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Tuesday, September 19, 2017
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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




 Symetrical Shunt Regulated Push-Pull:

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






 Common-cathode stage, unbypassed cathode:

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







 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 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







 Cathode follower ( calc. Load & Bias Resistor ) :

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.







 Working Backwards :

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




 Calculate Cathode Bypass Capacitor

   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





 Bias Settings For Plate Dissipation

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
Vdc
Bias (Hot) =  mA =  Watts
Bias (Avg) =  mA =  Watts
Bias (Cool) =  mA =  Watts





 Tube Data

Va (volts) Vg1 (volts) Ia (A) ra (ohms) S mu Tube Datasheet
250-20.001110005.560.512AT7Download
250-8.50.010577002.216.9412AU7Download
250-20,009625001.610012AX7Download
250-20,0023440001.670.412SL7Download
250-80,00977002.620.0212SN7Download
150?0,008264005.535.22C51Download
150?0,008264005.535.25670Download
250-30.001580001.269.65751Download
110-9.50.00221400000.5705755Download
250-20.0244007.5336N1PDownload   ( is a direct plug-in replacement for the 6DJ8, ECC88 or 6922 )
15000,008264005.535.26N3P  ( is a direct plug-in replacement for the 2C51, 6385 or ECC42 )
90-1.30.02265012.5336DJ8Download
250-20.0023440001.670.46SL7Download
90-1.20.0015265012.533.136922   ( is a direct plug-in replacement for the 6DJ8 or ECC88 )
75-10.01300011.5357586Download
250-20.01110005.560.5ECC81Download
250-8.50.01577002.216.94ECC82Download
250-20.002625001.6100ECC83Download
90-1.50.01240001.524ECC84Download
250-20.019700658.2ECC85Download
90-1.30.0015265012.533.13ECC88Download