MosFET or other semiconductor substitute for cathodyne?

[tikiroo]

They do. For a PNP or PMOS you need to run the preceding tube anode at ~ 2/3 B+ instead of 1/3 B+ which can allow them to run at a better operating point sometimes.

 

[b_force]

They do. For a PNP or PMOS you need to run the preceding tube anode at ~ 2/3 B+ instead of 1/3 B+ which can allow them to run at a better operating point sometimes.

Oh yeah, that's actually true.

This also leaves an interesting practical choice.
Because in some cases a quite high anode resistor needs to be used, otherwise a DC coupled cathodyne just doesn't work very well.

With a PNP, this works exactly the other way around!
So a lot less issues with nipple distortion! ©

It will be hard to find a PMOS that will play nice around 250-350V

 

[stocktrader200]

Generally a pnp transistor is not used here.
With emitter to b+ it amplifies power noise as well as tube signal.

 

[wg_ski]

So does a tube, and so does an NPN. Same voltages are developed at the emitter and collector. LTPs have CMRR/PSRR, these don’t. P supply noise here originates in the single ended driving stage anyway, so not much to do about it.

 

[b_force]

These days, I usually slap in a capacitance multiplier with a Mosfet on the B+ anyway.
Since it costs bugger all, is super easy to implement and will just make the ripple go away completely.

But I had a couple of occasions that I couldn't use the DC follower because the anode resistor was getting to big.
In general with a 12AX7/ECC83, you need at least above 150kOhm to make that work.
So in that case it would work without a problem with a PNP

I think a cathodyne is quite need, since the distortion is very low, so the sonic impact is also very low.

 

[wg_ski]

The cap multiplier….Makes ripple go away completely - and gets rid of pesky low frequency feedback paths through the supply. And if regulated (or semi regulated) the decoupling and bypass caps downstream only need to be rated for the reduced supply, not the full unloaded voltage. That can be a big deal.

PNPs do solve the problem of needing to elevate the heaters. It just doesn’t have one. Neither do N channels, but they’re typically under 100V on the source/emitter/cathode.