SET Asylum

RE: This long thread just tells me what commercial SETs aren't....

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In Reply to: RE: This long thread just tells me what commercial SETs aren't.... posted by tube wrangler on September 03, 2017 at 12:37:44

"Yes, it does, some of it. Some signal goes thru the resistor"

Sure, some of it does.

let's say you have a 1000 ohm cathode resistor and a 100uf bypass cap.

What portion of a 1000Hz AC signal flows through the resistor?

Let's do the math.

The reactance of the cap at 1000Hz is 1.592 ohms.

So you have a 1000 ohm resistor shunted by a 1.592 ohm resistor.

Let's just pick .1 volts AC at the cathode,

62.814ma flowing through the cap

1 / 1.592 = .062814 amps

And .1ma flowing through the resistor.

1 / 1000 = .0001 amps

"Each component has a profound effect on the sound qualities
of the tube's plate and cathode signals. "

So you are saying that the cathode resistor that is conducting less than 1/628th of the total AC current at 1000Hz has a profound effect on what the 1000Hz AC signal will sound like? Get real Dennis.

Maybe at lower frequencies where the reactance of the cap is higher but the whole point of a bypass cap is to make it's value such that the reactance at the lowest frequency of interest is a small fraction of the value of the cathode resistor.*

I'm sure in your design (where the cathode by pass cap is too small) the resistor does carry a lot of low frequency AC (a lot more than it should) and that resistor does make a profound effect on the sound but that's only because your design is flawed.

If you let too much (anything more than a tiny fraction) of the AC flow through the resistor instead of the cap then the bias will change.

That means you will have negative current feedback applied to your output tube and it's plate resistance will increase and the load line will rotate towards the vertical and that will cause an increase of the harmonic distortion as well as a loss of damping factor. (And none of that has anything to do with the sonic properties of the resistor used.)

but you knew all that, right Dennis?

* note, it's really the value of the cathode resistor in parallel with the impedance of the cathode but that discussion is for a different time.

Tre'

Have Fun and Enjoy the Music
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