High plate - low plate

[Mr. dB]

Correct no cathode bypass cap.

With a cap all the 100Ks would be the same, all the 220Ks, and all the 470Ks?

[norburybrook]

Doesn't the cap just act as a high pass filter, and have nothing to do with the gain?


M

[xtian]

Doesn't the cap just act as a high pass filter, and have nothing to do with the gain?

No. The bypass cap allows the cathode voltage to remain more steady under the influence of AC signal, providing a gain boost. With 100K/1K5, using a 22-25uF bypass cap is called "fully bypassed," meaning all audible frequencies, and you get a gain boost.

[norburybrook]

I was under the (wrong obviously) impression that the cathode bypass caps were high pass filters, and changing the values changed the amount of bass being passed.


M

[pompeiisneaks]

I was under the (wrong obviously) impression that the cathode bypass caps were high pass filters, and changing the values changed the amount of bass being passed.


M

You were completely correct. If you get down to about 25uF it's covering completely the range of guitar, so you're 'byassed" if you go up to say .1 or so it starts really cutting down the bass and even into the midrange at that point.

The treble is left intact. It is a high pass filter. A good show of this is using the ampbooks https://www.ampbooks.com/mobile/amplifi ... alculator/ that helps you see what bass frequency is lost at specific cap ratings as well as the anode, cathode and screen resistors values etc. (remember it's an RC filter so the resistance has some impact as well).

~Phil

[Mr. dB]

I was under the (wrong obviously) impression that the cathode bypass caps were high pass filters, and changing the values changed the amount of bass being passed.

It is, but it effectively shorts the cathode to ground at audio frequencies, above the cutoff frequency. At DC the cathode resistor still biases the tube for the desired current flow.

The cathode resistor is sort of a form of local negative feedback, or degeneration, without a bypass it affects the gain of the stage - as the cathode resistor value increases, there's more degeneration and less gain (also less current and higher output impedance). With a bypass, the gain at audio frequencies is the same as with a cathode resistor value of "0", a short.

[Mr. dB]

So in effect the cathode bypass isn't so much a bass cut, it's a high boost.