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 Saturday, September 22, 2018 3989 users online
 To read: Tube CAD Journal - Extrapolation from Plate Curves Tube CAD Journal - The Grounded-Cathode Amplifier The Valve Wizard - Heater / Filament Supplies

 SRPP Common-cathode stage, unbypassed cathode Common-cathode stage, bypassed cathode Cathode follower ( Load & Bias Resistor ) Working Backwards Cathode Bypass Capacitor Bias Settings For Plate Dissipation Tube Data

Tube gain (from tube manual or spec sheet) mu *
Ri (the internal plate resistance of the tube )  Ohms *
Rk1  Ohms *
Rk2  Ohms *
Gain (Not Bypassed)  =  dB
Gain (Bypassed)  =  dB
Impedance Output (Not Bypassed) Ohms
Impedance Output (Bypassed) Ohms
* = required values

Heater considerations:
Because the cathode of the upper triode will be at roughly half Vb+, the heater supply will probably need to be elevated to avoid exceeding the valve's maximum heater-cathode potential- always check the data sheet.

More about Heater / Filament Supplies

Tube gain (from tube manual or spec sheet) mu *
Rp (value of resistor between plate and power supply)  Ohms *
Ri (the internal plate resistance of the tube )  Ohms *
Rk (value of cathode resistor)  Ohms *
Gain  =  dB
Impedance Output [1] Anode Ohms
Impedance Output [2] Cathode 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.
nF
* = 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.

More about Heater / Filament Supplies

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 manual or spec sheet) 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.
nF
* = 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.

More about Heater / Filament Supplies

Plate voltage ( Va )  volts *
Plate current ( Ia )  mA *
Bias voltage ( Vg1 ) - volts *
Idle current ( use Tube datasheet Plate Characteristics )  mA *
Cathode load resistor ( Rk )  kOhms
Cathode bias resistor ( Rb )  Ohms
* = 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.

More about Heater / Filament Supplies

 The design of the cathode follower is similar to the gain stage. I have chosen a steeper loadline as I want to have the cathode to run at higher output current. Cathode load resistor ( Rk ) = 300V / 3mA = 100k ohm. Cathode bias resistor ( Rb ) = Bias voltage / Idle current = 1V/1.35mA = 740 ohm At idle point, the tube will drop about 163 V as seen from the graph. So the cathode idle voltage is equal to 300V - 163V = 137 V. The grid will be at 1 V below the cathode so it will be running at 136 V.

Sometimes we are presented with an existing circuit that lists the values of the plate resistor and cathode resistor, but not the operating voltages or idle current.
We can work backwards from the resistor values to the operating points (if the B+ voltage is specified).
Plate voltage ( V+ )  volts *
Rp (value of resistor between plate and power supply)  Ohms *
Rk  Ohms *
Ri (the internal plate resistance of the tube )  Ohms *
Tube gain (from tube manual or spec sheet)  mu *
Iq  mA
Vp  Volts
Vgk  Volts
* = required values

Known Lowest frequency to be amplified    Result
 mu (Tube gain) * Rk (Cathode resistor) Ohms * Rload (Load resistor) Ohms * Rp (Plate resistor) Ohms *
 fmin Hz * Attenuation at this frequency: dB
 Ck = uF    * = required value

You should have a good understanding of classes of operation before using this bias calculator.

For instance, it is accepted practice to bias a tube operating class AB to 70% of its maximum allowable plate dissipation at idle.
For class A, it is 90%.

Many amplifiers running a push pull output advertise as being Class A. They aren't. If you are uncertain, you should consult an amp tech.

When setting the bias, all tubes should be installed and the amp should be warmed to normal operating temperature. Consider the result of this calculator to be MAXIMUM bias current.

 Class AB 70%  -  Class A 90% Class Tube Type Plate Voltage AB A EL84 6K6GT 6V6 6V6GTA 6L6 6L6GC 6L6WGB 5881 EL34 5763 7189 7591 KT66 KT88 7027 6CA7 6BQ5 Vdc Bias (Hot) =  mA =  Watts Bias (Avg) =  mA =  Watts Bias (Cool) =  mA =  Watts