Lar/Mar PPIVM 68 Plexi 100w

[Ampzone]

Has anyone tried a 1meg dual pot instead of a 250k for the Lar/Mar PPIVM in a 68 Plexi circuit? I ask bc I am curious about this mod and only have 1meg dual pot. I imagine it will ramp up quicker, but will it work ok? Ive heard of people using 500k.

Thanks for any advice in advance

Lar mar/ layout
Lar mar master volume image by RussBert on Photobucket

[RedHouse]

Has anyone tried a 1meg dual pot instead of a 250k for the Lar/Mar PPIVM in a 68 Plexi circuit? I ask bc I am curious about this mod and only have 1meg dual pot. I imagine it will ramp up quicker, but will it work ok? Ive heard of people using 500k.

Thanks for any advice in advance

Lar mar/ layout
Lar mar master volume image by RussBert on Photobucket

I have not, but the 250k works great.

[Ampzone]

Thanks for the input. What I am really wondering, can a 1meg be used without it be too out of range? I have a 1meg dual pot now but the local supplier is out of 250k pots.
I wanna try this mod

[Chuck H]

If the "Lar/Mar" master vol. circuit is the same as the one that replaces the bias feed resistors with the pots then I would advise against using the dual 1M pot by itself. You could parallel a fixed resistors on each gang of the dual 1M pot so that the max bias circuit resistance doesn't exceed 250k and that would be fine but it will give the control an unusual taper.

And if this is a master vol. that replaces the bias feeds I would suggest changing any amps using a dual 500k pot to a proper dual 250k also.

Chuck

[pdf64]

If a 1M linear pot was used with 270k grid feed resistors, then the resulting taper would probably useful, similar to the fender 30% audio taper.

[Ampzone]

Thanks for the advice guys...I had removed the 220k feeders on board. Should I put them back to get the dual 1meg pot to be in a better range?

[novosibir]

You can't use a 1M pot and you can't use a 500K pot, if you want to benefit from my version of a PPIMV!
There are a lot of reasons why, which I don't want to explain again & again in the forums, but the main reason is, that the load on the PI plates doesn't drift much away from the ideal of 220K, no matter where the pot's wiper is set to.

Try it with a 1M pot and a safety R of 270K as suggested - dimed the PI plates are seeing 212K load - the pot nearly closed and the PI's plates are seeing 1M load - a totally different sound - and the sensitivity of the cables to the output tubes is enhanced by the factor FOUR for catching crap fields, crosstalk, RF, aso. - and then the people are whining, that their amp is oscillating, or is making funny noises, or is sounding thin and lifeless...

... only, because the guy already had a 1M pot laying around and was too lazy, to order the RIGHT ONE

Check my signature!

Larry

[Ampzone]

You can't use a 1M pot and you can't use a 500K pot, if you want to benefit from my version of a PPIMV!

Well ok...thanks.

There are a lot of reasons why, which I don't want to explain again & again in the forums, but the main reason is, that the load on the PI plates doesn't drift much away from the ideal of 220K, no matter where the pot's wiper is set to.

I searched here but found nothing regarding PPIV pot value variations...faulty search function perhaps? Thanks for explaning "again" here.

Try it with a 1M pot and a safety R of 270K as suggested - dimed the PI plates are seeing 212K load - the pot nearly closed and the PI's plates are seeing 1M load - a totally different sound - and the sensitivity of the cables to the output tubes is enhanced by the factor FOUR for catching crap fields, crosstalk, RF, aso. - and then the people are whining, that their amp is oscillating, or is making funny noises, or is sounding thin and lifeless...

If I am understanding you right the mod version PDF64 kindly suggested will not work. I will avoid that suggestion so we can avoid the "whining"

... only, because the guy already had a 1M pot laying around and was too lazy, to order the RIGHT ONE

Are you suggesting I am lazy? If so I dont think I could bear it

Check my signature!

I checked it...does this apply to you?

[novosibir]

Are you suggesting I am lazy? If so I dont think I could bear it

No, this was said in general. If you'd have been lazy, then your post wouldn't have appeared here

I checked it...does this apply to you?

Shure! Again and again this still happens to me, despite working on tube amps already 25 years.

Larry

[Ampzone]

I have to tell the truth...I am a bit lazy. But I did check the local supply house. Seems no one has a 220k. I will order one upon your suggestion. I am looking forward to trying this out as it sounds to be the most transparent MV variation out there.


I already had the shielded cable and installed it last night Now I have to find a dual pot at digikey. Sucks the 1meg will not work as it is a really nice Allen Bradley dual pot. They are such fine pots.


Stay tuned for a tone report Congrats for coming up with this version.

[pdf64]

Larry makes a good point, with a 1M pot the source impedance that are presented to the grid, are much higher at most intermediate pot settings, which might well cause problems.
I was trying to be helpful regards the taper, but didn't take account of other issues. Though I'm sure that I'e seen commercial designs, and read reports from builders and modders, that have used dual 1M pots.
I can report very good results with a dual 220k log pot with 2M2 'fail safe' resistors, as per the diagram above, in my Fender BF based build.

[Ampzone]

PDF64 -- I understand...you know it's going to be a week before the brown truck brings me some more parts. I am itching to try this. Do you think with the safety R's installed I should just try this 1meg since as others have. I mean what's the worst that can happen?

Noise, not sparks I would hope

Larry would you think I was crazy to try it?

[Ampzone]

Update:

Dual log 250k pot arrived. Hooked everything up with shielded wire. I unsoldered the green/orange wires coming of the removed 220k bias resistors at the tube socket lug but left them attached at the board lugs. Should they be removed entirely?

Powered up the amp has low hum now that gets louder as you roll up the PPIVM pot. Also when maxed I got a loud POP!


One thing to note I am using a KT88 tubes in a 68 plexi circuit. The bias was modded for the KT88's to work.

Any ideas?

[novosibir]

I unsoldered the green/orange wires coming of the removed 220k bias resistors at the tube socket lug but left them attached at the board lugs. Should they be removed entirely?

They MUST be removed entirely. Your amp isn't a transistor radio, but a highly sensitive animal!
Anything what's left in the circuit w/o function is acting like an antenna and catching hum, buzz and other several noises up to RF radiation

One thing to note I am using a KT88 tubes in a 68 plexi circuit. The bias was modded for the KT88's to work.

Your late with the details of your amp - very late!
When using KT88's you'd need a 100K log. double pot with 470K safety resistors on it, to get the 82K bias feed resistance, KT88's are requiring.

Any ideas?

Anytime enough

Larry

[Ampzone]

Hey Larry, sounds good, I will remove those two wires. I asked bc I was suspicous about them staying under the board and causing noise but did not see anything about their complete removal on the PPIVM thread.

About the KT88's damn! I should have thought about that How silly of me. Ok a 100k pot on order now.

I appreciate your help Larry. A big thanks. Stay tuned...I hope for a final tone report with positive results