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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 3rd Mar 2021, 6:20 pm   #1
G6Tanuki
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Join Date: Apr 2012
Location: Wiltshire, UK.
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Default Regen HF receiver supply stabiliser.

Over the last 5 decades I've always been fascinated by regenerative HF receivers, and have built loads [valve, FET, bipolar] with varying degrees of success.

One of the underlying issues is that to receive CW/SSB stations you need to have the receiver gently oscillating - but being able to get a receiver to slip gently into and out-of oscillation without there being undue hysteresis in the regen control, or the poor thing 'squegging', is always tricky. Part of this is due to the DC conditions of the valve/transistor changing between oscillating and non-oscillating states.

My latest toy is a 2N3819 receiver for 5MHz; it uses a Hartley configuration, with regeneration controlled by varying the supply-voltage. It's rebuilt-from-memory of a design first published in 'Practical Electronics' sometime in the 70s.

To minimise the change-in-DC-conditions issue I've focussed on minimising the DC resistance in the supply-feed; I replaced the design's original 10KOhm drain-feed resistor/regen-control with the primary-winding of a 'LT44'-type driver-transformer [whose secondary feeds a BC109C followed by a LM384 IC audio-amplifier] - the DC resistance of this transformer's primary is a twentieth of the originally-specified drain-load resistor.

Then, I added a BC109C and Zener to provide an emitter-follower-voltage-regulator [again, that low-source-resistance thing]. This allows me to control the drain-voltage from 6V up to 12V with a 10KOhm pot; the result is wonderfully-smooth and hysteresis-free regeneration. I'm currently 'optimising' the capacitors either side of the transformer-primary, in order to provide decent cutoff of signals above 3KHz.

Experimentation continues - now I know rather more-accurately the voltage-range I need to cover for the regen-stage I will consider replacing the Zener and 10K pot with different values - 9.1V for the Zener and 1K for the pot look likely trial-values.
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