LarMar PPIMV question

[Greg_L]

I've decided I'm gonna put a LarMar PPIMV on my 1973 Marshall Super Lead with 6550s. No big deal. I've done them before. But I don't really understand it. I've always just followed instructions and it works. Yay! Lol.

I'm spare-partsing my "kit" though.

The grid leak resistors are 150k for the 6550s. I have a 100k linear dual pot in a drawer. That should be ok, right? A 250k seems to be most often used pot, but it's also most often used with EL34s/6V6/6L6 tubes, which use 220k grid leaks. I'm thinking my 100k pot should be a good enough match for the 150k grid leaks. Am I wrong?

Also, this mod often uses 2.2M "safety resistors" strapped across the pot as a safeguard against pot failure. I don't have any 2.2M resistors, but I have a pile of 1Megs. Could those suffice?

The rest is pretty straightforward. I love this amp. I want to be able to use it without breaking windows.

[Chuck H]

If you love the amp then you really don't want to change the actual "load" on the power tube grids the way you're using it now. You could use a dual 250k with a 390k resistor across the voltage lug and wiper of each gang for a load of 152k (+/- spec not considered) with the MV at ten. All tweener settings not withstanding, but should be within a safe range for the tubes. AND... The 390k parallel resistor doubles as a safety resistor in this instance so there's no need for the 2.2M resistors.

JM2C

[Greg_L]

If you love the amp then you really don't want to change the actual "load" on the power tube grids the way you're using it now. You could use a dual 250k with a 390k resistor across the outer lugs of each gang for a load of 152k (+/- spec not considered) with the MV at ten. All tweener settings not withstanding, but should be within a safe range for the tubes. AND... The 390k parallel resistor doubles as a safety resistor in this instance so there's no need for the 2.2M resistors.

JM2C

Well it's strange....the schematic calls for 82k grid leaks for a MkII Marshall with 6550s. Conventional wisdom runs them at 150k. I thought maybe I could roughly split the difference with a 100k pot since I already have it.

How necessary are those "safety resistors" anyway? I'm not really looking for shortcuts, I'm just curious. I mean, how often to pots just totally fail?

I'm gonna go to my local electronics warehouse today and see what they have. It's a crapshoot most of them time but sometimes I get lucky. Maybe they'll have a 250k dual pot.

[Chuck H]

How necessary are those "safety resistors" anyway? I'm not really looking for shortcuts, I'm just curious. I mean, how often to pots just totally fail?

There's a reason most newer designs that have adjustable bias use multi turn trim pots rather than ordinary panel pots. They have a lower failure rate in this application. When a panel pot fails open it's most often at the wiper. How often do they fail? Not very. But it only has to happen once and a power tube loses bias completely. Which is a big deal. So the safety resistors are VERY good idea. Providing a fixed resistor to supply bias in the event of any pot failure and preventing the potential consequences of no bias on a power tube.

[Greg_L]

There's a reason most newer designs that have adjustable bias use multi turn trim pots rather than ordinary panel pots. They have a lower failure rate in this application. When a panel pot fails open it's most often at the wiper. How often do they fail? Not very. But it only has to happen once and a power tube loses bias completely. Which is a big deal. So the safety resistors are VERY good idea. Providing a fixed resistor to supply bias in the event of any pot failure and preventing the potential consequences of no bias on a power tube.

Understood. So let me see if I got this....in the event of a pot failure, those safety resistors become the grid leaks. And at 2.2Meg (for example)....that would bias so cold that tubes would just shut down. Is this correct?

[Helmholtz]

Understood. So let me see if I got this....in the event of a pot failure, those safety resistors become the grid leaks. And at 2.2Meg (for example)....that would bias so cold that tubes would just shut down. Is this correct?

Yes, the (normal) grid leak current would make the grids more negative with 2M2s.

But with somewhat gassy tubes or tubes running too hot the grid leak current can turn positive, which might cause thermal runaway and eventually catastrophic failure.
For this reason power tubes have an upper Rgk limit, e.g. 50k for 6550s vs. 500k for EL34s, which should not be vastly exceeded for safe operation.

[Greg_L]

Well my local nerdtronics warehouse didn't have a dual gang 250k pot. Closest I could get is a 1meg that actually measures 750k. So I got a few 200k resistors to go with it and that'll get me near the 150k grid leak value.

So my next question is...



This is the typical wiring scheme for a LarMar PPIMV. It shows the "safety resistors" going from input to wiper. Don't I need to strap the resistors from outer to outer on the pot to get it down to 150k?

[vintagekiki]

"safety resistors" are used that tubes not run without bias voltage if MV potentiometer intermittently loses contact.
I personally am fan TWMV type 3 because it symmetrically decreases both branches of PP, unlike relative matched MV dual gang pot.

[Greg_L]

"safety resistors" are used that tubes not run without bias voltage if MV potentiometer intermittently loses contact.
I personally am fan TWMV type 3 because it symmetrically decreases both branches of PP, unlike relative matched MV dual gang pot.

I actually have that type installed right now. It works but it seems all of it's adjustment is in a very narrow range.

[vintagekiki]

Adjustment depends from type of pot (lin, log ...)

http://www.geofex.com/article_folders/potsecrets/potscret.htm

[Greg_L]

Adjustment depends from type of pot (lin, log ...)

I'm not actually sure which type it is, but it seems like log pot. It's in there more as a proof of concept as opposed to a permanent mod. I might keep it though. It does the job.

[Helmholtz]

All these post PI MVs are fighting against the global NFB, which tries to keep gain and output constant.

[vintagekiki]

It does the job.

It's most important thing.

[Greg_L]

All these post PI MVs are fighting against the global NFB, which tries to keep gain and output constant.

Yeah but what else can ya do without redesigning the amp or using an attenuator?

They certainly sound better than a PrePIMV. These amps don't have enough preamp gain for that.

I've read about zener diode clipping MVs after the cathode follower, but no. I'm not getting into all that.

[vintagekiki]

https://www.guitarscanada.com/threads/help-picking-a-master-volume-ppimv-mod-to-install-in-my-vintage-amp.67761/
Help picking a Master Volume (PPIMV) mod to install in my vintage amp

1)
https://ampgarage.com/forum/files/vvr3a_507.pdf

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