james david low

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Train Wreck Type-2 Lar-Mar on a Victory V40

I’ve been looking at doing a post phase inverter master volume mod to my ’68 Fender Vibrolux. I was wondering about using what was on the Victory V40, since it is a American-style tube amp. It turns out they seem to use a Type-2 Lar-Mar but with a larger value pot, and smaller value safety resistors.

I think the advantage of this setup is that with the resistors in parallel, the lower resistance makes more difference when calculating the total resistance. The better tolerance of the fixed resistors keeps the total grid leak resistors more consistent between the two power tubes, as pot tolerances can be ±10% of the stated value.

Here’s a comparison:
1) 10% tolerance 250K pot in parallel with 1M resistor:
Min = 225K x 1.0M / 225K x 1.0M = 184K
Max = 275K x 1.0M / 275K x 1.0M = 216K

2) 10% tolerence 1M pot in parallel with 250K resistor:
Min = 0.9M x 250K / 0.9M + 250K = 196K
Max = 1.1M x 250K / 1.1M + 250K = 204K

As you can see, even though the pot tolerance is the same, the end result is a much smaller deviation between the 2 grid resistors with a 1M pot vs a 250K pot.