Convert MV Twin Reverb to PPIMV

[jondanger]

I'm back again with more SFTR questions. I was browsing RobRob's AB763 mods page and saw the PPIMV mod, which uses a 1M pot. This is the same value as the current MV pot. Can I just move that to after the PI to accomplish a PPIMV? In this schematic, if the pink line bridges those two points, and the leads from the wiper and the outside lug (left side of the schematic) go to the purple points, will that work?

Here's the link to Rob's page:

https://robrobinette.com/AB763_Modifications.htm#Master_Volume_Mod_

I'm not really going for a high gain thing, just wondering if I can get a juicier sound at lower volumes that way - maximum versatility.

 

[cboutilier]

I just did the same with my SFSR, but haven't got to test it yet. After you unhook your stock MV, you could easily alligator clip the ppimv in place to try it.

 

[jondanger]

Cool, hopefully I can give it a shot tomorrow or maybe even this evening. If you crank it up, let me know how it sounds? It seems so easy that I'm surprised there aren't a ton of threads on it - which is what gave me pause instead of just going for it.

 

[cboutilier]

Cool, hopefully I can give it a shot tomorrow or maybe even this evening. If you crank it up, let me know how it sounds? It seems so easy that I'm surprised there aren't a ton of threads on it - which is what gave me pause instead of just going for it.

Will do! I just did it because I had the chassis out and figured I'd try to make use of that idle MV pot.

 

[robrob]

Remove the input and wiper wires from your master volume and solder them together. Disconnect the pot's ground wire.

Then wire the two power tube grids to the pot's two outside terminals. Bridge the wiper to one of the other two terminals to create a variable resistor. You want the resistance to increase as you turn the master volume pot "up" or clockwise.

With the pot shaft pointing away from you and the terminals pointing upward, you would bridge the right terminal to the wiper to increase resistance as you turn up the pot.

 

[jondanger]

Remove the input and wiper wires from your master volume and solder them together. Disconnect the pot's ground wire.

Then wire the two power tube grids to the pot's two outside terminals. Bridge the wiper to one of the other two terminals to create a variable resistor. You want the resistance to increase as you turn the master volume pot "up" or clockwise.

With the pot shaft pointing away from you and the terminals pointing upward, you would bridge the right terminal to the wiper to increase resistance as you turn up the pot.

Thanks Rob! That's the kind of step by step I was looking for. I'll report back after I change it.

 

[faasie85]

Dont know about the schematic but a thing to keep in mind is that it probably is harder to boost your volume for solos with ppimv. I love my Marshall vintage modern and love the sound but using it for gigs a bit more difficult because of not being able to boost solos enough.

 

[jondanger]

Dont know about the schematic but a thing to keep in mind is that it probably is harder to boost your volume for solos with ppimv. I love my Marshall vintage modern and love the sound but using it for gigs a bit more difficult because of not being able to boost solos enough.

Cool, that is good to know. In the past when I used a MV JCM800 I plugged straight in and used the guitar volume knobs (SG) to change the dirt/volume. I'm not harboring any hope really of being able to do this with the Twin but it's always interesting to see how dirt pedals act with different amps. I don't really have a straight up boost on my board right now. I guess we'll see how it likes the RAT [emoji15][emoji3][emoji43]

 

[faasie85]

On the Marshall forums people said the boost problem could be worked around by

- eq pedal for just to get more cut trough
- footswitch operated attenuator
- install second mv or ppimv

I don't know if its the ppimv or something else but the Marshall does clean up nicely when rolling back volume.

 

[jondanger]

So should I take off this 120pf cap on the MV? It seems like it goes from the input side to a tapped lug or something like that.

 

[jondanger]

So I buttoned it up and it *works* but there is no noticeable difference between 2.5 and 10 in terms of overall volume, and then a sharp drop from 2.5 to 0. I guess this is a linear instead of an audio pot. A learning experience, but this might end up getting reversed when I do the bypass caps. I don't need overdrive from this amp, and the juicy low volume cleans aren't really there. A Twin isn't built for harbors anyway though, right?

 

[robrob]

You can't use that 120pF bright cap on a Type-3 Master volume so just remove it. The MV does need to be a log or audio pot to make it more usable.

I like this simple master volume. You can also use a push-pull 1MA pot and wire either grid wire through the switch for up=connected=MV on. Push in the pot and the MV is completely removed from the circuit.

