Tube input headroom

HI
I have been thinking about how the input headroom of a tube behaves.
Would this be a truthfull way of looking at it?

When a bypassed common cathode stage is used the operation  will be limited to Vgc = 0 and whereever the grid has its cutoff threshold. So if Vgc is -0.5V then we could say that 1Vpp input is the limit.

when the tube is unbypassed, the Vgc will change (not DC though)  due to part of the input AC signal now also is present at the cathode. How much is determined by the voltage divider - created by the cathode impedance and the cathode resistor
so if we have cathode Z of 1K and a cathode resistor of 1K, there will be a AC signal at the cathode that is half the amplitude of the inputsignal.
this will let the grid see double the amount of swing as the bypassed version - since a tube with 1V bias, and a 1Vpeak signal will give 1.5V at the cathode at the instance of the peak. and this peak translates to -0.5V Vgc. so we have another 6db to expose for the input in contrast with the bypassed.

my guess is when global negative feedback is tied to the input cathode, the input headroom increases even further. much further.

truth or false?

You are globally right. A good example is the cathode follower, where the cathode following the grid results in huge swing capability.

5v333 said:

my guess is when global negative feedback is tied to the input cathode, the input headroom increases even further. much further.

truth or false?

Click to expand...

In basic terms a common cathode stage followed by a further gain stage with overall  NFB to the first tube cathode acts in a manner similar to an op amp - the overall circuit acts to keep the ac difference between the the grid and the cathode very small. How small depends on the open loop gain and the feedback network. For instance if the open loop gain is 26dB and the feedback network is -6dB, the circuit will have a closed loop gain of close to 6dB. The 20dB of -ve feedback will ensure the input is with 10% of the input voltage. Hence the input can swing 10 times as far before grid current or clipping as the stage without NFB can.

Here is a tube circuit that does exactly this:

https://i.postimg.cc/26XyCxJ5/Eurocard-CCTsht3.jpg

When the variable resistor is open circuit, the feedback resistor is 47K and the resistor from the -ve input (cathode) to ground is also 47K so the gain is close to 6dB. The gain of the SRPP output stage is about 20dB and the 12AX7 stage gain is close to 10dB for an open loop gain of 30dB. This circuit will happily take a +20dB input and produce a +26dBu output even though the 12AX7 dc bias is about 1 volt.

Cheers

Ian

Thanks a bunch!
This clears things up for me alot.