08-11-2017, 09:30 AM
Well after a few hiccups I have made further progress.
One of these was very straightforward to sort as my use of harmonics made things obvious. As soon as I fed the oscillator stage into the buffer amplifier and checked the output I found that the oscillator output frequency had jumped from the 7th to 9th harmonic. It didn't take long for me to realise that the oscilloscope probe capacity was to blame. Although I don't know its capacity I was able to roughly work out it's capacity to around 45pf (see attached maths). Addition of a 51pf silver mica capacitor across the tuned circuit sorted. The issue with the oscillator stage was the low output, requiring the probe on x1. Not an issue further down the line. My lack of experience showing here! (see attached calculations)
I have constructed more modules, 2 x 1j29b (as one required for audio stage) and one 1j37b. During this process I decided to replace my original 1j37b. I think I may have flashed the heater somehow (probably when using original lashups) and destroyed its emission. It seems to work but low gain. It needs a thourough check so I replaced it. While doing so I found that it was connected wrongly, with g1b and g3 interchanged. I am not 100% on this, but if so, my original modulator experiment was using g1a for rf and g3 for audio.
Following this I sorted the audio stage then assembled things. As yet I am not certain on the final situation with the modulator, but I need to get something together for test purposes, then steadily work toward optimum (at least as required for this application). Shown are photos of output waveform (input audio 1kHz sinewave from a signal injector to input socket) and the current working circuitry. The valve layout (from left to right) are oscillator (1j24b), rf buffer (1j29b), modulator / rf output (1j37b) and audio amp (1j29b). I have only done a little testing and optimising of settings. With a short lead for aerial I have received a signal across the house, on frequency, as checked using my comms receiver (digital frequency readout).
I can see yet quite extensive work needed as yet, but am happy with progress so far.
One of these was very straightforward to sort as my use of harmonics made things obvious. As soon as I fed the oscillator stage into the buffer amplifier and checked the output I found that the oscillator output frequency had jumped from the 7th to 9th harmonic. It didn't take long for me to realise that the oscilloscope probe capacity was to blame. Although I don't know its capacity I was able to roughly work out it's capacity to around 45pf (see attached maths). Addition of a 51pf silver mica capacitor across the tuned circuit sorted. The issue with the oscillator stage was the low output, requiring the probe on x1. Not an issue further down the line. My lack of experience showing here! (see attached calculations)
I have constructed more modules, 2 x 1j29b (as one required for audio stage) and one 1j37b. During this process I decided to replace my original 1j37b. I think I may have flashed the heater somehow (probably when using original lashups) and destroyed its emission. It seems to work but low gain. It needs a thourough check so I replaced it. While doing so I found that it was connected wrongly, with g1b and g3 interchanged. I am not 100% on this, but if so, my original modulator experiment was using g1a for rf and g3 for audio.
Following this I sorted the audio stage then assembled things. As yet I am not certain on the final situation with the modulator, but I need to get something together for test purposes, then steadily work toward optimum (at least as required for this application). Shown are photos of output waveform (input audio 1kHz sinewave from a signal injector to input socket) and the current working circuitry. The valve layout (from left to right) are oscillator (1j24b), rf buffer (1j29b), modulator / rf output (1j37b) and audio amp (1j29b). I have only done a little testing and optimising of settings. With a short lead for aerial I have received a signal across the house, on frequency, as checked using my comms receiver (digital frequency readout).
I can see yet quite extensive work needed as yet, but am happy with progress so far.
