Lar/Mar PPIMV

[Pacosipulami]

For the shielded cable part - I used RG174/U type cable

I did this type of mod on many marshall 1987/1959 type heads over the past years! Works great!

[sepulchre]

Did you connect the shield to the Bias voltage, or to ground?

Do you know why the shield is connected to the bias voltage in the plan/instructions?

[Reeltarded]

Not to ground! The shield carries the bias voltage.

[sepulchre]

To where? I only see it connected at one end, to the pots.

Am I missing something? The diagram I have shows the shields connected to the bias voltage at the pots on one end and to nothing on the other end.

[Reeltarded]

http://ampgarage.com/forum/download/file.php?id=25515

This is not exactly as I installed one. I have only installed two, and removed one because of the NFB circuit going bye byes.

Ahh so Google PPIMV and look at layouts and pictures of installs. I have a small wire under the board from the 220k junction (removed resistors) that goes under the board and terminates at the left lugs of the dual 250k pot if looking from the back. The blk and red wires from the grid feeds are terminated as shown, and the shield itself terminates to the left lugs just as shown in that layout ^^^^ up there.

[sepulchre]

That's the diagram I have too. I wired it exactly as it shows but I can see how the shielding could be used to convey the bias voltage to the pots. However, since I gave the bias its own wire would it matter if I hooked the shielding to ground instead of to the bias V?

Earlier in this thread I said how, since installing the PPIMV it started feeding back, or some kind of low Freq parasitic oscillation, about 300Hz, when turned half way up. It was fine before but I'm looking for a way to use it in the house without replacing windows. I'm also looking into an Airbrake, but that will take a little longer.

Also, I'm concerned about the NFB problem. Does it remain even when the MV is dimed?

(Btw, Happy New year . . . a little early)

[martin manning]

That's the diagram I have too. I wired it exactly as it shows but I can see how the shielding could be used to convey the bias voltage to the pots. However, since I gave the bias its own wire would it matter if I hooked the shielding to ground instead of to the bias V?

Bias voltage is AC ground in that grid circuit, and that is the reason for the shield going to Vbias. Using the shield to carry the bias voltage is an interesting idea that was not part of the original design. Off-hand I can't see a problem with that. Reeltard, did you do one that way? Did it work out?

I have a Lar-Mar in my express, with the leads unshielded but twisted, and it works fine. See: http://ampgarage.com/forum/viewtopic.ph ... 948#205948 It's a non-traditional layout though, and the MV runs are pretty direct.

Also, I'm concerned about the NFB problem. Does it remain even when the MV is dimed?

The beauty of this MV is that when it is dimed, the circuit is 100% back to the original. The odds are that your oscillation is due to some other lead dress issue. For low volume settings I suppose you could increase the NFB with a smaller FB resistor.

[Reeltarded]

Yes I did but was also different values from Metro Amp kit

[M Fowler]

I put Martin's wiring to work in my recent Express but used single lead coax to keep noise down. There is a lot wire going to one dual pot but it works great and once the chassis was in the head cabinet I had no issues from the presonance control.

Mark

[sepulchre]

I used single lead coax too. Yes, there's a lot of wire going to the pot but they are not too long - 2 to 3 inches.

I didn't have the feedback problem before installing the MV. But I'll check everything again. Like Reel, I have a wire running from where the two 220K joined (now gone) to the left side of the pots. That's also where I connected the shields - kind of a mess with all those single lead coax shields coming together. Maybe I'll redo those, connecting them at the board end to clean up the pot area. It's rather crowded with the 2M resistors and all.

[dorrisant]

While on the Lar/Mar subject....

Does anyone see why a 1M dual pot strapped with 250K resistors wouldn't work in place of the usual?
It is strapped to roughly 224K either way... right?

Also... don't know where to source it yet but we have some shielded single pair twisted wire at work... I think it would be the cat's meow. Only have to comb/pick out the ends of one cable...

Tony

[martin manning]

While on the Lar/Mar subject....
Does anyone see why a 1M dual pot strapped with 250K resistors wouldn't work in place of the usual?
It is strapped to roughly 224K either way... right?

It's the same when it's turned all the way up, but as you lower the volume the resistance to ground will increase, which will change the load on the PI.

[dorrisant]

It's the same when it's turned all the way up, but as you lower the volume the resistance to ground will increase, which will change the load on the PI.

Thanks Martin. I think I will try it both ways and listen for the difference... dual 250Ks are delayed in transit.

Tony

[novosibir]

It's the same when it's turned all the way up, but as you lower the volume the resistance to ground will increase, which will change the load on the PI.

Thanks Martin. I think I will try it both ways and listen for the difference... dual 250Ks are delayed in transit.

Martin is right and it's worth the wait!
You'll be disappointed with a dual 1M pot and probably the PO will be driving you nuts

Larry

[novosibir]

Using the shield to carry the bias voltage is an interesting idea that was not part of the original design. Off-hand I can't see a problem with that.

Usually I don't use shielded wires by putting in a PPIMV into an amp, I just twist the cables and additional I'm running the twisted bundles close to the chassis - and never have had PO problems.

Hence I haven't recommended, using the shield as for carrying the negative bias voltage.

But there's NO reason, not using the shield for this purpose

if you're using shielded cables.

Larry