VK5SRP 2013 projects
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A valve radio with a Regnerative detector
This all started one Thursday night while I was watching the late news on television. While thinking about my next technical session at the NERC, all of a sudden a ligh bulb come on in my head flashing, "make a simple radio". It is a long time since I built a regen set, perhaps fifty years. Next morning looking through my folders of project ideas collected over quite a few years, I did not find exactly what I wanted to build. Time to see what I have in the junk boxes and to see what I can make in two or three days.
The first problem was what sort of chassis would I make it on ?, do I have suitable sheet metal at hand or do I have anything that is adaptable to the project ?. I have some used dicast boxes that came from a "former life" and I found one without too many holes. What holes it did have could be adapted to the new project. The next decision was the tuning capacitor and a dial mechanism. This was easy, I had a BIG single section capacitor of about 30 to 400 pF and a Philips friction dial mechanism from the 1950's. I bought this at a swap meet and "Harry" had chompted bits out of the backing plate so it would fit into whatever he was making.
Remembering these simple radios could resolve SSB signals I decided it would have plug in coils for the Broadcast Band so I could demonstrate it anywhere and at least 80 and 40 metre Ham Bands. As they are notorious for radiating signals out on the antenna I planned on having an RF stage in front of the Regen detector but it would have to be untuned as it would take too long to get a full set of coils going for both antenna and RF stage. I was not looking forward to winding the broadcast band RF coil but I was rescued by finding an Agis RF coil with Regen in one of my boxes of coils. The only probem was I would have to adapt it to plug into an octal socket.
I soon decided if it was going to be stable enough for SSB it had better have a voltage regulator tube (VR150) and as the lower frequency HF bands are so noisy, whay not include a noise limiter circuit, a good old 6H6 with contact bias to set the noise threshold. The last decision to make was what to use for the audio output. I have in the past used half a 6SN7 as an output tube so the audio stages are not two halves of a twin triode.
It took me two days to put it together and get it going but it took me another day to sort out the coupling between the grounded grid RF amplifier stage and the regenerative detector. I did a lot of tests with different pentodes for the first valve and I had wanted to use an variable u 6SJ7 but this did not have enough gain. I connected a sig gen to the input and a CFRO on the input to the detector stage and experimented with all the metal cased octal pentodes I had available. It was soon obvious the 6AC7 had by far the highest gain. Once I sorted that out I had to experiment with the coupling method and I soon found less was better. I had started out with a 2.5mH choke as a load on the RF Amp but 1mH gave better gain.
The circuit diagram, still a little bit of work to do here. Right click to save in high resolution.
Transmission line test meter by Lloyd Butler
I had been thinking of making the 2009 version of this device for some time and when Lloyd published his 2013 version I decided I should make both version using a rotary switch instead of the toggle switch so it could make four measurements.
Here is a PDF file of my version of the instrument with details of the cores.
Modulation tester trapezoidal pattern generator
For many years it has been common to check AM modulation using a trapezoid pattern generated by looking at the RF waveform from the transmitter with the Vertical (Y) input of the CRo and the demodulated audio on the horizontal (X) input with the time base off so the X input is not a sweep. To do this we need to do two things. Greatly attenuate the RF output from the transmitter and demodulate the RF sighnal so we have an audio signal for the X input. This simple little box does just that. In trying to use an old CRO I had round the shack I discovered not all Oscilloscopes would work, some have delay lines in the Y amplifier and this destroys the trapezoid pattern.
Ceramic or Crystal microphone for BoatAnchor rigs
I am looking into building some old transmitter circuits from the 1950's and they either use a carbon or a crystal microphone. Crystal microphones can still be bough on Ebay but I found there are also ceramic microphone inserts available from an Ebay store in the UK (globemusicworkshop). They have an item listed as Ceramic Microphone Element/ Capsule Harmonica, Harp, Mic, Hi Z, Blues, Crystal. Their full description is:
Globe Music have managed to find a limited supply of Ceramic Microphone Elements perfect for your Harp Mic project. These Elements use the same old school technology as the original crystal microphones from the 50's and are sourced from Japan. They are High Impedance so no Audio Transformer is needed. The element is approx 1 1/4" diameter or more accurately 31mm, Sensitivity -60 dB Frequency range 300 - 10 000 Hz, Dimensions 31 mm x 9 mm, Impedance 200 kO
I am sure other suppliers will have the same or similar items, it would be worth trying your local musical instrument shop. I bought two from the UK shop and have fitted one into the case from an old Philips two way radio microphone. I had to cut a block of foam to hold the insert in place.
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Written by and Copyright, Phil. Storr © Last updated 25th December 2015