The PPIMV (Post-PI-Master-Volume) thread.

[ledvedder]

The thing about the Chupa is that it has two bias circuits so that you can bias each output tube accurately if they are mismatched.

You will need:
A dual 100k pot and knob
2 feet of dual core (2 different colours, i.e. red and black) screened wire.
some shrink wrap correct size for the screened wire
2 2M2 carbon film 0.5w resistors

You apply the commonly found diagram with my amendments below:

n.b. these instructions are very specific to this amp because of the bias arrangement
Remove the two 110k resistors from the board (where J & K wires are hooked up) and the wires between both J & Ks.
Install the pot in one of the output jack holes (the extended one - not one with feed wires!)
Take the shielded wire and solder one end (i.e. red) to J on the board and other (black) to K turret, solder the red wire to the outer right (as you look at the amp front to back) and the black wire to the same lug on the other pot. Shrink wrap the shield at the board end so it cannot touch/short on anything. Extend the shield and solder to the back of the pot only.
Next take the other piece of shielded wire and solder red to J on the swamp resistor and black to K. Again solder the shield to the back of the pot. Take the red wire and solder to the wiper (middle lug of the pot you already have a red wire on and the black wire to the wiper of the other pot.
Next you take the bias feed to each pot, take a piece of wire, ideally red with a tracer from the turret where the other end of the 110k from K went and solder to the far left lug on the 'red' pot. Do the same with a wire from the other end of the 110k from K and solder that to the left hand (empty) lug on the black pot.
Last thing to do is solder a 2M2 safety resistor between the wiper on each pot and the individual wire from the middle of the board.
Check you work and you should be good to go.
Points to note, your individual wires are the bias feed to each half, the shield is not used as bias feed as you would with one bias pot. If you can get a high quality PEC pot I am advised these work best (by George)
Standard ones are here:http://valvestorm.com/Products/Componen ... /Amplifier
Other stuff here:http://valvestorm.com/Amp%20Mods/LarMar

Thanks for this explanation. I have a dual 250k pot and 300k resistors to connect to the pot. This is what Nik from Ceriatone told me to use. Would these also work?

[neikeel]

It depends if Nik has changed his spec. since he posted that layout on his site. He is normally very helpful.

If you were adding a PPIMV to a normal (lead spec.) Marshall or accurate clone which uses 220k grid leak resistors then a 250k pot is correct.

Maybe Nik does not have any 100k pots and wants you to use 250k and parallel the 300k resistors across the pot to get closer to the 110k he uses on the board? (1/250 + 1/300 = 1/136). So near enough?

Did he send you the 2M2 safety resistors too?

[ledvedder]

It depends if Nik has changed his spec. since he posted that layout on his site. He is normally very helpful.

If you were adding a PPIMV to a normal (lead spec.) Marshall or accurate clone which uses 220k grid leak resistors then a 250k pot is correct.

Maybe Nik does not have any 100k pots and wants you to use 250k and parallel the 300k resistors across the pot to get closer to the 110k he uses on the board? (1/250 + 1/300 = 1/136). So near enough?

Did he send you the 2M2 safety resistors too?

I'm guessing that's the reason he said to use the 250k pot with the 300k resistors.

I purchased the parts myself. I didn't get any 2M2 resistors. I wasn't aware I needed them. Are they shown on any of the layout diagrams?

[neikeel]

It depends if Nik has changed his spec. since he posted that layout on his site. He is normally very helpful.

If you were adding a PPIMV to a normal (lead spec.) Marshall or accurate clone which uses 220k grid leak resistors then a 250k pot is correct.

Maybe Nik does not have any 100k pots and wants you to use 250k and parallel the 300k resistors across the pot to get closer to the 110k he uses on the board? (1/250 + 1/300 = 1/136). So near enough?

Did he send you the 2M2 safety resistors too?

I'm guessing that's the reason he said to use the 250k pot with the 300k resistors.

I purchased the parts myself. I didn't get any 2M2 resistors. I wasn't aware I needed them. Are they shown on any of the layout diagrams?

Yes they help protect the tubes if you get a pot failure - you can see them on the pics if you google LarMar PPIMV diagram.

[ledvedder]

It depends if Nik has changed his spec. since he posted that layout on his site. He is normally very helpful.

If you were adding a PPIMV to a normal (lead spec.) Marshall or accurate clone which uses 220k grid leak resistors then a 250k pot is correct.

Maybe Nik does not have any 100k pots and wants you to use 250k and parallel the 300k resistors across the pot to get closer to the 110k he uses on the board? (1/250 + 1/300 = 1/136). So near enough?

Did he send you the 2M2 safety resistors too?

I'm guessing that's the reason he said to use the 250k pot with the 300k resistors.

I purchased the parts myself. I didn't get any 2M2 resistors. I wasn't aware I needed them. Are they shown on any of the layout diagrams?

Yes they help protect the tubes if you get a pot failure - you can see them on the pics if you google LarMar PPIMV diagram.

Yes, but I'm replacing the 2M2 resistors with 300k resistors.

[neikeel]

I do not think that you should (well if you do you may) have problems.

