6SN7 GT Low - MU Twin Triode
The 6SN7 GT is an indirectly heated twin triode. with two individual triode units sitting beside each other, having separate heater but common heater pin connections so it is possible to use each unit for different functions or both in cascade.
CATHODE Indirectly heated oxide coated
MAXIMUM RATINGS(each triode unit)
Maximum anode voltage Va 300 V
Maximum heater current If 650 mA
Maximum anode dissipation Wa 2.5 W
Maximum Cathode Current Ik 20 mA
Maximum Average Grid current 1 mA
(Measured without shield)
Maximum height 90 mm
Pin1 Grid )
Pin4 Grid )
TYPICAL OPERATING CONDITIONS
Heater voltage 6.3 6.3 v
Resistance Capacity Coupled Amplifier:
The valve is very suitable for use as a resistance capacity coupled amplifier and below is a table giving a summary of useful valves for two different supply voltages for one triode unit.
The relationship between the valve parameters under conditions of resistance capacity coupling is shown on the graph below. Showing an anode supply voltage of 250 volts with anode load resistance values of 50,000Ω, 100,000Ω and 250,000Ω, plotting the anode current, mutual conductance, amplification factor and anode impedance against grid voltage. The correct grid bias (cathode resistor) , the stage gain calculated and an estimate made of the distortion can all be obtained from this graph. The graph is limited to the commencement of grid current and the grid cut off region.
An example of using the graph –
To find the grid bias it is noted that the graph shows a slightly linear portion of the curve of anode current/ grid voltage over the range -0.5 to -8.5volts with the mid point being -4.5volts. Here the anode current is 1.5mA and therefore the cathode resistor is approximately 3000Ω and the peak input voltage 4.0 volts and the r.m.s. input 2.8 volts. If we then follow the grid bias voltage up we see that with an anode load of 100,000Ω the amplification factor is 19.2 and the anode impedance is 14,500Ω. The anode load is effectively on parallel with the succeeding valve grid leak with regards to the signal but not to the anode current so the effective signal value of the anode load is 100,000Ω in parallel with 500,000Ω, or 83,000Ω
The stage gain is shown as
µR = 19.2 x 83,000 = 16.3
The peak input voltage was 4volts, therefore the output voltage will be this multiplied by the stage gain - 4 x 16.3 = 65.2 (46 volts r.m.s )
Cascade Resistance Capacity Coupled Amplifier:
The two triode units of the valve maybe used in cascade if required but precautions are necessary to avoid instability. It is essential not to use a common resistor but that a suitably decoupled separate bias resistor be used for each cathode. Grid and anode leads should be neither over long or too close together and adequate anode supply voltage decoupling is required.
The circuit shown below indicates two sets of typical values together with figures of output voltage, gain and frequency response. These figures indicate an output of approximately 55 volts peak, an overall voltage gain of approximately 270 and a frequency response within 3 dB from 50cycles to 25Kc/s.
There are many applications that require a push-pull input from an input having one side earthed. If it is preferable not to use a transformer for obtaining the two phase output can be conveniently obtained from a resistance capacity phase splitting circuit.
Three suitable circuits are described below –
a) Normal Paraphase : The first circuit shows a paraphrase amplifier where the first triode unit feeds the output of the second triode unit. To reverse the phase, the input is adjusted so the gain is the same. Typical values are given in the table below, together with figures of output voltage, gain and frequency response. These figures indicate a peak push-pull output of around 110 volts with an input for this output of 6.5 volts peak.
The condenser across the common cathode bias resistor may be omitted, but if so, the balance of the higher frequencies will be adversely affected. In this circuit the potentiometer tapping down the grid of the second triode unit is critical. if an accurate balance of the output is required, this should be variable.
b) Anode-Cathode Load Phase splitter : The push-pull output on this circuit is found by splitting the load into two equal parts, the first half the anode and one half in the cathode of the same triode unit. the first unit is used as a straight amplifier with the second unit giving no gain after it. Typical values are given in the table below, together with figures of output voltage, gain and frequency response. These figures indicate a peak push-pull output of around 100 volts with an input for this output of 6.5 volts peak
It is not essential to fit the condenser across the cathode resistor of the second unit as the resultant loss of gain is only about 0.5 dB. There will be minor changes in the output but not enough to be very noticeable.
If an accurate balance of push-pull is required it is essential to match of R1 and R2 and to a lesser extent, R3 and R4.
c) Cathode & Anode Coupled Phase Inverter: Connecting the two cathodes of the units together gives the push-pull output of this circuit shown below. The grid of the second unit is driven from part of the anode of the first unit at R3, which is also common to the load of the anode of the second unit. This produces negative feedback in both anode and cathode circuits. Typical values are given in the table below, together with figures of output voltage, gain and frequency response. These figures indicate a peak push-pull output of around 60 volts with an input for this output of 4 volts peak.
It is noted that this circuit gives less output than previous circuits. The resistors R1 & R2 are not as critical on this circuit and can allow for up to 20% tolerance without affecting the balance of the push-pull output. It is preferable that R3 is a variable resistor to allow for adjustment to the balance. R3 is not affected by frequency.