Cathode follower > Master Volume > LTPI

[stephenl]

It sounds like some her may have tried this. I'm curious what people's experiences were with this set-up.

I would like to try the KOC bootstrapped MV in this position on my KF50ish build.

I mostly need it as a PA drive control in conjunction with the powerscale circuit.

I have tried a few different post PI circuits but didn't love them.

[Firestorm]

The bootstrapped MV from TUT 3 works as advertised; just don't also bootstrap the EQ as shown because this makes the tone stack even less effective than it was to begin with. The single pot bootstrapped MV has the quirk of never letting the volume get quite to zero. Some of the signal gets through, but just a little. Pre-PI masters with output tube scaling eliminate the ability to generate distortion in the PI, a problem if that's part of the sound you're trying to get at lower volume.

[stephenl]

Thanks Firestorm,
I actually need to reduce the signal into the PI a little anyway. I didn't use the KF50 PI. I was thinking about trying a split load on the CF's cathode resistor - I'm not sure how well that will work. The PI is getting hit pretty hard early on.

[stephenl]

Has anyone tried powerscaling both the PA and PI?

[stephenl]

I ended up installing the LP bootstrapped MV and it works great. It's pretty transparent unlike the other MV types I tried including the LarMar. It also makes the PowerScaling much more useful at low volumes.

[steveneddy]

Hi - dummie here.

Please interpret some of the nomenclature for me please:

LP

The bootstrapped MV from TUT 3

bootstrap

PA - I'm assuming NOT public address

Thanks - :^)

[jhaas]

LP - London Power, a Canadian company from London Ontario owned by Kevin O'Connor who wrote a series of books called "The Ultimate Tone" (TUT).

PA is Power Amp
PI Phase Inverter

I'm familiar with the bootstrapping of a Cathode Follower described here:
http://www.valvewizard.co.uk/dccf.html
but I never got my head around why it's called that.

As far as a boostrapped Master Volume (MV), I'm not familiar with that (I don't own TUT 3 either)

Edit: I just checked my facts and London Power is in "Thunder Bay" Ontario.

[LOUDthud]

I think a "Bootstrap" can be defined as positive feedback of a signal to a circuit element with a gain slightly less than one. This effectively raises the impedance of the circuit element because less current flows when a signal is applied.

Example: A triode has a 1 Meg resistor to ground on the grid. The input impedance is 1 Megohm. Now disconnect the 1 Meg from ground and insert a 47K. The input impedance looks like 1.047 Megohm. Now connect a capacitor from the unbypassed cathode of the the triode to the junction of the 1Meg and 47K. Positive feedback from the cathode at a gain of slightly less than one feeds a signal to the bottom of the 1Meg. If the amplitude of the feedback is 0.9 times as much as the signal on the grid, the input impdeance looks like 10 Megohms because only 1/10 as much current is flowing in the 1Meg resistor.

Below is a schematic of the bootstraped master volume. Feedback to the bottom of the pot makes it look like a higher impedance. KOC recommends that the bottom of the Mid pot also be connected to the point where the low side of the Master Volume is connected.

[LOUDthud]

Double post

[V2]

I use a version of the bootstrapped MV that Kevin sells as a kit (and explained in TUT4):
http://www.londonpower.com/amp-mods/lon ... ter-volume

It works very well, except that it interferes with NFB and controls inserted in the NFB loop (e.g., Presence). So, like a PPIMV, this MV makes the most sense if you eliminate the NFB loop.

edit: the bootstrapped MV shown in the photo above (also found in Kevin O'Connor's books) wouldn't interfere with the NFB.