Probably not...

[Earl]

Just wondering if anyone uses partially bypassed pre-amp stages.

I saw a design the other day where the Rk was 1000 + 500 ohms (series), and the 500 ohm leg was bypassed with the typical large value cap.

I reckon this is done as a means to reduce the stage gain.

[Joe Bee]

I have seen that done in the 30W combo I butchered for my SLO project. I am wondering if really is just about reducing gain, since that could be done by using a smaller plate resistor with the added benefit of reduced output impedance. And would anyone really care about a 1 or 2dB gain reduction?

[CarlZ]

It's not about gain reduction. It's about changing the eq curve of the particular gain stage. partially bypassing a cathode resistor like that has a completely different response curve than either changing the coupling cap on the output, or bypassing the plate resistor.

-Carl

[Don Moose]

So, does it move the curve lower in frequency if you do that? (I'm thinking so, but ready to be corrected)

[clyde]

cathode bypass

generally speaking, using smaller value caps for bypassing the cathode resistior will brighten the signal. using the smaller value will produce what's called a shelving effect. lower frequencies are attenuated and higher frequencies are not.

[Earl]

If we're speaking in terms of the 'typical' 1500 ohm/25uF triode circuit:

If you make the 1500 ohm Rk out of a 1000 and a 500 ohm resistor, and then bypass the 500 ohm resistor (leaving the 1000 connected between there and the cathode) - the gain of the stage drops by ~32%. The lower -3dB roll off point moves from the fully bypassed frequency of ~8hZ to ~17hZ.

Just from an acedemic standpoint, what would be more obvious, a gain change from ~60 to ~37, or a -3dB point of 8 hZ to 17hZ....?