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Automatic bias supply module for two push-pull (PP) tube AB-2. Dimensions 74x42 mm

Automatic bias control module for two triode output stages or two pentode push-pull (PP) tube AB-2

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modul AB-2
pc
Manufacturer:TES
Price : 62,00 €

Automatic bias control module for triode output stages or push-pull (PP) amplifiers

Designed for class A, AB, or B amplifiers.

The module controls bias in two tubes.

The module controls automatic bias settings to maintain a strict symmetry of currents through both tube branches in the output stage, and to completely avoid flow of DC current through the output transformer. The DC saturates the output transformer which results in increased distortion of low-frequency sounds.

Bias control is executed by maintaining a constant voltage drop on the output tube cathode resistors, independently on the driving signal.

The module may be supplied by the voltage for tube heating (6.3V or 12V AC) in the amplifier. However, this heating must not have a center tapped winding. An independent 6-12V AC power supply may also be used.

User Manual AB-2

Automatic bias control module for two AB-2 tubes

The modules are suitable for all amplifier types (classes A, AB, or B), as well as single-acting amplifiers with A-class tubes connected individually or in parallel.

Their use is strongly recommended in existing amplifiers with fixed bias of output tube control grids, as aging of the tubes causes the need for a constant re-adjustment of their operating points.

The connection secures an absolute symmetry of anode currents through both branches of primary winding of the output transformer. This way, saturation of the the output transformer core with DC current with subsequent signal distortion and dearease in available power is reliably avoided.

Furthermore, tube operating points need no further re-adjustments. The module constantly creates optimum conditions for the output stage, independently of tube aging and consequent changes in their parameters.

Features:

  • No need to constantly re-adjust tube operating points
  • Current regulation not influenced by music signal
  • Regulation of cathode current of the tubes from 1mA to 250mA with a 10Ω cathode resistor. Current match between tubes depends on cathode resistor tolerance. The typical mismatch is << 0.1mA with ± 1% resistors. Perfect current balance for accurate bass sounds
  • Setting of anode currents through a standard miniature metallic resistor or a multiturn potentiometer
  • Noise reduction by at least 40db (100 times) resulting with brilliant sound and clearer high-frequency tones
  • Clear undistorted sound for a great listening experience
  • Turn on delay of 45 s
  • Slow ramp up of current over a further 45 s
  • Fault indication via LEDs (red) on the module board; the LEDs may be also be re-mounted on the amplifier chassis
  • Fault indication via TTL compatible open collector output – pin 3 (the output may also be used for other protection outputs)
  • No maintenance or additional setup
  • Suitable for all types of output transformers (namely toroidal transformers which require automatic control)
  • Safe operation due to minimized danger of runaway tube current – even when the amplifier is left unattended
  • Universal use: suitable for all connection types
  • Fully analogous control: the module has no digital elements

Electrical data:

  • Maximum current per tubes with 10Ω cathode resistors: up to 250 mA
  • Maximum output voltage: up to -150 V (depending on the original voltage of bias source in the amplifier)
  • Outputs can sink at least 2mA current or much more
  • Requires a single 6.3 V to 12.6 V AC supply available in the amplifier (i.e. tube filament supply of 60-100mA) for the actual module, and the original negative bias supply of the amplifier (-70 to -150V).

Installation and wiring:

  • The module is designed for easy installation – one module may controll two tubes.
  • Connect only the existing DC 70-130 V bias voltage and the 6.3-12V AC voltage (taken from the filament supply of the tube. Note: it must not be a center tapped winding and if one side of the AC winding is grounded, then that side must be connected to pin 4 of the AB-2 module). The current draw is 60mA at 6.3V and 100mA at 12.6V. The grid and the cathode of each output tube is to be connected at the same time.

Connection requirements:

In order to use the AB-2 module, the amplifier must have the following:

  1. At least one 6.3 V....15 V AC winding that is either one side grounded or floating (not connected to ground at all; may be tested using a multimeter). If one side of the AC winding is grounded, this grounded side must be connected to pin 4 of the AB-2 module. Centertapped windings will not work unless the center tap is disconnected.
  2. The bias supply voltage must be sufficient to bias any tube you may wish to use with this module. For instance, some EL34 amplifiers have insufficient bias voltage under load. All KT88 and 6550 tubes thus require at least 70V of reliable bias at anode voltage of 430V.

