My construct is based on a circuit design from cool386 found at: 6BL8 Super-Regen Receiver
A well detailed article describes the circuit and how it functions. (No need to repeat here)
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For audio output I used a pre-fab LM386 audio module (cost $1 ea)
As suggested by John (cool386) I have inserted a JFET source follower between the 6BL8 detector and the audio amp module.
This matches the high impedance output of the 6BL8 valve (500 KΩ) to the low impedance input of the LM386 module (10 KΩ)
The gain pot' is only "just off the stop" for good listening volume.
My fabrication uses mostly recycled componentry.
Mains transformer is ex color TV 240v : 110v + 6.3v (fils)
Tuning gang is ex colour TV UHF manual tuner:
4 x 15pF gang with 20:1 reduction, anti-backlash gears tuning mechanism, only one 15 pF section being used.
Valves ex B/W TV: 6U8 6BL8 6DX8 6EA8 (all types worked well)
It's good to retain all this "old junk" you see!!
Initially using a short indoor wire aerial produced no results,
but connecting my exterior masthead amplified vertical whip antenna (scroll down)
all local FM broadcast stations can now be received!
Using the receiver outside my all-metal workshop is possible with a whip antenna
however the recovered audio is lower than when using a masthead/elevated whip setup.
"FM-Audio" is a bit "raspy" but considering its only a 1 valve super-regen it does a good job!
Super-Regenerative Receiver (Mk 2)
A second variation where the LM386 audio-amp module has been replaced by a
conventional valve design audio o/p stage, providing far greater audio volume.
I have used one half of a Russian 6N16B (wire ended) twin triode, (voltage amp)
followed by a 6AQ5 (7 pin) beam power tetrode (power amp)
An "ultra-linear" type design is used, with a suitably configured Jaycar M1900
5W Line Speaker Transformer, to satisfy both the impedance transformation:
5 K Ω primary to 8 Ω secondary, and screen grid tap-off point @ 46%
Some background:
Ultra-Linear operation provides a compromise between the high efficiency of tetrodes
and the low distortion and uncritical load impedance of triodes.
Screen voltage supply is obtained "automatically" from the transformer.
Screen grid tappings: 43% gives minimum distortion 20% gives maximum power.
Source: RSGB handbook 4th Edition (1968) pp9.19
Alan Blumlein�s Patent 496,883 dated 5 June 1937 is known as the "Ultra - Linear" amplifier.
With so many of Blumlein�s circuits the design is deceptively simple.
It shows a Pentode output stage of a single ended audio frequency amplifier
with a tap on the primary winding of the output transformer.
This tap provided feedback to the screen grid to improve the linearity of the amplifier.
Blumlein realized that if the tap was placed at the Anode end of the primary winding,
the valve would then be connected as a Triode,
and if the tap was at the supply end, it would be connected as a Pentode.
It was well known that when a pentode was connected as a triode it would be far less efficient
and therefore provide a lower power output.
But if however the tap was placed at say 15-20% from the supply end of the transformer,
the valve would combine positive features of both the triode and the pentode.
Source: The life and works of Alan Dower BLUMLEIN:
Robert Charles Alexander (2000) pp213
Negative feedback from speaker to cathode resistor is supposed to 'flatten' the frequency response curve.
No audio-gain control is included.
Super-regen pot can be used to throttle-back any excessive volume.
I had restricted "upgrade" space but managed to fit 2 valves & other components beneath the chassis.
Otherwise a 6BM8 triode/pentode circuit would be an obvious choice.
Power supply has been altered to a voltage-doubler type circuit giving 140 & 280 V rails
Larger filter capacitors are needed, so 470uF/200VW electrolytics (after reforming)
were re-purposed from a PC SMPS board (switch mode power supply).
Click to enlarge
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Article scanned from "Second thoughts on Radio Theory" M G Scroggie 1956
Originally published in Wireless World June 1946
Similar concept:
A single valve HF regenerative receiver (link)
© 4Q 2018