Cathodyne Mu Follower Line Out

[merlin]

6922 different than a 12AX7 in that regard?

Oscillation is uncommon in single stages, especially when the signal source is physically close to the tube. A couple hundred ohms should suffice, I would think.

Yes, that and 4:1. Pretty much everyone makes one.

Do you have a preferred part?

 

[atavacron]

Oscillation is uncommon in single stages, especially when the signal source is physically close to the tube. A couple hundred ohms should suffice, I would think.

Great, normal stopper.

Do you have a preferred part?

An API 2623 backwards. 30dB max primary like that. But Jensen, Lundahl, Cinemag, Crimson all do this, with varying flourishes. Edcor too. @ruffrecords did a deep dive into the distortion of a few mfr options a few years ago, I’m not sure what the link is but I recall an excellent PDF.

 

[merlin]

Just playing around in simulation, this puts 20dBu into 600 ohms (Ra and Rk trimmed for unity gain). Not stellar but certainly economical.

 

[atavacron]

Just playing around in simulation,

Thanks for looking into it. Why fixed bias vs self?

 

[atavacron]

View attachment 124851

I see your circuit and raise you a lower output impedance and Av closer to 1. Back to you.


edit: slightly higher gain coming off the plate instead of the follower, also obv then the mosfet is just a ccs and not in the signal path.

and: i don’t get why cathodyne phase splitters are often set up around a lower-than-mid-supply voltage. does it have to do with cathode stress?

 

[merlin]

also obv then the mosfet is just a ccs and not in the signal path.

A follower is not in the signal path? That's a bold claim!

and: i don’t get why cathodyne phase splitters are often set up around a lower-than-mid-supply voltage. does it have to do with cathode stress?

It's because triodes cannot swing rail to rail (they need more voltage across themselves), unlike transistors or pentodes.

Why fixed bias vs self?

Generally preferred because it makes the input impedance independent of the load impedance.

 

[ruffrecords]

Just playing around in simulation, this puts 20dBu into 600 ohms (Ra and Rk trimmed for unity gain). Not stellar but certainly economical.
View attachment 124851

Definitely struggles at +20dBu output into 600 ohms but then we don't need that. PSRR at -10dB is pretty poor. Almost as bad as an SRPP but definitely more economical Could be a serious contender with adequate HT smoothing.

Cheers

Ian

 

[atavacron]

PSRR at -10dB is pretty poor.

Maybe there’s a way to inject the power supply noise in reverse phase, a la many Broskie examples over the years. Series cap and resistor from B+ to the cathode? Something in that direction, null it out…

Oh yeah here’s a 2010 TCJ entry on this

Question is, what’s the simplest way with a single supply. Dual supply is out of scope in most scenarios.

 

[ruffrecords]

Maybe there’s a way to inject the power supply noise in reverse phase, a la many Broskie examples over the years. Series cap and resistor from B+ to the cathode? Something in that direction, null it out…

Definitely possible but it would need another tube. Since we only have the one and the current draw is modest then perhaps the simplest solution is an RCRCRC filter in the HT. It is not too hard to achieve 90dB ripple reduction with this method

Cheers

Ian

 

[atavacron]

Definitely possible but it would need another tube.

I edited the post, curious to know thoughts on that TCJ post.

It is not too hard to achieve 90dB ripple reduction with this method

Or possibly better with a Maida.

 

[ruffrecords]

I edited the post, curious to know thoughts on that TCJ post.

TCJ? Can't seem to find that

Or possibly better with a Maida.

But a lot more expensive and regulation is not really needed.

Cheers

Ian

 

[atavacron]

TCJ? Can't seem to find that

https://tubecad.com/2010/12/blog0196.htm

 

[merlin]

PSRR at -10dB is pretty poor. Could be a serious contender with adequate HT smoothing.

Some boost converters offer bipolar outputs. If the ripple is symmetrical it would sidestep the PSRR issue. Just a thought.