Historic Designs


This page contains summaries of valved designs common in the 1950s and 1960s and would make fun projects for home constructors. There is no reason why they cannot be customised for the additional HF bands that we currently can use.

1. VFO - Buffer - Multiplier - PA Transmitter

I built this as a penniless student in 1960 when first licenced and made thousands of contacts on CW (full break in) and rather less on AM running around 5W output initially - the first PA was an LS50, later replaced with a 2E26. The VFO can be either a Clapp or high-C Colpitts type and should be run from a stabilised 150V supply. It was not unusual to get a 599X CW report. The VFO must be biased off on receive otherwise it will be heard. The PA must be biased off on receive to prevent wideband noise being heard. Any stage not keyed must have fixed negative bias or cathode resistors to control the maximum anode current during key up conditions.

Switching has been omitted for simplicity. Grid blocked keying was used which is shown elsewhere on this site. The PA was modulated on AM using a simple design based on a pair of 6L6 valves in push-pull with a high level clipper and low pass filter in the PA feed.

For a really minimilist design the first buffer can be omitted but this requires multiplying by two for 28Mhz in the VFO anode circuit. A drive level control will be required and the grid current in the PA must be metered. The intermediate tuned circuits can be ganged together as shown or tuned with individual controls.

Tunable IF Receiver with Crystal Controlled Converter

By today’s standards this design has a number of issues which could be significantly improved, particularly in respect of gain distribution. However, in 1960 it performed much better than its predecessor which used 6K8 mixers with no SSB filter. The new design allowed me to operate in the presence of nearby strong transmissions on the same band with significantly reduced intermodulation and blocking problems.

The fast break-in and muting allowed me to listen through my own 25wpm CW transmissions with no clicks or other irritating problems. The critical issue is to mute the receiver so that the AGC received signal time constant does not have time to charge up. The AGC has to be very fast which requires the received signal time constant to be fully decoupled from the muting signal. I use the same grid block keying system and AGC based muting in my SB310/401 combo with very similar results.

Details of the grid block keying, receiver mute and electronic T/R switch can be found here on this site.

Details of circuits suitable for break-in use are shown here on this site.

There are alternative beam deflection mixers to the 7360 at rather lower prices although the noise performance may not be quite as good and the biassing of the deflection electrodes will be different. The 6AR8 is fine as a transmit mixer or balance modulator.

The principles shown here are equally applicable to a crystal mixer VFO scheme where a single band VFO is mixed with the output from a switched crystal oscillator to obtain multi-band operation. Band pass filters will be required for each band to minimise any spurious responses.