LarMar MV

[leodiditright]

How to install it on my JTM-45 without drilling a hole into the frontplate.
I've seen that Granger amps is putting a pot in place of one of the 4 inputs (into the low input of the normal channel) This is where I'd like to install the LarMar. Any experience with this mod? thanks.

[eddie25]

If you don't want to drill it's either that or a speaker jack hole. Speaker jack hole is recommended because near the inputs is potentially noisy, but try it if you'd like. It's a good mod for a jtm45 for sure.

[leodiditright]

If you don't want to drill it's either that or a speaker jack hole. Speaker jack hole is recommended because near the inputs is potentially noisy, but try it if you'd like. It's a good mod for a jtm45 for sure.

Thanks. I thought about an input jack because it's more convenient to have the pot on the front and I've seen Granger amps doin' it, though I don't know if it's a LarMar or something else that they install in that spot.

[Structo]

Do you belong to the Metro forum?

They are the originator of the that MV (although I think they borrowed heavily from Ken Fischer).

Tons of guys there have installed it and have talked just about everything concerning the installation and operation of it.

[leodiditright]

Do you belong to the Metro forum?

They are the originator of the that MV (although I think they borrowed heavily from Ken Fischer).

Tons of guys there have installed it and have talked just about everything concerning the installation and operation of it.

Thanks. I've been lurking there but I'm not a member yet.

[Colossal]

Yes, the LarMar MV is simply a Ken Fischer Type II master volume. Dual ganged 250kA pot with 2M2 between the wiper and one leg. It replaces 220k grid leak resistors and Vbias is applied to the same leg as the 2M2 is connected to. Typically mounted in Marshall style builds by removing one of the speaker jacks. Use of shielded coax is strongly recommended. Supposedly, this MV works very well to preserve power amp distortion while dealing with excessive volume. The Metro crowd laud this design. Martin Manning posted a schematic on this site.

[David Root]

I have this MV on a pair of 8417s with 270K instead of 2M2, gives about 125K. I like it.

[paolojm]

Yes, the LarMar MV is simply a Ken Fischer Type II master volume. Dual ganged 250kA pot with 2M2 between the wiper and one leg. It replaces 220k grid leak resistors and Vbias is applied to the same leg as the 2M2 is connected to. Typically mounted in Marshall style builds by removing one of the speaker jacks. Use of shielded coax is strongly recommended. Supposedly, this MV works very well to preserve power amp distortion while dealing with excessive volume. The Metro crowd laud this design. Martin Manning posted a schematic on this site.

The Ken Fisher version did not have the 2M2 resistors.
Without them, if the pot wiper fails, there will be no bias voltage applied to the power valves.
That's why many techs recommended avoiding the Fisher MV.