixed with glue or what have you (after verified working). Wires would run down to the ampeg board as required.
From the sounds of it, they have ticking issues, so you may have to try mounting in different locations anyway.

[mikeskory]

The original tremolo had 8 through hole type leads. I mounted the new one positioned the same as the old but with short jumpers because there are no leads or pins on the new vibrato. HOWEVER...the problem is exactly the same. It "tries" to tremolo for a few few seconds then quits. I'm going to pull the 3 effects tubes and hand check each component. Maybe there is a bad resistor or maybe I made a mistake changing out the caps or maybe a solder joint I missed. The audio signal comes from the treble control and is getting to the effects board.

[g1]

Trem that starts but then stalls out is usually caused by caps in the oscillator circuit, or sometimes a weak tube.
Chances are that the original module is still good and not the issue.
The caps circled in red should be replaced.

[mikeskory]

Thank you. Yes those capacitors have been replaced. I can see the voltages on pin one of V7 change with the foot switch. After cleaning the VR7 control again and resoldering it, the vibrato works a little more. It comes on at full but as I turn the control down, it abruptly turns off. Turning it backup it will go back to full.again. very consistent. I ordered 2 500k pots just now from Flip tops. If I get it working I had planned to put the old tremolo back in

[Tom Phillips]

...It comes on at full but as I turn the control down, it abruptly turns off.

Which control are you referring to? Speed or intensity.

Can you scope the low frequency oscillator waveform and verify that the speed is within a normal range (within approximately 4 to 11 Hz) and a is a nice stable sine wave.
If the overall LFO circuit gain is low then it wont sustain the oscillation. Could be a weak tube as g1 mentioned.
I think it's unlikely that the pots are causing the described problem. You can measure the status of the speed and intensity pots before you order replacements.

[mikeskory]

Finally got back to this one. Pulled the vibrato and intensity pots out and cleaned the, Cleaned and resoldered the small board too (that holds the pots). The vibrato then started working week then stronger as I kept cleaning and soldering. Now seems fine. Reverb works too with a new tank. Actually the amp sounds pretty good now but not tremendously loud. It's only getting about 426Vs at the the 6L6s. Schematic says it should be 480V. Ugh!

[g1]

It's only getting about 426Vs at the the 6L6s. Schematic says it should be 480V. Ugh!

How hot are the power tubes idling? Do you have around -50VDC at 6L6 grids?

[mikeskory]

Yes. -49v at each grid

[Tom Phillips]

Yes. -49v at each grid

It would be best to measure the actual plate current and calculate the actual dissipation at idle. Some "Modern" 6L6s require much more bias than that. Could be as much as -55V.

[mikeskory]

Ok. I will study that and get back. It does not sound bad but not loud like a Twin or even a Bassman. Thank you -Mike

[Tom Phillips]

...It does not sound bad but not loud like a Twin or even a Bassman. Thank you -Mike

Ampeg claimed 40 W output. It's hard to tell if it's working to full potential without measuring the power and evaluating the speakers. It's great if it sounds good to you but it will never measure up to a Twin (Assuming you are talking about a Fender Twin) and it's a bit lower powered than a Bassman.

[mikeskory]

Hey Tom! I'm back to this Ampeg. I had my variac in circuit before. I used 2 Euro bias probes at the same time.

Voltage at V2 is 475v at pin 3. Voltage V2 is 480 at pin #4. .043ma
Voltage at V1 is 468v at pin 3. Voltage V1 in 480 at pin #4. .082ma

Using only one probe at V1 I still get .082 ma

[Tom Phillips]

...Voltage at V2 is 475v at pin 3. Voltage V2 is 480 at pin #4. .043ma
Voltage at V1 is 468v at pin 3. Voltage V1 in 480 at pin #4. .082ma...

Well... now we know that V1 is dissipating about 38 watts and V2 is dissipating about 21 Watts. Pretty unbalanced and at 38 watts V1 is in distress and maybe even redplating a bit. I would adjust the bias so the power tubes run cooler first and consider another set of power tubes.

Do you have the capability to measure power output?