Valve Circuits (Battery) B5

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1.4v Valve Detectors

There are a number of passive detectors detailed elsewhere on this website. This page concentrates on active device detectors.

1. Product Detectors

These are used to detect Morse (CW) and Single Side Band (SSB). A number of circuits were tested for conversion gain and the results shown below. Distortion was assessed from the scope display.

1a. Pentode with filament/cathode injection. This circuit shows high gain and low distortion with similar results from three different valves. An equivalent circuit using the DC90 single triode was built but the audio output distortion on the positive peak could not be reduced to a reasonable level so the circuit is not shown. All versions were tested at 3.4MHz. All voltages are peak to peak.

Valve V1

BFO Level

IF Level

AF O/P

Conversion Gain

1T4

230mV

40mV

2.5v

x63

DF97

230mV

40mV

1v

x25

1L4

230mV

40mV

2.6v

x65

1b. Heptode with direct BFO injection to grid 1. This circuit shows lower average gain than the above circuit but with no visible distortion.

The onset of detection is visible at a BFO injection level of about 100mV pk-pk and the conversion gain does not increase significantly above an injection level of 500mV pk-pk.

The 1R5 is an equivalent to the DK91.

Valve V1

BFO Level

IF Level

AF O/P

Conversion Gain

Supply Voltage

DK91

520mV

80mV

1.12v

x14

67.5v

DK91

520mV

80mV

1.6v

x20

90v

Page under development

All signal voltages are peak to peak.

Heptodes will be noisier than pentodes but this should not be a problem with the signal levels present at the output of the IF amplifier.