03-07-2018, 01:24 PM
Just a minor update on rod pentode circuits for those interested.
I am in the process of working on my TRF radio, but broke off to order some new valves so went back to my pantry Tx as I had never really got the record deck into it. (see last post). I have now tried the 1j37b mixer technique and found that it does work. I used my audio processing circuit, mentioned previously, which I made from the RSGB VHF UHF Handbook many years ago, putting these with a PSU in a case. I have to say that, although the system works, modulation depth cannot be raised anywhere high enough using the deck for optimum reproduction. I can see some reasons why not, but I think using old modules in less than ideal arrangements bring bigger problems than a new, clean design, would. I will give it further thought. Trying to design around a specific circuit, particularly not having the original designers notes, is not easy.
One side result of this though was my organisation of negative grid bias. I used a couple of capacitors from the AC 12V input to the PSU (I used a strange wall wart having seperate 12 and 24V AC out) to a seperate bridge to provide a negative supply. Then using a regulator and trimmer pots I obtained the needed adjustable bias. Thinking further, I thought about Trevor's work on rod tubes with isolated heater power. This would perhaps be an easy way of providing any number of isolated heater supplies.
Tracy
I am in the process of working on my TRF radio, but broke off to order some new valves so went back to my pantry Tx as I had never really got the record deck into it. (see last post). I have now tried the 1j37b mixer technique and found that it does work. I used my audio processing circuit, mentioned previously, which I made from the RSGB VHF UHF Handbook many years ago, putting these with a PSU in a case. I have to say that, although the system works, modulation depth cannot be raised anywhere high enough using the deck for optimum reproduction. I can see some reasons why not, but I think using old modules in less than ideal arrangements bring bigger problems than a new, clean design, would. I will give it further thought. Trying to design around a specific circuit, particularly not having the original designers notes, is not easy.
One side result of this though was my organisation of negative grid bias. I used a couple of capacitors from the AC 12V input to the PSU (I used a strange wall wart having seperate 12 and 24V AC out) to a seperate bridge to provide a negative supply. Then using a regulator and trimmer pots I obtained the needed adjustable bias. Thinking further, I thought about Trevor's work on rod tubes with isolated heater power. This would perhaps be an easy way of providing any number of isolated heater supplies.
Tracy
