Post Phase Inverter MV

[cicone]

Anyone have experience with putting one of these in a JCM 900 MkIII? Seems like it would be pretty simple if I knew what pot to use and were to solder it in. Also, do you think this would sound nice in a Master Volume amp? I think the only thing you would give up is slamming el34's and the output tranny, no??

[WaltC]

a true PPIMV isn't particularly easy to retroactively install safely in any amp because of the need to ensure the delivery of proper bias voltage to the grids. On a fairly complex pc-board-based amp like the 900 its even harder. Do-able but not necessarily simple or easy. A one-wire MV (ala Fischer) is somewhat easier and safer and doesn't sound bad, but, that said, the 900 already has separate gain and "master" volume for each channel, so what would you be trying to achieve?

I doubt that you gain much on that particular amp and I'd recommend against it if you were a customer of mine.

[cicone]

thanks

[epluribus]

Just caught up here...seems to me the one advantage, since this amp already has pre-PI MV, would be the ability to control for saturation in the PI more or less independently of the rest of the circuit. While I've found having both flavors of MV to be useful from a learning standpoint, I've kind of come around to PPIMV generally anyway.

According to the MV vendor in question here, it's a good mod on this particular amp. Just from reading the schematic, I'm not sure whether changing the gain structure of the PI would have all that much impact after all the other distortion-generating gear in the circuit, but it certainly would be enlightening to try it out. I could, that said, see such a mod being more useful under moderate-drive conditions such that you could actually hear it. :)

Anyhoo, from a technical standpoint I'd find it very interesting to have so much control over the way you drive the PI.

On a slightly different tack with this amp, what's your opinion of using op amps to control MV? (I'm only just now learning about op amps.) Seems to me it wouldn't be too hard to tweak them up to make them very transparent, possibly moreso than a pot. Alternatively, seems to me you could rig them to overdrive more or less like a tube-driven gain stage. I'd love to know if Marshall uses them as linear or non-linear gain stages, or if they're just rigged to run at unity when wide open, providing no gain at all.

Anyhoo, it's an interesting question for sure. Color me tuned in.

--Ray

[cicone]

This will sound crazy, given the flames regarding the effects loop volume switch.....but....after mapping the circuit path, it would appear that one could simply plug a cord into the loop and turn down the effect level on the back panel??? Am I nuts to think that the sound(gain stage shaping)prior to the loop will pass through intact, but at a lower level and then I could use more MV to goose it up??

[JJman]

I'm not sure what model the MKIII would be. I had a 4100 model and it had only one knob for the loop level.

http://www.schematicheaven.com/marshallamps/jcm900_25xx_45xx_50w.pdf

As apparently confirmed in the above schematic, the loop knob increases the send while decreasing the return at the same time, and vice versa. I recall that moving the knob does not change the sound level you hear out the speaker. Rather it controls the signal level going to the loop, AND resets the level back to parity before it comes out the loop. I recall that this allows the user to adjust the signal level that the effects in the loop will see. I placed my EQ in the loop since I used it to boost volume as well as contour the EQ. Placing it in front would not boost the volume during high gain settings on the amp.

I would confirm that it's possible to set the dials so that the PI clips while the preamp is clean. Without that scenario, there's no potential benefit of having a post PIMV. My guess is that the preamp clips 1st but who knows.

[brett s]

I've got a JCM800 with one - when purchased it had David Bray's "plexi" mod, I changed the preamp back to near stock but left the PPIMV on it.

It does give me more flexibility having both, preamp gain & phase inverter clipping don't sound quite the same.

[cicone]

It does give me more flexibility having both, preamp gain & phase inverter clipping don't sound quite the same.

You are absolutely correct about that. With the MKIII 2500, I have a Preamp knob to hit one gain stage, a Gain knob(with diode clipping circuit)to slam the next one and the MV to push the output stage. You can do a lot of twisting with those 3 and get quite a range of sounds. In reality, it sounds great to me and I have no complaints from neighbors or police---but it can get a bit loud, if you get my drift. Thanks for the input. I think I'm grasping at straws. I should just play and enjoy it.

[cicone]

Just to clarify---I don't want to move the existing MV from before PI to after. I want to leave the original in place and use the post inverter add on to tone down the output volume while the original MV saturates the PI----a built in attenuator, if you will.

[WaltC]

I doubt you'll see much difference. Using an attenuator will let you get power tube saturation as well and you may notice a difference there, but.... <G> you'll age your tubes and the amp faster that way. It's worth it for some. And for some the real magic is in the complete package, pre-, PI, Power tube and speaker/cab being pushed to the limit and connecting w/ the guitar/strings and your fingers directly. NO substitute for that IMHO... <G>