5v333
Well-known member
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?
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?