Adding PPIMV

[dude]

I'm very happy with the 6V6 Plexi, Sluckey's schematic with the MV before the PI, works great but only problem, MV works on the preamp mostly and I'd like to try a MV after the PI, like the Allen Old FLame I have.


The Old Flame schematic was drawn with the PPIMV, so I don't know what value the caps would be without it.  I assume the two caps would be have been .1uf ?


This Trainwreck MV Type-1, I came across is the same as the Old Flame but adds two .1uf caps, doesn't say what the other two were.


Bottom line, if I copy the Old Flame PPIMV what would the value of the two added caps would be? The existing PI caps without a PPIMV are .022uf. in the Plexi 6V6. I was going to just add two more .022uf but now I'm not sure sure from what I see here with the trainwreck layout..?

[PRR]

It will work (whatever).

It is VERY much a voicing choice.

If deep bass seems shy, use bigger caps. If bass is big and flubby, use smaller caps. This will also depend on the speakers used. A pair of E-130 won't get flubby. Some of the small jobs sure will.

[dude]

Thanks PRR


I figured with master on 10 the coupling caps would be in full series, cutting the bass response and brightening the amp but wasn't sure. There are several types of PPIMV, some use a dual 250KA pot and ditch the 220K grid leak R's and one set of coupling caps so no tone differences. Some use a 100KL pot if 100K grod R's. Just didn't know from inexperience which is the best sounding, always used the pre PI MV and they seem to only get pre amp distortion at lower volumes. I was hoping for some other suggestions but I guess this topic has been beat'n to death.


I appreciate the answer, I wasn't sure if the fixed bias would be affected when turning the MV. I assume not.


al   

[Big_Mike]

I asked about this as well in another thread.  I am planning on building a 6v6 plexi and would like to add a PPIMV to it.  One of the best sounding ones for Marshall-style amps is the Rich Mod 2 with a dual 500K PEC potentiometer.  Do a search on Youtube for "PEC potentiometer master volume" and you will find the clip by George Metropoulis on his 50 watt clone, it sounds great.

Also, check the Valvestorm site and it will show you a wiring diagram.

As best as I can tell from the Hoffman schematic, you would place the pot after the .022 capacitors after the phase inverter and return it to the 100K resistors on the board, which also go to the power tubes.  Not sure if you would need to add anything else to the circuit.

[Big_Mike]

Bumping an old thread... I am thinking of adding a ppimv to the 6v6 plexi in addition to the regular master on the Hoffman layout.  The stock amp has 100K grid resistors, and the Trainwreck pages recommend a dual 100K linear pot for a type-2 master.  Do I need to add any safety resistors to the dual 100K pot?

[sluckey]

I would.

[Big_Mike]

What size resistors would you recommend adding to the dual 100k linear pot?

[dude]

Same as any LaMar PPIMV, 2.2Meg.

[d95err]

Are you sure you want a linear pot? It would work well for small volume reductions (e.g. -6 db or so). If you want to make bigger volume reductions (home or bedroom level), I’d use an audio taper pot.

[Fiat_cc]

What about the Frondelli MV?
https://robrobinette.com/5F6A_Modifications.htm#Frondelli_Master_Volume

Use a 100K dual gang pot instead of the 250K mentioned here.
No need for safety resistors with this version.

[pdf64]

I wouldn't trust power tube bias to pot tracks. That's 2 mechanical connections (where the solder lugs are riveted onto the track ends) which are way more likely to fail open circuit than regular resistors, especially when folks bend the solder lugs using the rivets as an anchor point.

[Big_Mike]

I am going to order a PEC dual gang 100k audio pot and install a Lar/Mar with the typical 2.2 Meg safety resistors.  I will report back on how it turns out.

[Fiat_cc]

I wouldn't trust power tube bias to pot tracks. That's 2 mechanical connections (where the solder lugs are riveted onto the track ends) which are way more likely to fail open circuit than regular resistors, especially when folks bend the solder lugs using the rivets as an anchor point.

I don’t think they’re any more likely to fail than a resistor leg that gets bent to fit a turret, eyelet, or pcb. There’s more metal, so actually probably less likely.

[sluckey]

I wouldn't trust power tube bias to pot tracks. That's 2 mechanical connections (where the solder lugs are riveted onto the track ends) which are way more likely to fail open circuit than regular resistors, especially when folks bend the solder lugs using the rivets as an anchor point.

So, what about bias pots?

[pdf64]

I've not come across one myself, but I've have seen forum reports from repair techs of Fender type bias pots failing. Not sure whether that was due to poor connection at the wiper or track end lug though.
Hence with the Fender arrangement, it's a good idea to use a safety resistor R3 across bias pots to mitigate for this, see http://www.valvewizard.co.uk/bias.html


The Marshall bias arrangement is inherently 'fail safe', in regard of an o/c bias pot at least.

[Auke Jolman]

With regards to the PPiMV, I've used a Frondelli type to my latest build with great results (https://robrobinette.com/5F6A_Modifications.htm#Frondelli_Master_Volume). Might be something to look at.

[Big_Mike]

With regards to the PPiMV, I've used a Frondelli type to my latest build with great results (https://robrobinette.com/5F6A_Modifications.htm#Frondelli_Master_Volume). Might be something to look at.

What kind of amp did you install the Frondelli master volume?

[Big_Mike]

Also, what is the difference between the Frondelli and Lar/Mar?  They seem basically the same.

[sluckey]

Also, what is the difference between the Frondelli and Lar/Mar?  They seem basically the same.

Inputs and outputs are reversed.

[Auke Jolman]

I've used in in the COS-build I'm working on. You can find it here: https://el34world.com/Forum/index.php?topic=21804.0

You can find the the schematic on Rob Robinette's site. The Lar/Mar uses a 1M dual pot. 2 220K resistors are left in place to carry the bias voltage. In case of the Frondelli the 2 220K resistors are replaced by a 250K A dual pot (pin 1 and 3).