What is this called? "Triode with active load"?

[DF96]

Is the half-mu stage meant to be done with a cathode bypass cap?

No. If you bypass the lower cathode then you get gain nearer mu than mu/2.

Very roughly, a valve used as an active load (with just an unbypassed cathode resistor, as used here) will present a similar impedance to a resistor dropping the same DC voltage at the same DC current. If you bypass the upper cathode then you get worse AC performance than a resistor.

To get a significantly better result than a resistor you need to do the trick used by the mu-follower: separate AC and DC cathode resistors.

I was asking about the topology, not about the limitations of a 12AX7.

The topology gives you mu/2 gain and ra/2 output impedance.

 

[rongon]

No. If you bypass the lower cathode then you get gain nearer mu than mu/2.

Very roughly, a valve used as an active load (with just an unbypassed cathode resistor, as used here) will present a similar impedance to a resistor dropping the same DC voltage at the same DC current. If you bypass the upper cathode then you get worse AC performance than a resistor.

To get a significantly better result than a resistor you need to do the trick used by the mu-follower: separate AC and DC cathode resistors.

Thanks for the explanations. Much appreciated.

The topology gives you mu/2 gain and ra/2 output impedance.

Now *that* has possibilities. I often find that the only time a line stage is useful is if it has gain of about 5x to 10x at most, and Zout of 2k or less.

A 5687 or 6N6P in this 'half-mu' configuration should have gain of about 7x and Zout of about 1.5k ohms. As long as its distortion isn't awful, that strikes me as a useful line stage that might compete with something like a 12B4A or 6V6-triode line stage. An added benefit is that no hot-running, high wattage wirewound resistor is needed in the plate. Well, you do get a hot-running, high wattage tube running, but that would be mounted above the chassis and it doesn't expose its high voltage to the outside world.

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[TonyTecson]

You know me, I'd use a good current source, then couple to a follower. 12AX7 is a tricky tube to use well, despite its ubiquity.

yes SY, but me i use the 12ax7 on guitar amps only, for gain i use the 12at7,
i know what you mean by a follower....😀

i recently made a line amp out of the 4kn8 twin triodes...
using active loads on both the gain stage and the cathode followers....

 

[SY]

Your work is always so much neater than mine. Sigh.

 

[TonyTecson]

thanks SY, this is the final top view...