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Old 11th Sep 2016, 3:39 pm   #1
Variometer
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Default Heath-Robinson-Style One Valve Receiver

Heath-Robinson style one valve receiver.
I did not really intend to build this. A few days ago, I wanted to test some old headphones that I had renovated, so I built a crystal set. It consisted of a centre-tapped coil wound on a piece of plastic drainpipe, 50mm diameter, mounted on the back of a variable capacitor. The germanium diode was contained inside an old glass fuse and holder mounted on the back of the tuning capacitor. A bracket was bolted to the front of the capacitor to take the 1/4 inch headphone jack socket. It did work, but not very loud.
I then decided to add a valve amplifier. I put an aluminium angle bracket between the variable capacitor and the coil. This was fitted with a 4-pin DIN socket for the valve. The valve is a CV122 triode that came out in the 1940s. I believe they were used in the electronic guidance systems of certain artillery shells. I have a number of these. They are wire-ended, but it was an easy job to fit a 4-pin DIN plug on one end, and use a 4 pin DIN socket as a valve-holder.
The completed receiver did not work at all! I therefore removed the germanium diode from the glass fuse, and replaced it with a 220pF capacitor, and added a 1M grid leak resistor between the grid and earth. The receiver then worked, but was not much louder than the original crystal set version.
So, I wound a reaction coil around the lower half of the grid coil. I did this without taking it to pieces, by threading it between coil and capacitor, and passing it round the coil - it was all very laborious!
Initially, I had a 47K pot and a 150pF feedback capacitor in the reaction circuit, and it then worked very well with quite strong reception, and very selective.
The only drawback was that the reaction was "crackly," so I dispensed with the 150pF capacitor and pot. and replaced them with a small, modern, variable capacitor. The reaction then became very smooth.
The reaction works back to front though, as the capacitor decreases as it is turned clockwise, but I quickly got used to that.
With 54 Volts HT, it draws 0.38mA anode current. When I increased it to the valve's maximum, of 100 Volts, it drew 1.46mA, and a significant gain in volume. The headphone volume is quite adequate, thanks to the addition of the reaction circuit.
It is something of a Heath-Robinson affair, but I enjoyed building it!
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Old 11th Sep 2016, 8:09 pm   #2
Radio Wrangler
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Default Re: Heath-Robinson-Style One Valve Receiver

I like it!

For your next one use more of those triodes, lots of gears and brass. But for the full steampunk thing, you'll need the matching costume and blunderbuss.

David
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Old 11th Sep 2016, 8:40 pm   #3
Colin G8TMV
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Default Re: Heath-Robinson-Style One Valve Receiver

For those of us that don't understand reaction circuits could you post a circuit diagram please
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Old 12th Sep 2016, 8:22 am   #4
Variometer
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Default Re: Heath-Robinson-Style One Valve Receiver

Here is the circuit.
The reaction circuit is L3 and VC2. RF is fed back into L2 from the anode of V1. The amount of feedback is controlled by VC2. As its capacitance is increased, the volume gets louder until it bursts into oscillation. VC2 should then be backed off a bit until the oscillation stops. A reaction circuit can quite easily double the volume of a grid leak detector circuit. It also increases the selectivity of the set.
I am not presuming to tell granny how to suck eggs here, I have put the above explanation on simply because I was asked to, although I am pretty sure that most of you will know anyway! Practically any triode, directly or indirectly heated will work in this circuit. If you wish to use a pentode, connect G3 to the cathode and connect G2 to the anode, thus running it as a triode. This circuit works very well with 54 Volts HT and 1.5 Volts LT. An increase in volume can be obtained by increasing the HT to 100 Volts. The LT should be left at 1.5 Volts.
PS
The headphones should be high impedance. If you haven't got any, use an LT44 miniature output transformer to match it to normal low impedance headphones. The high impedance winding of the LT44 goes in the anode circuit, and the low impedance headphones to the low impedance winding.
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Last edited by Variometer; 12th Sep 2016 at 8:25 am. Reason: Correct spelling mistake
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Old 12th Sep 2016, 8:48 am   #5
Colin G8TMV
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Default Re: Heath-Robinson-Style One Valve Receiver

Thanks, that is exactly what I needed. I've been a ham for nearly 40 years but have only very recently started looking at valve circuits. Since I'm not an electronics engineer all of this is new to me, but I'm finding it very interesting.

Colin
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Old 12th Sep 2016, 9:28 am   #6
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Default Re: Heath-Robinson-Style One Valve Receiver

It's also called regeneration, basically you design the detector as an oscillator with a means to control the amount of feedback needed to take it in and out of oscillation, the Armstrong and Hartley configuration are the two most common types used but other types such as Colpitts, Vackar etc will work.

The most common arrangement for the control of the feedback with the Armstrong type is a variable capacitor and for the Hartley a variable resistor which controls the screen voltage when a pentode is used.

Controlling the anode's DC voltage is also sometimes used but less so.

There can be many variations.

Lawrence.
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Old 12th Sep 2016, 10:03 am   #7
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Lightbulb Re: Heath-Robinson-Style One Valve Receiver

As a development idea, it might be worthwhile making C1 a variable, so that a trade-off can be made between selectivity and gain. The length of the aerial can have a damping effect on the tuned cct. at various frequencies, depending on the length of that aerial and thus the impedance it throws across the tuned cct.

Al.
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Old 17th Oct 2016, 2:16 pm   #8
Colin G8TMV
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Default Re: Heath-Robinson-Style One Valve Receiver

After letting it languish for a while I got back to my version of the HR over the weekend. I'm pleased to say that it now works nicely using half of an ECC81. The whole thing, apart from a couple of cable ties came from my junk box which explains the massive transformer!
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Old 17th Oct 2016, 2:33 pm   #9
ParcGwyn
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Default Re: Heath-Robinson-Style One Valve Receiver

I like it, very minimalist.

Dave

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Old 17th Oct 2016, 3:04 pm   #10
Bazz4CQJ
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Default Re: Heath-Robinson-Style One Valve Receiver

Thinking about the lateral design of the HRO four-gang tuning condenser, if you got a 'good length' of floorboard, adding the 9 valves and all the gubbings would take this approach to the max .

B
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