Paul C P.I. mod on 5E3 P.I. driver

[Mark]

Hi

I have a 5E3 clone on the bench and it has the Paul C P.I. mod done to the driver stage. I have heard that the 470K resistor to the input of the triode is recommended (which this amp doesn’t have) but I’m not so sure about the Paul C mod in the amp.

I was wondering what the consensus about the P.I. stage?

Thanks for your assistance.

IMG_2744.jpeg

[sluckey]

The original pi circuit used bootstrap biasing. The Paul C mod simply converts to fixed biasing. I don't know how the sound is affected. Your mod looks to be done correctly.

[Mark]

Thanks again for your reply and assistance. Much appreciated.

[pdf64]

I think adding a gridstopper to that fixed bias cathodyne should have the same effect / benefit as it would to the stock self biased version.
Grid stoppers on the output valve control grids would probably be beneficial too.

Merlin advocates for fixed bias etc in this rather more extensive cathodyne document https://valvewizard.co.uk/cathodyne.pdf

[Helmholtz]

The original pi circuit used bootstrap biasing. The Paul C mod simply converts to fixed biasing. I don't know how the sound is affected. Your mod looks to be done correctly.

The Paul C mod in the pic on the right uses the full 56k cathode resistor for autobias.
It's not fixed bias as only the grid voltage is fixed but the bias Vgk is not.
Essentially the same as with a normal gain stage, where the grid voltage is fixed to 0V but the cathode voltage adjusts the bias.

[pdf64]

The original pi circuit used bootstrap biasing. The Paul C mod simply converts to fixed biasing. I don't know how the sound is affected. Your mod looks to be done correctly.

The Paul C mod in the pic on the right uses the full 56k cathode resistor for autobias.
It's not fixed bias as only the grid voltage is fixed but the bias Vgk is not.
Essentially the same as with a normal gain stage, where the grid voltage is fixed to 0V but the cathode voltage adjusts the bias.

That's a very fair point!
I'm not sure if Merlin originated the 'fixed bias' term for the circuit in question, but certainly he uses it.
Have you got any thoughts on a more accurate way of describing / identifying it?
eg fixed grid, non bootstrapped?

[Helmholtz]

Have you got any thoughts on a more accurate way of describing / identifying it?

I'd describe the principle as cathode autobias with a fixed, elevated grid voltage.
(Cathode voltage is grid voltage + bias voltage required for the current Vk/Rk.)
Same as with a DC coupled cathode follower or LTPI (like Hiwatt).

I think the grid voltage should only be called bias with a grounded cathode.