trinityamps.com

Stephen, I've looked at this schematic but (amazingly) can't really "see" what needs to be done here.Sorry

For the post-PI MV, you should be working off the schematic from 18watt.com, that has the dual-gang 1M pot.

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Is the 1M pot better than the 500K version Phil? and Why?

ZP, I need a layout to work from, I can't read the schematics well enough to know how to do it, I'll check though.

http://www.google.com/images?q=PPIMV&rl ... 88&bih=684

Can't find an 18watt specific latout here but perhaps you can get the "idea".

The schematic I'm referring to is this one (note: the resistor shown as 82k is actually 100k)



First of all, since the 18W is a cathode-biased amp, the power tube grids sit at 0V and there is no need for isolation caps after the MV pot wipers. By comparison, the Mark Huss MV in that google link you posted, is the equivalent for a fixed-biased amp - http://www.amparchives.com/album/Marsha ... 0PPIMV.jpg.

Second, you will see that each pot is in parallel with a grid reference resistor, so that the effective resistance is the two in parallel. The stock 18W design requires 470k grid reference (aka grid leak) resistors. A 1M resistor in parallel with a 1M pot gives 500k at maximum travel, which is about right. If you want to be more accurate to the 18W design you could use 910k fixed resistors on the grids in parallel with the ganged pots.

At the max setting, this MV is exactly the same as a stock 18W.

I will also point out once again that in a Classic/Trem or 18W Lite amp, most of the distortion comes from the power stage, and a post-PI MV, won't really behave like an MV - as you turn the MV down the distortion will decrease with the volume, so that the tone cleans up. That's why I prefer to call a post-PI MV a "Drive" control when it's used in 18W amps. In a TMB style amp it will behave more as expected for the TMB channel, but still act as a Drive control to the Normal channel.

Edit: Was this the MV you were refering to Stoo? download/file.php?id=237&mode=view It's actually the same MV, but uses a couple of huge resistors on the EL84 grids to allow use of 500k pots, instead of the 1M. I think it's mainly a matter of what pots you can most easily get hold of. Just make sure to use metal film resistors for those 2.2M, to keep noise levels low.


This mod is so simple it barely needs a layout. You're swapping the two 470k resistors on the EL84 grids with 1M (or 910k) ones, then inserting a 1M pot in parallel with each of those resistors.

HTH

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