2M2 are high resistance so as not part of the equation they are there to stop run away bias in the event of wiper failure on your pot.

The 300k (I would have thought 270k as that gets you closer to 110k - but Nik will tell you for sure) are, as far as I am aware to put between wiper and the output of the pot, to get the correct value in lieu of the correct sized pot.

Have a read of this whole thread. It morphs from talking about the 'Rich Mod' Mk2 PPIMV into the LarMar etc. You will see posts by Larry (Lar) aka Novosibir who talked Mark Mar) aka Rockstah through the process to make the nice layout diagram of what was probably first put up by Ken Fischer (he of Trainwreck fame).
I quote Larry from page 4:

Larry talk to me more about the 250k dual pot and the 2.2m resistors - not sure where they go

Ok - step by step:
- The coupling caps are connected to the input lug's of the pot
- The bias supply voltage (which usually feeds the both 220K) is connected to the output lug's of the pot (of course to both)
- The pot's wipers are connected to the power tube's grids (or to their swamp resistors)
- Now simply solder each a 2.2M on each section of the pot from the wiper to the output lug - a small 1/4W resistor will do it
These are only for safety, because if the pot's wiper fails, then the output tube's grid is w/o bias voltage and will immediately jump over the Jordan.
But with the resistor applied and a failing wiper, although this side can't be regulated anymore, but nevertheless there's bias voltage on the output's grid - and the tube(s) will survive
Larry

[MrRGecko]

Is this a Rich Mod with a PEC pot? Does anybody know the exact PPIMV circuit for this?

[ledvedder]

I do not think that you should (well if you do you may) have problems.

2M2 are high resistance so as not part of the equation they are there to stop run away bias in the event of wiper failure on your pot.

The 300k (I would have thought 270k as that gets you closer to 110k - but Nik will tell you for sure) are, as far as I am aware to put between wiper and the output of the pot, to get the correct value in lieu of the correct sized pot.

Have a read of this whole thread. It morphs from talking about the 'Rich Mod' Mk2 PPIMV into the LarMar etc. You will see posts by Larry (Lar) aka Novosibir who talked Mark Mar) aka Rockstah through the process to make the nice layout diagram of what was probably first put up by Ken Fischer (he of Trainwreck fame).
I quote Larry from page 4:

Larry talk to me more about the 250k dual pot and the 2.2m resistors - not sure where they go

Ok - step by step:
- The coupling caps are connected to the input lug's of the pot
- The bias supply voltage (which usually feeds the both 220K) is connected to the output lug's of the pot (of course to both)
- The pot's wipers are connected to the power tube's grids (or to their swamp resistors)
- Now simply solder each a 2.2M on each section of the pot from the wiper to the output lug - a small 1/4W resistor will do it
These are only for safety, because if the pot's wiper fails, then the output tube's grid is w/o bias voltage and will immediately jump over the Jordan.
But with the resistor applied and a failing wiper, although this side can't be regulated anymore, but nevertheless there's bias voltage on the output's grid - and the tube(s) will survive
Larry

So, I should put 2 resistors between the wiper and output lug of each side of the pot? A 300k and a 2M2? I'm confused.

[neikeel]

Yes

2M2 safety from middle lug (wiper) to the bias feed off the board and 270k (0r 300) between middle lug (wiper) to the other lug.
Same for each pot.

Alternatively use a dual 100k pot and lose the 270k/300k resistors.

[harleytech]

I have the Rich-mod PPIMV's in all 3 of my builds - JTM45, 1986 and a superlead built to Bass specs(1992 I think..)
I have had them in over a year on all 3 and no problems. I bought everything from Valvestorm (pec pots etc..)
and used the wiring diagrams on their site. They all work great , No problems !

[ledvedder]

I have the Rich-mod PPIMV's in all 3 of my builds - JTM45, 1986 and a superlead built to Bass specs(1992 I think..)
I have had them in over a year on all 3 and no problems. I bought everything from Valvestorm (pec pots etc..)
and used the wiring diagrams on their site. They all work great , No problems !

What's the difference between the Rich-mod and the Larmar?

[neikeel]

The Rich Mod keeps the 220k grid leak (bias splitters) and uses a 500k dual pot. It does work well but is never quite out of circuit and tends to add a little distortion of its own.

In your case you have 110k grid leaks and two dedicated bias feeds. You could simply use your dual 250k pot, leave the resistors on the board and flip the output couplers and wire them almost PTP style to the pots (like George does on some of his builds).

It is not quite as pure as the LarMar which is totally out of circuit when dimed but might work for you.

[harleytech]

What Neikeel said... If it does add a little distortion , I don't notice it..
And the same with it not being all the way out when dimed, I don't notice it at all...
They sound great ! And they do what they are supposed to do...

[ledvedder]

The more that I think about it, I'm not sure if my Chupacabra really needs a ppimv. A lot of folks are telling me not to bother installing it.

[neikeel]

The more that I think about it, I'm not sure if my Chupacabra really needs a ppimv. A lot of folks are telling me not to bother installing it.

I suspect they are correct.