Replacing Cathode Resistors With Chokes

[xcortes]

Would this be a good idea?

[Paul Birkeland]

Hello Xavier,

For a voltage amplifier, you (generally) want the cathode resistance to have a very low AC impedance to ground.  In the Quickie, this is why we bypass this resistor with a capacitor.  In other kits, the LED functions to set the bias voltage while maintaining very low impedance to ground.

With a choke, the reactance results in higher impedance than the DCR alone, which is great for a power supply or plate load, but not so good for a cathode resistor in a voltage amplifier.


-PB

[xcortes]

Thanks Paul,

I'm trying to figure out what to do with three pairs of EXO-01s!

Saludos

[VoltSecond]

I don't have a copy of the schematic so I'll generalize:

If the audio path routes across the resistor, the resistor can be replaced with a choke if the DC resistance stays the same. It is normally best to have any added resistance to maintain the bias point placed between the choke and the tube.

If a C4S would be used in that location, an inductor can be a good choice.

--An emitter follower will benefit from choke bias.
--The bias resistor for a class A biased push pull pair of tubes will benefit.  Class B push pull will not.

If an LED or a bypass cap would be used in that location, an inductor is a bad choice.
--The emitter bias resistor for a voltage gain stage will not like an inductor.

[Paul Joppa]

You can use a choke for the upper cathode resistor in an SRPP configuration, making it a mu follower with fewer parts and no capacitors. My experience is that it sounds good but a C4S sounds better.

You can use a choke in the cathode of a cathode follower, providing bias from the DC resistance and a high impedance load from the inductance. This is most useful when the grid must be at ground potential, like a unity-gain buffer or a class A2 grid drive.