Valve Circuits (Battery) B2

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1. RF Amplifier

The 1T4 is a remote cut-off pentode suitable for use as a gain controlled RF or IF amplifier in an LF/HF receiver or transmitter. Both input and output impedances are high so to get the highest gain the anode tuned circuit should have a high Q and a high L-C ratio to get the highest dynamic resistance.

The anode current was about 2.8mA.

Reducing the HT supply from 90v to 67.5v shows a gain reduction of about 20%. All of the following results were obtained with a 1T4 running on a 90V supply and with no AGC applied.

The measurements were made with a home made low capacitance 10:1 scope probe shown here on this site and allow for the probe capacitance of 4pF and an additional 7pF for the typical valve anode capacitance.

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Ref.

Frequency MHz

Anode tuning capacitor

Anode inductor

Voltage gain

1

170KHz

470p

1.83mH

x105

2

365KHz

82p

2.1mH

x170

3

6.3MHz

100p

5.9uH

x16

4

11MHz

22p

7.0uH

x27

5

22.8MHz

22p

1.04uH

x17

6

28.5MHz

10p

1.73uH

x21

7

50MHz

10p

0.56uH

x26

Results

The effect of the L/C ratio in this very low power consumption amplifier can clearly be seen in refs 1 & 2 which are an unrealistic set of values but demonstrate the point. The gain in ref 7 is a little higher than expected. Tests were also carried out using a 1L4 and a DF97 with very similar results.

The gain of an RF amplifier in an HF bands receiver should be adequate to overcome mixer noise on the higher bands but is much less important on the lower bands where sky and man made noise predominate.