How to have control on the swing of a Cathodyne ?

[kagliostro]

The idea is to maintain a Preamp + Cathodyne PI from a certain circuit (simply the Wangs Mini 5) using as power tube a 6N6P instead of a 12BH7

The 6N6P has a gm of about 3x or 4x the gm of the 12BH7

I considered a MV and also a PPIMV

The Master Volume has the disadvantage that by limiting the input signal to the PI it limits the possibility of distortion of the PI itself and this alters the sound response

The Post Phase Inverter Master Volume, on its part, lets the PI distort following the preamp signal level but also, constitutes a rather heavy load for a Cathodyne PI which although resulting in an increase in distortion and is not the effect I would like to obtain

What I would like to achieve is a reduction in the amplitude of the signal arriving at the G1 of the power amplifier without altering the original timbre of the preamplifier and the PI but maintaining the possibility of pushing, if desired, the final valve to different sound levels, increasing the signal arriving at the more sensitive G1

The solution with a PPIMV would have been the simplest if the PI had been an LTPI but here we are dealing with a sensitive Cathodyne and therefore it does not apply very much to what I would like to achieve

I then tried to find another way and one of the things that came to mind was to use a VVR to lower the B+ voltage on the PI plate only, thus making the swing more reduced and emulating the use of a Master Volume, but I fear that this too could then take on the colouring of a PPIMV

This is because the initial idea was to equip the Cathodyne with two buffers, one for each branch, made up of two MOSFETs, but I don't have enough know-how to design the practical implementation

What do you think? Would a VVR work as hoped or would it color the signal in an unwanted way?

Franco

[Merlin]

I don't see why the type of PI matters. Just use a dual pot PPIMV, like in your first schematic?

[kagliostro]

Hi Merlin

agreed

but in this way the cathodyne will not be too affected by the load represented by the PPIMV and will tend to alter the tonality of the "original" sound ?

I've read that the Cathodyne is not as the LTPI in relation to the load

Thanks

Franco


p.s.: Which one will be the better way, the version with Grid Leak or the version where the Grid Leak are represented by the PPIMV Pots ? ??

[pdf64]

A 90k load on a 500k pot will mess up the pot's taper.
Why not use 100k pot with a 500k to 2M2 load?

[kagliostro]

Because I have 500k Linear pots and no 500k Log pots, the 90k was calculated to obtain a Log taper from the Linear taper

Franco