23-10-2017, 08:10 PM
It's about time I updated on my progress.
I had found that using more than one of my little modules with a 'lashup' of wiring got very complicated. With this and also feeling that I had generally sorted my circuit power supply I decided it was time to use my most usual approach to design of larger projects. In this I decide how much space is likely to be taken up by circuitry (often visualising), then obtain a suitable box / enclosure. The circuits, as I produce them, are then fitted into place. This often means working within limits, but, usually, it works.
With the pantry Tx design I bought a plywood box from The Works (£4). I have varnished it internally, and will decorate the outside when the internals are sorted.
Apart from the power supplies, I have done little beyond sorting a buffer amplifier for the RF oscillator. One of the photos shown is of the unit with output of the buffer amplifier shown on the oscilloscope. The oscillator is fixed in position, but the buffer is not fully accepted at present. It uses 1J29b.
The other photograph shows the box containing the oscillator, plus 3 other modules, which shows the layout I am thinking of. Final details will be sorted as time goes on, as will any additional shielding etc, needed.
Looking at the box, apart from the valve modules, to the left of them are the upper boost converter and negative bias battery packs (NiMh for convertor and standard alkaline for negative supply). To the front of these is the on/off switching and audio I/P socket box (contains fuses). Under that is the convertor box. To the right of this is a 6V gel battery for heater and lower boost convertor box, There is a board in front of this for distribution and negative bias setting for valves which require it. To the right of this is the 1.24V heater regulator board (with heatsink). Fine setting of 1.1V is done on each valve module.
My next task will be to sort audio input / amplifier for audio drive.
I have yet to fully decide on modulation. There are specific ways as mentioned here for rod pentodes, plus there are recomendations for 'standard' techniques. I have been reading around and found a few pointers.
The only modulator using the 1J37b I have found is one by Joe Sousa, which appears to use the method described by Mike, but the valve is used in Gammatron mode with anode being g1b. I don't intend to go this way as this, although maybe efficient, is 'off at a tangent' from common design. I have noticed a similarity of feeding grids differentially with 1J37b to similar looking circuity for VHF double tetrode circuitry like with QQV03-10. It seems to deserve a closer look, later if not now.
Traditionally (just from what I have read so I am not 100% on this), combined anode / screen modulation appears the most efficient, but needing high AF drive, and grid modulation, rather less efficient but likely less issues of distortion and also lower drive needed. With the intended application, high output power is not required (and would be bad), but audio quality and RF purity is. With grid modulation the valve impedance varies so matching is not as precise, but at the low power envisaged this may be less of an issue. The choice appears to me to be between simple grid modulation and the differential application described. The screen modulation described being similar to grid modulation. The differential approach may well be far better, and easier, at higher frequencies with easier wound coils etc. As I said, I have yet to finalise so anything may come to pass.
More to follow!
I had found that using more than one of my little modules with a 'lashup' of wiring got very complicated. With this and also feeling that I had generally sorted my circuit power supply I decided it was time to use my most usual approach to design of larger projects. In this I decide how much space is likely to be taken up by circuitry (often visualising), then obtain a suitable box / enclosure. The circuits, as I produce them, are then fitted into place. This often means working within limits, but, usually, it works.
With the pantry Tx design I bought a plywood box from The Works (£4). I have varnished it internally, and will decorate the outside when the internals are sorted.
Apart from the power supplies, I have done little beyond sorting a buffer amplifier for the RF oscillator. One of the photos shown is of the unit with output of the buffer amplifier shown on the oscilloscope. The oscillator is fixed in position, but the buffer is not fully accepted at present. It uses 1J29b.
The other photograph shows the box containing the oscillator, plus 3 other modules, which shows the layout I am thinking of. Final details will be sorted as time goes on, as will any additional shielding etc, needed.
Looking at the box, apart from the valve modules, to the left of them are the upper boost converter and negative bias battery packs (NiMh for convertor and standard alkaline for negative supply). To the front of these is the on/off switching and audio I/P socket box (contains fuses). Under that is the convertor box. To the right of this is a 6V gel battery for heater and lower boost convertor box, There is a board in front of this for distribution and negative bias setting for valves which require it. To the right of this is the 1.24V heater regulator board (with heatsink). Fine setting of 1.1V is done on each valve module.
My next task will be to sort audio input / amplifier for audio drive.
I have yet to fully decide on modulation. There are specific ways as mentioned here for rod pentodes, plus there are recomendations for 'standard' techniques. I have been reading around and found a few pointers.
The only modulator using the 1J37b I have found is one by Joe Sousa, which appears to use the method described by Mike, but the valve is used in Gammatron mode with anode being g1b. I don't intend to go this way as this, although maybe efficient, is 'off at a tangent' from common design. I have noticed a similarity of feeding grids differentially with 1J37b to similar looking circuity for VHF double tetrode circuitry like with QQV03-10. It seems to deserve a closer look, later if not now.
Traditionally (just from what I have read so I am not 100% on this), combined anode / screen modulation appears the most efficient, but needing high AF drive, and grid modulation, rather less efficient but likely less issues of distortion and also lower drive needed. With the intended application, high output power is not required (and would be bad), but audio quality and RF purity is. With grid modulation the valve impedance varies so matching is not as precise, but at the low power envisaged this may be less of an issue. The choice appears to me to be between simple grid modulation and the differential application described. The screen modulation described being similar to grid modulation. The differential approach may well be far better, and easier, at higher frequencies with easier wound coils etc. As I said, I have yet to finalise so anything may come to pass.
More to follow!
