Lar mar master

[phsyconoodler]

Looking at the Lar-Mar master volume,it shows the left lug of the pot having the shield wires grounded there.Does the pot then go to the pot casing?I'm just wondering if anything gets grounded with this master?
I've enclosed the diagram I have.I'm thinking not.

[VMS]

I think the shield wires are connected to the negative bias voltage, which is shielding wise basically the same thing.

[sluckey]

That depends. Notice where the shield connects on the board. If this is a cathode biased amp, that point is ground and you would be OK to ground the left pot lugs to the back of the pot. It's not necessary though because the shield should be grounded at the board.

However, if this is a fixed bias amp, the shield will be connected to the bias voltage on the board. In this case you cannot connect the left pot lugs to the case.

So, either way, don't connect to the case.    

[phsyconoodler]

Yeah it doesn't gt any ground reference.Tried it and it works.However,it oscillates like a stuck pig when turned up.I only used shielded wire to the board and not from the pot to the grids of the power tubes.It has 5.6k grid stoppers already.Might have to up those quite a bit or use shielded wires?Not sure why it would squeal.At low settings with the master it sounds great.Even with the volume off and you turn up the master it squeals.Hmmmm.....

There is no shield connection on the board,it uses a two conductor cable.The grey wires are the shield in the diagram.
This is a 2203 Marshall amp.Still not sure why it's squealing.I've used the type 2 master many times but not the Lar Mar.Guess it needs shielding for all the wires??

[dynaman1]

Huh. I've used that MV dozens of times with no probs in fixed bias amps. I use shielded 2-conductor cable and have always used the shielding braid as the carrier for the bias voltage to the pot. Unless my output tubes are close enough to the pot to run straight grid resistors, I also use the same type of cable to the tubes. I also tie this shield to the bias voltage. Nothing to ground. Give a holler if you want some pics of examples.

[phsyconoodler]

I'm going to try using shielded wires to the grids to.The wires as is are very short,shorter than they originally were and I twisted them.I put the master in the extra speaker jack hole.
  A picture of yours would be nice! Thanks!

[phsyconoodler]

Still squealing.It works great but it still squeals when you try and turn up the master.Odd.
   The amp sounds killer otherwise.
 I think I'm going back to the type 1 master if I can't get this one to stop squealing.
No real reason that I can see right now.

[gldtp99]

I've used this MV in three amps so far without any problems----only in the last one did i use any shielded wire (from MV pot to output grids with 10k grid stoppers). Twisted wire seems ok in the other two--- these three amps were built and working ok before the addition of this style of MV.
Are you sure you haven't switched output polarity resulting in a pos feedback condition ? This could cause the squealing when the MV was turned up...................gldtp99

[dynaman1]

Damn, I can't find the closeups I specifically took of this MV. Eh, here are some not so close pics. All of these amps use the MV you speak of. I never call it "LarMar". I think "Ken Fischer's MV" would be more appropriate.

http://www.myspace.com/andryamp/photos/7891567

http://www.myspace.com/andryamp/photos/7923194

http://www.myspace.com/andryamp/photos/19255818

[phsyconoodler]

I didn't change the output polarity,haven't touched anything else,but that is a possibility.Thanks!

[phsyconoodler]

Solved!
  I had to reverse the OT leads.It still squealed even reversing the wires to pin 5 of each pair of tubes but not as much,so I reversed the plate wires and it works flawlessly.
 Thanks for pointing me in the right direction.It should not be doing what it's doing,but I did a thundertweaks mod as well for more gain.Maybe that has something to do with it?Unknown.All i know is the amp is working 100% at all volume levels.
  Gainiac special.

[Tone Junkie]

Phsyconoodler which thundertweeks mod did you do. Bill

[phsyconoodler]

I removed the bright cap and the treble peaking circuit and installed a 68k resistor in it's place,added a 100uf cap across the 10k on V1,a .68uf across the 820 on the next stage.I tried a 50k mid but didn't like it so it came out.
  Then the Lar-Mar master in the extra speaker jack.Tons of gain on tap;not like a boogie(thank god!)
The amp still has a useable range of tones.The low input now kicks out some coll blues tones.
 Crank the preamp,original master and turn the Lar-Mar down and you get some interesting saturated gain tones.Sounds better than any attenuator I've heard.