Ampeg reverbojet J12R

[glebert]

I think the meters measure mojo...

[Chuck H]

I think the meters measure mojo...

That might be really upsetting for a player that didn't know it was busted.

[johnhoef]

I agree, the meter is more or less useless, but, it does indicate in the manual that the red zone on the meter indicates clipping. The customer wants the meter working, so I'll make it work! And indeed, I did make it work. I bought the cheap Chinese meter. It's a 200 microamp meter. It was way too large to fit into the amp control panel, and the scale was not right. Using my dremel tool, I cut the new meter down to size, glued the old dial plate on, glued the old plastic bezel on, then modified the series resistor to make the red zone clipping.....I have not yet returned it to the customer, but I'm sure he will be thrilled to watch that needle bobbing around while he's playing some exotic Hendrix riff!
Mozz, thanks for digging around in your meter bucket....one of those on the right side may have worked, but my cheap meter did the trick.
Now just need to replace the main filter cap, and install a grounded power cord.
Thank you, everyone, for your thoughts.
John

[Chuck H]

The customer wants the meter working,.. I'm sure he will be thrilled to watch that needle bobbing around...

Or, just as likely, pegged in the red zone! Maybe include a zener or something in the meter circuit so that it only ever sees it's max red zone drive. That'll probably buy you a lot of reliability.

[johnhoef]

Chuck,
Indeed, you're probably right - pegged in the red zone! I am a musician, so I do know all about guitarists!! Good thought modifying the meter circuit to add some protection. I don't think that's necessary. Even at full volume, outputting square waves, I doubt it will be enough to damage the meter......although, the old meter WAS burned out.

[Enzo]

although, the old meter WAS burned out.

Well well. Nothing like a burned out meter to tell us the meter is not in danger.

[johnhoef]

Enzo....You're a funny guy........OK, OK....you're right..........I'll put a zener across the meter.

[vintagekiki]

https://ampeg.com/support/files/Schematics/J%20Series/J-12R%20(Original)/J-12R_Tube%20Location%20Schematic.pdf
_{Ampeg J-12-R with level meter schematics}

1)
Calibration (level meter unknown sensitivity)
Disconnect resistor 33k from OT, and connect it to a 12V AC voltage source (12V AC at 8 Ohm = 18W)
By changing the resistor 33k set the level meter to max.
Who like precision, instead resistor 33k, connect resistor 22k in series with a 22k trimmer pot.

[johnhoef]

vintagekiki,
Thank you for your thoughts....the method you describe would make the meter read wattage at the beginning of the red zone. A better method would be to simply inject a 1khz sine wave, increase volume to the onset of clipping, then adjust the 33k resistor so the meter is just entering the red zone. This way the meter is indicating distortion. This is in line with the meter description in the user manual.

Enzo,
Diode is installed across the meter.....I feel a lot better now!

[vintagekiki]

In that case inject a 1khz sine wave, at phase inverter (point H), and increase volume to the onset of clipping ... ...