 

[GCKelloch]

You can't use that 120pF bright cap on a Type-3 Master volume so just remove it. The MV does need to be a log or audio pot to make it more usable.

I like this simple master volume. You can also use a push-pull 1MA pot and wire either grid wire through the switch for up=connected=MV on. Push in the pot and the MV is completely removed from the circuit.

Old thread, but I'm considering this for a '74 SF Twin. So, the pot can be wired b4 the 1500 Ohm grid R's without somehow screwing up the PI sound?

 

[Pete Farrington]

Live in hope but prepare for disappointment. Who knows, it may be just the ticket for your needs?

 

[MrBassmans]

@GCKelloch

As Pete says prepare for disappointment. Type 3 cross line PPIMV's don't sound that great, especially in a 12AT7 PI fender.

I do have one in my BF AA864 bassman head clone, however. I use it to slightly reduce volume when playing thru high efficiency 99db+ speakers. (I prefer lower db speakers so using the MV sparingly makes the cab less efficient). A high quality PEC 1meg audio pot will reduce the volume jump of a type 3.

Edit: Type 3's can easily be switched completely out of the circuit compared to other PPIMV's as posted by @robrob

I sometimes sub a 12AX7 in the PI socket, that along with reducing line voltage down to 110 volts using my Brown Box. With a Les Paul I can get a bit of a Bluesbreaker breakup at low volume.

 

[GCKelloch]

Thanks, guys. I'm still wondering if wiring a dual-gang 1M audio pot after the 1500 ohm R's would be better in some way, i.e., possibly not altering the bias V, or the corner freq of the HP-filter into the power tubes? I suppose it wouldn't hurt to wire it that way with such a pot. Even if it's not so great, I imagine it's an improvement over the stock MV b4 the PI tube, and having both MV's will increase flexibility.

This is for a restoration my brother is working on. I've also suggested he build the John H fixed 9~10dB attenuator. He could also do a B+1 V drop mod. I had recommended The Tube Store PS series 6L6WGC tubes that take 550V-plate/500V-grid, but he got the JJ 6L6GC in a rush to see if the amp at least works. He's about to replace the electrolytic caps and maybe some other components that should offer minor improvements.

 

[GotA24Fretter]

The LarMar isn't a bad option. I'm in the middle of putting one in a 1970, so a bit of a different animal--but I'll add a few photos later when I get back in the shop. The bias voltage change is very slight (<1V) from min to max.

Edit: adding photos

 

[Pete Farrington]

The bias voltage change is very slight (<1V) from min to max.

The actual bias voltage doesn't change. You're measuring a 1V change across the 10M input resistance of your meter. Connecting the meter is loading the circuit, changing conditions slightly.

 

[2L man]

Old thread, but I'm considering this for a '74 SF Twin. So, the pot can be wired b4 the 1500 Ohm grid R's without somehow screwing up the PI sound?

Grid Block block resistor first role is to block high frequencys enter to tube and in theory there should be minimum conductive material after it. GB also slightly change how Blocking Distortion behave but 1k5 does not.

1MA10 (10% logarithm) has significant effect to BD. Turn from 10 to 9 make potentiometer "voltage divide" 50%. Mathematically volume drop -6db representing 75% power drop. Not exactly because potentiometer it is in a circuit which also has effect but for understanding it does

If playing is so loud that it produce BD turning in this additional 500k series resistance to signal chain practically prevent (screw) the BD. I think it is the reason why many guitarists don't like PPIMV especially type-2 (Larmar etc)

 

[GCKelloch]

Grid Block block resistor first role is to block high frequencys enter to tube and in theory there should be minimum conductive material after it. GB also slightly change how Blocking Distortion behave but 1k5 does not.

1MA10 (10% logarithm) has significant effect to BD. Turn from 10 to 9 make potentiometer "voltage divide" 50%. Mathematically volume drop -6db representing 75% power drop. Not exactly because potentiometer it is in a circuit which also has effect but for understanding it does

If playing is so loud that it produce BD turning in this additional 500k series resistance to signal chain practically prevent (screw) the BD. I think it is the reason why many guitarists don't like PPIMV especially type-2 (Larmar etc)

Thanks, but it's not clear to me what your last sentence means. The additional R prevents BD? Isn't that a good thing?