03-03-2018, 05:04 PM
think it's about time for an update. I have been busily working away and made a little progress.
The only real thing I have done with my pantry transmitter is dyed and polished the outside of the box as shown in the photo. From the outside it perhaps only looks like an antique box now as there are no external controls. I did a little work on nonitoring modulation depth etc and found a few old circuits, with a view to updating. On close examination though, they did have limitations (mainly, I think, due to them being designed for use with higher power equipment). I am still thinking on that one.
I had thoughts about increasing the frequency of operation, and did think about making a VHF convertor. Initially I re-designed one of my 1J29b modules as a 20MHz crystal oscillator. My intention to drive a 1J29b frequency multiplier stage up to 80MHz. The 20MHz oscillator worked, although tuning was critical (peaky). After this was set, it worked well. My efforts to create the 80MHz tuned stage though came to nothing. I think mainly due to tuned circuit issues and my limited experience at that frequency. I was not helped by the fact that my dip oscillator seems to have a flat spot between 50 and 100MHz.
Some time back I bought a 50MHz oscillator module. It seemed a good idea to use this to feed a 50MHz signal into my multiplier, just using it as a fundamental amplifier for test purposes. With this in mind I added a two transistor buffer (straight from ham radio book, but with change of transistor) to the module (see photo). I constructed a new coil for the 1J29b amplifier using tinned copper wire, with intention of lower Q so easier tuning, but with sleeving to avoid shorted turns. On test this worked without problem (see photo of test setup).
From this, I then reverted to my 20MHz oscillator module. This now worked on the 2nd harmonic (40MHz) and the 3rd (60MHz)(maybe at the limits of tuning). I think I could go from this to my original 80MHz, but have, in the meantime, lost my drive for the convertor, so maybe won't. I have far more interest in HF, and the results indicate to me that I should not have major problems there.
During the last two months I have designed and constructed a new power supply unit. This provides HT, heater and bias supplies, plus a further positive supply. These are provided via four separate transformers so individually isolated from each other. The purpose of this is a utility PSU for lower voltage valves (both Russian and western), plus 5V digital IC's and transistors. This, I was also able to test (see test rig photo – the top connected box).
One problem I have realised in my tests is that I have poor test equipment for VHF. My scope is 50MHz and, although I have tested it with signal up to 100MHz, at which time sync appears to fail, it is obviously on its last gasp (my waveforms shown here are using X x 5 option to show clearly at minimum time [50nS]). My dip meter is useful. But has it's limitations. I have started to look at my SDR. It looks like it may be useful but I'm not sure how much. Testing with the 50MHz oscillator plus amp (with SDR in an adjacent room), signal was received, but I suspect the input was being overloaded as the harmonics content seemed too high. If anyone has used these for test and has any initial advice (or links to) I would appreciate it. (See photos). Not intending extensive work at VHF and higher I don't intend to accumulate a lot of additional test equipment.
As can be seen from the photos- I have created a higher frequency version of my rod tube module for VHF use. This is my first attempt, and does seem to work. Unlike at lower frequencies, I cannot be certain how well as I am using a pickup loop on my scope rather than probe. The main design aim of the module was to separate input and output circuits. The DC components are relatively unchanged. The circuit design of these modules (at least those described in this post) follow normal design, not using any special quirks etc of these valves. By and large I find the module approach useful, particularly with these Russian tubes.
The only real thing I have done with my pantry transmitter is dyed and polished the outside of the box as shown in the photo. From the outside it perhaps only looks like an antique box now as there are no external controls. I did a little work on nonitoring modulation depth etc and found a few old circuits, with a view to updating. On close examination though, they did have limitations (mainly, I think, due to them being designed for use with higher power equipment). I am still thinking on that one.
I had thoughts about increasing the frequency of operation, and did think about making a VHF convertor. Initially I re-designed one of my 1J29b modules as a 20MHz crystal oscillator. My intention to drive a 1J29b frequency multiplier stage up to 80MHz. The 20MHz oscillator worked, although tuning was critical (peaky). After this was set, it worked well. My efforts to create the 80MHz tuned stage though came to nothing. I think mainly due to tuned circuit issues and my limited experience at that frequency. I was not helped by the fact that my dip oscillator seems to have a flat spot between 50 and 100MHz.
Some time back I bought a 50MHz oscillator module. It seemed a good idea to use this to feed a 50MHz signal into my multiplier, just using it as a fundamental amplifier for test purposes. With this in mind I added a two transistor buffer (straight from ham radio book, but with change of transistor) to the module (see photo). I constructed a new coil for the 1J29b amplifier using tinned copper wire, with intention of lower Q so easier tuning, but with sleeving to avoid shorted turns. On test this worked without problem (see photo of test setup).
From this, I then reverted to my 20MHz oscillator module. This now worked on the 2nd harmonic (40MHz) and the 3rd (60MHz)(maybe at the limits of tuning). I think I could go from this to my original 80MHz, but have, in the meantime, lost my drive for the convertor, so maybe won't. I have far more interest in HF, and the results indicate to me that I should not have major problems there.
During the last two months I have designed and constructed a new power supply unit. This provides HT, heater and bias supplies, plus a further positive supply. These are provided via four separate transformers so individually isolated from each other. The purpose of this is a utility PSU for lower voltage valves (both Russian and western), plus 5V digital IC's and transistors. This, I was also able to test (see test rig photo – the top connected box).
One problem I have realised in my tests is that I have poor test equipment for VHF. My scope is 50MHz and, although I have tested it with signal up to 100MHz, at which time sync appears to fail, it is obviously on its last gasp (my waveforms shown here are using X x 5 option to show clearly at minimum time [50nS]). My dip meter is useful. But has it's limitations. I have started to look at my SDR. It looks like it may be useful but I'm not sure how much. Testing with the 50MHz oscillator plus amp (with SDR in an adjacent room), signal was received, but I suspect the input was being overloaded as the harmonics content seemed too high. If anyone has used these for test and has any initial advice (or links to) I would appreciate it. (See photos). Not intending extensive work at VHF and higher I don't intend to accumulate a lot of additional test equipment.
As can be seen from the photos- I have created a higher frequency version of my rod tube module for VHF use. This is my first attempt, and does seem to work. Unlike at lower frequencies, I cannot be certain how well as I am using a pickup loop on my scope rather than probe. The main design aim of the module was to separate input and output circuits. The DC components are relatively unchanged. The circuit design of these modules (at least those described in this post) follow normal design, not using any special quirks etc of these valves. By and large I find the module approach useful, particularly with these Russian tubes.
