What is the reason of having a cathode follower in Marshall type preamp circuit?

[Alan0354]

Alan - is it possible that what you are seeing is the negative going slew rate limited waveform caused by the capacitive load of the TS? Lift the connection to the TS and see what happens.

It's the loading of the TS.

I want to be specific, if I remove the TS, you get rid of the clipping at the bottom!!! It's the loading of the TS that cause the problem. But the whole point is to have a low impedance to drive the TS, but that turn out to be the limiting factor. The result is like you hitting the wall both at the top and at the bottom. Changing +B or Rk just move the window up and down when over driven.

Everything can be explained in theory here, I am not question any of the theory, just an observation. ChuckH is right, maybe it does generate even harmonics as it is not symmetrical clipping.

[Chuck H]

Alan - is it possible that what you are seeing is the negative going slew rate limited waveform caused by the capacitive load of the TS? Lift the connection to the TS and see what happens.

Well, since the question was about CF's in Marshall amps is it fair to judge the issue outside of the normal operating conditions? I don't know anyone who's lifted the tone stack from the CF for improved amplifier performance!?!

[Alan0354]

Well, since the question was about CF's in Marshall amps is it fair to judge the issue outside of the normal operating conditions? I don't know anyone who's lifted the tone stack from the CF for improved amplifier performance!?!

well, lifting the TS, the amp won't work!!! Lifting the TS definitely getting rid of the clipping due to current starving of the Rk. But you'll hit the bottom of the gain tube also. The plate don't go below 100V so you bottom out one way or the other.

BTW, the current starving of the CF is not limited to tubes, you see the same thing with transistors common collector ( emitter follower) or common drain ( source follower) also exactly like this. The way to improve it is lowering the pull down resistor Rk to pull more current.

[nickb]

Well, since the question was about CF's in Marshall amps is it fair to judge the issue outside of the normal operating conditions? I don't know anyone who's lifted the tone stack from the CF for improved amplifier performance!?!

Chuck - it was just for a diagnostic test.