Max signal input to a 12AX7 (or any tube)

[Tonewood]

Trying to learn about tube amps. I'm missing something very basic. An answer to one of the following may answer all of them.

When looking at the anode characteristics graph of a 12ax7, it looks like the most AC input swing you could get without clipping is around 4 volts pp. Correct?

If so, if no clipping is desired when cascading 12AX7s, do you have to keep attenuating back to < 4v for the input of the next triode ?

If so, is the cascading just for tone shaping via some controlled clipping other tone manipulation?

The questions that got me wondering-
I have guitar input to half a 12ax7 which then goes into the other half which is a cathodyne PI. Like a cathode follower, it has a gain of about 1 per output (i think). If I can only put 4v into the PI then can I only get 4v out?

If true, then how do I get enough voltage to overdrive a pair of 6V6s?

Thanks
-Scott

[Old Tele man]

...you are correct, the maximum class-A input signal will be constrained by the plate voltage and bias point conditions...as best "seen" on the plate and grid curves.

...unfortunately, those curves usually only show "typical" voltages, not maximum or limiting voltages.

[Steve Conner]

Hey Tonewood

The 4 volt limitation doesn't apply to the cathodyne PI stage. The curves in the data sheet say that you can apply about 4 volts maximum signal between grid and cathode before the tube goes into grid conduction or cutoff. But the cathodyne's cathode follows its grid (it's closely related to the cathode follower) so you can apply considerably more signal between grid and ground.

Here's how the signal levels in your circuit would stack up. Your guitar puts out, say, 0.5V if you have hot pickups and strum it hard. The first 12AX7 stage amplifies that by a factor of about 50, so you have 25V. The cathodyne PI amplifies that by +1 on its bottom output and -1 on its top, so each 6V6 grid gets 25V of signal, enough to get them good and dirty.

When cascading gain stages in a modern-style high-gain amp, you do need to attenuate the signal in between stages, otherwise the tubes get driven too hard and it sounds like a faulty Fuzz Face.

[Tonewood]

Thanks Steve and OTM. You cleared things up for me. I'll go read about cathode followers next.