Mu Follower Driver for Push-Pull

[scott17]

I'm sure this has been done before, but does it look feasible?

Scott

[DF96]

It is feasible in the sense that it will amplify audio signals. Possibly sub-optimal in the sense that a simple active load for the first triode would probably work just as well.

Driving an EL34 ultralinear output could require the phase splitter to use a significant part of its characteristic, so some distortion could occur here.

[tinitus]

Originally Posted by DF96

It is feasible in the sense that it will amplify audio signals.

not that I know much about it
but when using the low side out output of the mu-follower, which is high impedance, is there something about loosing gain, when 'receiver' load is less than in the mega ohm range

'upper' output on mu-follower is of very low impedance, and thus not a problem
but high impedance low side might be used fore DC coupling
why that is I dont know

[DF96]

Sorry, my mistake. Yes it is just an active load, not a mu follower. I should have ignored the thread title.

The 12AT7 will see an anode load of about 1M so it will be as linear as a 12AT7 can be.

[scott17]

Originally Posted by tinitus

not that I know much about it
but when using the low side out output of the mu-follower, which is high impedance, is there something about loosing gain, when 'receiver' load is less than in the mega ohm range

'upper' output on mu-follower is of very low impedance, and thus not a problem
but high impedance low side might be used fore DC coupling
why that is I dont know

Actually the output impedance of the lower output is "higher", not too high. About 12K in this case. The input impedance of the cathodyne is very high so the gain of the 12AT7 should not be affected.

[tinitus]

how do calculate/choose the right phase splitter tube, to ensure good balance between anode and cathode ?
(honestly I dont understand why theres not more gain from upper anode side, but apparently both sides are unity gain)

I understand that the limited drive of cathodyne phase splitter can be ok with easier driven tubes like el84
but EL34 can be a bit more tough to drive, or ?

also, cathodyne phase splitter might be more easily 'overdriven', from what I have read
considering the very high gain of mu-follower, would this cause 'issues' ?

[scott17]

Originally Posted by tinitus

how do calculate/choose the right phase splitter tube, to ensure good balance between anode and cathode ?
(honestly I dont understand why theres not more gain from upper anode side, but apparently both sides are unity gain)

I understand that the limited drive of cathodyne phase splitter can be ok with easier driven tubes like el84
but EL34 can be a bit more tough to drive, or ?

also, cathodyne phase splitter might be more easily 'overdriven', from what I have read
considering the very high gain of mu-follower, would this cause 'issues' ?

The Morgan Jones book suggests a low mu tube for better output balance. Other sources suggest this as well.

There may very well be too much gain in the mu-follower. It is direct coupled through a voltage divider to match the calculated grid bias of the phase splitter. The 6N1P is pretty much pushed to the limit in this case, and may not (probably won't) drive the EL34s very well at all. Another stage could be added. But this is an experiment.


Here is a good article by Albert Preisman: http://www.aikenamps.com/cathodyne.pdf

[TheGimp]

If you are going to use a 6N23P, why not use both stages for the Mu Stage as it is designed for Cascode operation per the data sheet.

[scott17]

Originally Posted by TheGimp

If you are going to use a 6N23P, why not use both stages for the Mu Stage as it is designed for Cascode operation per the data sheet.

My thinking was that the higher mu of the 12AT7 would provide better gain. But you may be right.

[DF96]

Originally Posted by tinitus

how do calculate/choose the right phase splitter tube, to ensure good balance between anode and cathode ?
(honestly I dont understand why theres not more gain from upper anode side, but apparently both sides are unity gain)

In this phase splitter circuit balance comes simply from having equal loads on cathode and anode. As both are the same total impedance and carry the same current they develop the same voltage. The voltage gain is a bit less than one.