 Setting the referrence current through the tubes – “Rref resistor”:

The AB-2 module is equipped with a resistor used for setting the desired current through the tubes. This resistor is to be replaced by the customer. Prepare a standard “reference” resistor (the small leaded resistor Rref in the front part of the module as shown in the diagram above) with 3k3 resistance. This resistor produces a constant current of 38mA with 10Ω between the cathode and the ground. Below is a table of values for R14 resistors for the given current and reference voltage value using the nearest E48 and E24 series resistor. In place of the Rref resistor, the supplied multiturn trimmer may be wired. If the resistance value is too high, it can be wired in parallel with the Rref resistor to limit the amount of adjustable current.

Iref

Vref

Rref       E48

Rref      E24

 

Iref

Vref

Rref       E48

Rref      E24

 

Iref

Vref

Rref       E48

Rref      E24

mA

V

 

mA

V

 

mA

V

10

0,1

750

750

 

95

0,95

11k0

11k

 

180

1,8

44k2

43k

15

0,15

1k1

1k1

 

100

1

11k5

12k

 

185

1,85

51k1

51k

20

0,2

1k54

1k5

 

105

1,05

12k7

13k

 

190

1,9

56k2

56k

25

0,25

1k96

2k0

 

110

1,1

14k0

14k

 

195

1,95

61k9

62k

30

0,3

2k37

2k4

 

115

1,15

15k0

15k

 

200

2

71k5

68k

35

0,35

2k87

2k7

 

120

1,2

16k2

16k

 

205

2,05

78k7

75k

40

0,4

3k32

3k3

 

125

1,25

17k8

18k

 

210

2,1

90k9

91k

45

0,45

3k83

3k9

 

130

1,3

18k7

19k

 

215

2,15

110k

110k

50

0,5

4k42

4k3

 

135

1,35

20k5

20k

 

220

2,2

127k

130k

55

0,55

4k87

4k7

 

140

1,4

22k6

22k

 

225

2,25

162k

160k

60

0,6

5k62

5k6

 

145

1,45

23k7

24k

 

230

2,3

205k

200k

65

0,65

6k19

6k2

 

150

1,5

26k1

25k

 

235

2,35

274k

270k

70

0,7

6k8

6k8

 

155

1,55

28k7

27k

 

240

2,4

422k

430k

75

0,75

7k5

7k5

 

160

1,6

30k1

30k

 

245

2,45

866k

820k

80

0,8

8k25

8k2

 

165

1,65

34k8

33k

 

250

2,5

open

open

85

0,85

9k09

9k1

 

170

1,7

36k5

36k

         

90

0,9

10k

10k

 

175

1,75

40k2

39k

         

Example: Maximum required current: 60 mA. Theoretical reference – resistor Rref, 5K62. If using the 20k trimmer, place the 8k2 resistor in parallel with the trimmer to achieve a combined resistance of 5K815 (the maximum current value is thus only 60 mA). 

The circuits of the first grids of the output tubes must be equipped with isolation resistors (marked red with the value 100k) and the cathodes with 10Ω resistors.

 LED indication

  manual_ab-2_2

Function

  • The reference voltage is developed across an exchangeable resistor or an multiturn trimmer. An external potentiometer may also be used.
  • The control loop adjusts the bias so that the current through the sampling resistor (10Ω) produces the same voltage as the reference voltage. This is hapenning very slowy by rejecting the dynamic variations caused by the music.
  • During the startup, the Active Bias System is set limit the distortion during this stage to just 15 s. During this time the “Start Up” LEDs are lit up (they may flash briefly as the control loops stabilises).
  • The the Active Bias System starts ramping up the bias voltage slowly, reaching full bias within approx. 45 s.
  • If the tube connected to the control system is prevented from reaching the bias and the desired nominal anode current, but continues to have increased current, the “Start Up” LED will re-light and the “Stop” output will be pulled low (open collector – output 2) to allow the external protection circuits to protect the amplifier from the defective tube.

 manual_ab-2_1

 

 

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