PPIMV Resistor Values?

[Amped]

Hi all.  I'm building one of Sluckey's amps-a Dual Lite 18W and am hoping to include his PPIMV mod.  The values I've calculated for the dual-gang pot resistors seem quite large and I'm hoping that I can trouble the "brains trust" to double check my work.

Firstly, what function do the resistors perform?  From my reading, I think these resistors preform the following functions;
1) as the pot is replacing the grid leak resistors - and the pots are not exactly the same value as the resistors, these resistors in parallel with the output (wiper to output) make the pot the same resistance when the volume = full up (effectively this is setting the grid reference voltage correctly at full volume).
2) sets a baseline resistance for reference grid voltage when volume = full down (i.e. not shorted to earth, volume will never = "0").
3) provides an electrical path for the grid if there is a failure in the pot so there is always a referenced path to earth for the grid (sounds like a great safety feature). 

Am I right so far?

If so; can I use a parallel resistor calculation so the resistor value+pot value = the grid leak resistor value, or am I missing something?  Is there any "artistic flair" that can/should be applied here?  Or am I over thinking it?

Calculation:
The resistor being replaced is 470K, the pot replacing it is 500K.  The closest resistor I could find was an 8M2, which is quite a large value.  Am I right?
1/r = 1/500K + 1/8200K
1/r = 0.002 + 0.000122
1/r = 0.002122
r = 1/0.002122
r = ~470K

In case I've missed anything attached is some screen grabs of the original circuit, schematic and PPIMV mod.  The resistors are circled.

Thanks everyone.

[shooter]

Or am I over thinking it?

  You can never over-think something, but in the end you might find it was a waste of time

build it without mods, get it "the way you like it"
add the mod as suggested, tweak it to "the way you like it", or remove because you didn't like it, THEN think retrospectively since it's ALWAYS 20/20 

[sluckey]

Or am I over thinking it?

WAYYY overthinking it. 

Just use the 2.2M resistors as shown in my drawing. Those are just a safety resistors. Their only purpose is to maintain a grid to ground reference in case one of the pot wipers fail. The value is not critical but should be high enough to not affect the pot resistance too much and low enough to still maintain a safe grid return so the grid stays at zero volts. You will find that 2.2M is a commonly accepted value for these resistors.

[Big_Mike]

I would throw out another option that I did to my 6v6 plexi.  Instead of using a PPIMV, which affects NFB, simply use a standard pre-phase inverter master volume like on the Hoffman 6v6, but add some frequency compensation for lower volumes.  What I did was add a resistor (220K) in series just prior to lug 1 of the master pot, then add a small capacitor (between 5-10p) between lugs 1 and 2 of the master pot.  The resistor preserves bass frequencies at lower volumes, and the capacitor preserves high end.

I tried all sorts of PPIMV on my 6v6 Plexi, and I just did not like the changes to NFB.  I also was able to get plenty of overdrive and clipping with a standard pre-phase inverter master using this setup. 

Just my 2 cents!

[sluckey]

There is no NFB in the amp being discussed. And a pre PI master such as used in the Plexi6V6 is no good for this amp either. Take a look at the schematic and you'll see why.

[Amped]

 
Yea thought that could have gone either way...  Thanks to you all for your input and ideas.  At least one of my theories was close....

100% Sluckey, 2.2M comes up often.  When looking around online I found information was a bit disjointed - these resistors almost seemed an afterthought for some designs, hence the attempt to maths my way out.

The best resource I found was robrobinette's republished Ken Fisher article "The Trainwreck Pages":
https://robrobinette.com/The_Trainwreck_Pages.htm

The section "MASTER VOLUME Mods For MARSHALLS: Four Types" has the following note added:

Rob adds: The Master Volume Type-2 Is the most "transparent" and my recommended MV because at max volume the amp circuit is unchanged. The Type-2 simply replaces the power tubes' grid leak resistors (upper set of resistors) with a dual 100KL pot (100 kilohm linear pot, dual gang--1 shaft turns both pots together). The grid leak resistance does not change when the master volume is adjusted so there is no change in bias voltage on the power tube grids. Note in the diagram above the grid leak resistors have been removed (see the Type-1 diagram for comparison). The pots function as variable power tube grid leak resistors. The pots' left terminals are tied together and connected to the old junction of the original grid leak resistors. That junction is either tied to ground for cathode biased amps or to the bias power supply for fixed bias amps. If you are building a new amp I recommend installing this MV during the build. I recommend a 100KL dual gang pot when the original grid leak resistors are 100K. For amps with 220k grid leaks like most Fenders I recommend a dual gang 220KA pot like this. Another option is placing two 1.8M resistors in parallel with the 250KA dual gang pots. Solder the resistors across both pots--center terminal to input terminal (as shown below). This will improve the pot's taper and drop the pot's max resistance from 250k to 220k. The "Lar Mar" MV is simply a Type-2 MV using 2.2M resistors added to a 250KA dual gang pot. The 1.8M resistors reduce a 250K pot's total resistance to 220k and act as a safety path for the bias voltage in case the pot wiper fails.

Anyway, that's why I went down the rabbit hole.  You know the one thing I didn't think of doing?  Reading the resistor bands on the drawing (D'oh!).

Thanks again everyone.

[Big_Mike]

Sorry about that, I missed the lack of NFB in this circuit. 

[Amped]

Not a problem Mike, I appreciate you taking the time to respond.

Sorry about that, I missed the lack of NFB in this circuit.