Cathode follower question

[J Rindt]

I guess i am thinking something like a Cathodyne or LTP phase inverter.
If the plate resistors are equal, will the amplitude of the output at the cathode be the same as at the plate.?
I understand the Cathodyne is just one triode, and 1 plate resistor .....but i have the same question.
Thank You

[PRR]

Question unclear.

[tubeswell]

Your topic post heading mentions ‘cathode follower’, but the stuff you talk about in your post is about a cathodyne inverter. Cathodyne inverters need two load resistors to work (not just the plate resistor). And they deliver their lowest possible output impedance if both load resistors are the same.

[J Rindt]

    Yeah Sorry.Let me reword this.
On the Cathodyne PI  the amplitude of the output of the Cat will be equal to the plate if the load on each is the same. Is that correct.?

But the typical LTP i see does not do that, or maybe i do not understand how to calculate the load on the Cat of a LTP.
Are the loads the same at cathode and plate for that first triode of a LTP.?
Is that why a LTP will frequently have different value  plate resistors.?
I hope that makes more sense.
Thank You

[tubeswell]

The output impedance of a cathodyne plate and cathode are the same if the load resistances are the same. The more imbalance there is between the two, the higher the output impedance gets. The output impedance is the same for each electrode as it would be if you were using the same type of tube in a cathode follower configuration with the same overall (summed) load resistance. This unusual situation arises because the gain at each output electrode is unity gain (if the loads are balanced).


Calculating output impedance for a LTP is different because you need to factor in the effect of the shared unbypassed tail resistance. Without going into all the integrals and hand-waving, the upshot is that the gain from each plate of a LTP is about 1/2 the gain of what you’d expect from using the same tube type in a typical common cathode gain stage typology that you find in most Fender blackface amps, all other things being equal.

[J Rindt]

OK... GreatThank You

[HotBluePlates]

Is that why a LTP will frequently have different value  plate resistors.?

The gain-to-output is different on each side of the long-tail pair, so the plate loads are trimmed a little to compensate (and bring them closer to equal-gain).

You should read this.