Train wreck Master Volumes?

[pdf64]

Sounds they are arguing a technicality, but nobody bought the argument anyway, cuz it still wasn't what the people wanted!

I remember reading accounts that Fender's chief engineer, though extremely competent in audio etc, wasn't a guitarist; so though he tried to give marketing what they asked for, it never ended up quite right. I think that over the intervening decades, the type 4 master has proved to be by far the predominant arrangement, probably because it facilitates (clean/overdriven) channel switching, and the implementation of fx to the overdriven signal, rather better than the others.

You could stick a Type 4 anywhere where there is only one signal to be attenuated, and it'd "work." Not that it would meet any expectations, but that's okay... I like wacky and am willing to try it. I agree that it's not a "Master Volume" in the normal way we understand it..

'Master volume' usually indicates the last significant gain control in the signal path (there may be later level controls eg fx loop send/return levels, boost).

Theoretically, could I not stick a type 4 before the first gain stage (right after the input), after it, and another after a second gain stage? Assuming only two gain stages you would be able to stick 3 in? Anywhere before the PI... I ask not expecting a result, but that a Type 4 is stupid-simple and easy to implement almost anywhere...

Yes, it's common to have several such pots in a high gain pre-amp, eg http://bmamps.com/Schematics/fender/super_112_schem.pdf
They are all gain controls, the earlier ones may be called overdrive, distortion etc, the last, as above being the master (may be referred to as channel volume).

The Type-2 will work with fixed bias amps but you may get some pot scratchiness when you adjust the pot. You can't add coupling caps to the Type-2 because the DC bias must travel through the circuit to the grids.

The additional 2M2 resistors of the LarMar arrangement http://forum.guitarplayer.ru/attach/290612-975044.jpg may act to mitigate the scratchiness when the control is operated, and also the risk of loss of bias should the pot fail and the wiper lose its connection to the bias supply.
I think that the latter definitely makes such back up resistors well worth adding to any gain pot feeding a power tube control grid.

[Steve A.]

The Type-2 will work with fixed bias amps but you may get some pot scratchiness when you adjust the pot. You can't add coupling caps to the Type-2 because the DC bias must travel through the circuit to the grids.

While we could avoid adjusting the MV during a song I would think that the scratchiness might lead to a dead spot which would allow a tube to run too hot. There was an old AMPAGE thread about adding resistors across the MV to make sure that did not happen.***

As to the value and taper of the pot I remember Randall Aiken recommending that 220k bias resistors be replaced with 100k. It might not be a bad idea to use a 250k pot with 220k or 270k resistors across it. Linear taper has worked great for me.

*** Apologies to pdf64 for not reading his reply first. My bad!

Steve Ahola

P.S. It should be noted that Ken Fischer never used master volume controls in his amps but he did design the Trainwreck Air Brake attenuator which he eventually licensed to Dr. Z.

P.P.S. Type 3 MV's get very gritty when turned down low as the clean signal is attenuated but not the differences (mainly distortion) between the two phases.

Type 4 MV's work best with a high gain preamp that already has enough distortion without adding that from the phase inverter, which is why the Type 1 or 2 MV's are usually recommended for a vintage amp. (With the Type 1 MV you want to add large caps like 0.1uF to minimize the effect on tone. One drawback is that with the dual gang pot and large capacitors they do take up quite a bit of space.)