Crazy Idea?

[hossymandias]

I have owned a Music Man 212-75 for about 20 years and I love it. It was my first tube amp (even though it's "merely" hybrid, I don't care). Somewhere along our journey together, the on-board phaser has stopped working. I've spent the last 5 years trying to figure out what's wrong with it. I've replaced the VTL5c4/2s, no go. I've replaced all op-amps. I just recently replaced ALL capacitors in the signal path and measured all resistors. I even took it to my amp tech and he was stumped. We got it to make a kind of vague tremolo effect, but that's it. Because the vactrols are now unobtanium (I'm not payed $30 for two more components that still might not solve the problem), I tried rolling my own vactrols. I was able to confirm that the LFO circuit still works, as the LEDs fired when I connected them to the board. All that to say...I've resorted to a desperate strategy.

I have all the makings of several MXR phase pedals, so I would like to try to bypass the phasing section of the Music Man circuit board, and add a Phase 90 phase section daughter board into the amp.

Basically, I would take this circuit:



and jack it in to replace this part of the Music Man circuit:



What I don't know is quite how the difference in bias voltages will affect the JFETs. I don't think there too much voltage (the Music Man supplies +16 and -16), but I don't know if this will get the JFETs moving right to start phasing.

I'm also not clear on how the Music Man powers the opamps. Before I disconnect I will check to see if there is voltage on pin 8 of the JRC4558s I have in there. The schematic doesn't show any connections to these opamps. But in other Phase 90 schematics, I see the opamps powered at 9v.

If anyone has the patience to read through this mess, any thoughts on this endeavor is appreciated! I will update as I move along.

BTW, I did consider just sticking an FX loop in the amp and running a phaser pedal...which would make way more sense. But I want to be able to use my Music Man with an onboard phaser. I know that is completely illogical. So be it.

[martin manning]

No, not crazy... pretty interesting actually. The circuits are so similar that with a little ingenuity you might be able to replace the Vactrols with FETs. You said you already tried replacing the Vactrols with the correct parts and it still doesn't work? There is a service bulletin that describes the procedure for setting the trim pot, and the resistance range for the LDRs. Did you run through that? Did you look at the LFO output on a scope?

In the Musicman all of the OpAmps will get +16 on pin 8, -16 on pin 4, and it would be worth checking IC5, 6, and 7 to be sure that they all have correct +/- voltages. In the Phase 90, the OpAmps are run from a single positive supply which is divided in half (roughly) for the reference voltage, and the pin 4 negative voltage is ground. It also has an adjustable bias voltage for the FETs, which you would have to create in your Musicman if you were to go that route. Can you post a good photo of the area around the Vactrols?