Common-Cathode Stage,
bypassed Cathode

Recommended to read:
Using Tube Curves - Load Line Calculations

Tube CAD Journal - Extrapolation from Plate Curves

Tube CAD Journal - The Grounded-Cathode Amplifier

The Valve Wizard - Heater / Filament Supplies

Common-Cathode Stage, bypassed Cathode

By placing a capacitor in parallel with the cathode bias resistor any instantaneous rise in cathode current will be diverted into charging the capacitor, and if cathode current falls, the capacitor will supply the deficit from its own charge. Another way of looking at it is to say that the capacitor shunts or ‘bypasses’ to ground any AC signals on the cathode so that signal current does not flow in the cathode resistor, while the DC bias voltage remains unchanged. With either explanation the result is the same: the cathode bypass capacitor ‘smoothes out’ changes in cathode voltage, helping to hold the cathode voltage constant, preventing cathode feedback and allowing full gain to be realised.

A capacitor will allow greater current flow at high frequencies than it will at low frequencies. If we want the stage to have maximum gain at all audible frequencies then the capacitor must be large enough* to smooth out the lowest frequencies of interest, and the stage could be described as being ‘fully bypassed’. If the capacitor is made relatively small then only high frequencies will be smoothed out while lower frequencies will not. Therefore the stage will have maximum gain at high frequencies and minimum gain at low frequencies, producing a treble boost, and the stage would be termed ‘partially bypassed’. To the designer, this is an extremely useful consequence of using cathode bias. If the stage has no cathode bypass capacitor it may be described as ‘unbypassed’ and will have minimum gain.

Tube gain (from tube datasheet) mu *
Rp (value of resistor between plate and power supply)  Ohms *
Ri (the internal plate resistance of the tube )  Ohms *
Rload (resistance of the next stage)  Ohms
Gain  =  dB
Impedance Output Anode Ohms
Input coupling capacitor: Because the grid is now at a high DC potential it will require a coupling capacitor to block the DC from upsetting previous stages.
I will choose an arbitrary reactance of 1Meg at a low frequency of 5Hz and calculate the value.
* = required values

Heater elevation:
It is very important not to exceed the maximum heater-to-cathode voltage. Since the cathode is at a high voltage it is often necessary to elevate the heater supply to ensure safe operation and long valve life.

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