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22 July 2017 


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6SN7 GT Low - MU Twin Triode

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General Information

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.

CHARACTERISTICS

CATHODE    Indirectly heated oxide coated                                         
                                                                       
Heater Voltage                                        6.3                    V
Heater current                                        600                   mA
Max D.C. Heater –Cathode Potential  250                   V

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

CAPACITIES (Approx)

(Measured without shield)
                       
First triode Unit               Second Triode Unit
Grid – Anode                             3.4                    3.5        pF
Grid – Cathode                         2.15                  2.15      pF
Anode – Cathode                     0.9                    0.9        pF
Heater – Cathode                     4.0                    3.6        pF
Grid 1 – Anode 2                                                0.20      pF
Grid 2-  Anode 1                       0.20      pF
Anode 1 – Anode2                   0.50      pF
Grid 1 – Grid 2                           0.10      pF

DIMENSIONS

Maximum height                        90                     mm
Maximum diameter                   33                     mm
Base                                            International Octal 8 pin

BASE CONNECTIONS

Pin1      Grid                  )
Pin2      Anode              )---- Second Triode Unit
Pin3      Cathode          )

Pin4      Grid                  )
Pin5      Anode              )-----First Triode Unit
Pin6      Cathode          )

Pin7      Heater
Pin8      Heater

6SN7 GT base connections diagramme

TYPICAL OPERATING CONDITIONS

            Heater voltage               6.3                    6.3        v
            Anode Voltage                100                   250       v
            Grid Voltage                   0                      -8         v
            Anode Current               10.6                  9          mA      
            Mutual Conductance        2.5                    2.6        mA/V
            Amplification factor          20                     20
            Anode Impedance            8,000                7700     Ω     

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.


Anode supply voltage  Va  100V

 

 

Anode Load Ra (MΩ)

0.5

0.1

0.25

 

Grid Leak (succeeding valve MΩ)

0.1

0.25

0.25

0.5

0.5

1

 

Cathode Resistance Ω

2000

2500

4000

4700

9700

11000

 

Output Voltage (peak)

14

17

17

19

18

20

 

Voltage gain

12

13

13

13

13

13

 

 

 

 

 

 

 

 

 

Anode supply voltage Va (b) 200V

 

 

Anode Load Ra (MΩ)

0.5

0.1

0.25

 

Grid Leak (succeeding valve MΩ)

0.1

0.25

0.25

0.5

0.5

1

 

Cathode Resistance Ω

1500

2000

3000

3300

7000

8000

 

Output Voltage (peak)

30

35

34

38

36

40

 

Voltage gain

13

13

14

14

14

14

 

 

 

 

 

 

 

 

 

Anode supply voltage Va (c) 300V

 

 

Anode Load Ra (MΩ)

0.5

0.1

0.25

 

Grid Leak (succeeding valve MΩ)

0.1

0.25

0.25

0.5

0.5

1

 

Cathode Resistance Ω

1300

1500

2500

3000

6000

7000

 

Output Voltage (peak)

51

60

56

64

57

64

 

Voltage gain

14

14

14

14

14

14

 

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 –
Using a valve –    Supply voltage     250V
                                Anode load        100,000Ω
                                Valve grid leak    500,000Ω

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
    Rp    +  rp                            83,000  +  14,000

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 )

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6SN7 GT Low - MU Twin Triode

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.

Cascade Amplifier Diagram

 

CONDITION

CONDITION

 

1

2

R1  (Ω)

100,000

220,000

R2 (Ω)

2,200

6,800

R3 (Ω)

220,000

1,000,000

Voltage gain at  1Kc/s

269

273

Max R.M.S. output voltage at  1k C/S (volts)

40

40

Gain at 50c/s compared with 1Kc/s  (dB)

-2.6

-0.4

Gain at 10 Kc/s compared with 1Kc/s (dB)

-0.6

-0.8

Gain at 20 Kc/s compared with 1Kc/s (dB)

-1.7

-3.7

Paraphase Amplifier:

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.

Normal Paraphase Diagram

 

COND.

COND.

 

1

2

R1 & R5 (Ω)

100,000

220,000

R2 (Ω)

1,000

3,300

R3 (Ω)

235,000

1,000,000

R4 (Ω)

14,500

68,000

R6 (Ω)

250,000

1,000,000

Voltage gain at  1Kc/s

16.8

16.8

Max R.M.S. output voltage at  1k C/S Grid-to-grid (volts)

80

80

Gain at 50c/s compared with 1Kc/s (dB)

-0.5

-0.9

Gain at 10 Kc/s compared with 1Kc/s  (dB)

-0.7

-0.4

Gain at 20 Kc/s compared with 1Kc/s (dB)

-1

-6.5

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.

Anode-Cathode Load Phase splitter Diagram

 

COND.

COND.

 

 

1

2

 

R1 (Ω)

100,000

220,000

 

R2 (Ω)

2,500

7,000

 

R3 (Ω)

220,000

1,000,000

 

R4 & R5 (Ω)

47,000

100,000

 

R6 (Ω)

2,000

6,000

 

R7 &R8 (Ω)

100,000

470,000

 

Voltage gain at  1Kc/s

15

15

 

Max R.M.S. output voltage at  1k C/S Grid-to-grid (volts)

70

80

 

Gain at 50c/s compared with 1Kc/s (dB)

-2.7

-0.6

 

Gain at 10 Kc/s compared with 1Kc/s  (dB)

-0.4

-0.5

 

Gain at 20 Kc/s compared with 1Kc/s (dB)

-0.6

-0.9

 

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.

Cathode and Anode Coupled Phase Inverter Diagram

 

COND.

COND.

 

1

2

R1 (Ω)

50,000

100,000

R2 (Ω)

75,000

150,000

R3 (Ω)

10,000

20,000

R4 (Ω)

680

1,500

Voltage gain at  1Kc/s

13.5

15

Max R.M.S. output voltage at  1k C/S Grid-to-grid (volts)

50

40

Gain at 50c/s compared with 1Kc/s (dB)

-0.3

-0.8

Gain at 10 Kc/s compared with 1Kc/s  (dB)

-0.3

-0.5

Gain at 20 Kc/s compared with 1Kc/s (dB)

-1.1

-2

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