29-08-2021, 07:58 PM
Hi.
Perhaps some will remember I made a two valve TRF using 1J42A rod pentodes. The radio worked amazingly but then I popped one of the filaments and when I replaced it the performance could never be replicated. Its bugged me for a year or more.
So I decided to have another go. These low voltage 1J42A's are like hens teeth and I've never seen any more than the 10 I bought a good few years ago.
I have now three valves in circuit and although it works very well it is not as good as the original that was literally ear splitting.
I did substitute the output with a 1J18B and it's ear splitting, I'd think if all the valves were the higher voltage versions the radio would be really excellent. Reaction is very smooth with one setting covering half of the medium wave band. Sensitivity tails off though at lower frequencies.
I include the diagram. The audio stages share a common screen feed and decoupling, I've purposely not used potential dividers so I don't need to add an HT switch, valve voltages are low and initially I had 2v on the RF amp anode with good results, it's now at 4v with a slightly higher gain, lowering the anode load resistor although giving more HT its negated by damping the RF, a choke works but the reaction is harder to get working smoothly I may try a choke in series with a lower value resitor, a bit of trial and error.
Overall pleased with the result.
Perhaps some will remember I made a two valve TRF using 1J42A rod pentodes. The radio worked amazingly but then I popped one of the filaments and when I replaced it the performance could never be replicated. Its bugged me for a year or more.
So I decided to have another go. These low voltage 1J42A's are like hens teeth and I've never seen any more than the 10 I bought a good few years ago.
I have now three valves in circuit and although it works very well it is not as good as the original that was literally ear splitting.
I did substitute the output with a 1J18B and it's ear splitting, I'd think if all the valves were the higher voltage versions the radio would be really excellent. Reaction is very smooth with one setting covering half of the medium wave band. Sensitivity tails off though at lower frequencies.
I include the diagram. The audio stages share a common screen feed and decoupling, I've purposely not used potential dividers so I don't need to add an HT switch, valve voltages are low and initially I had 2v on the RF amp anode with good results, it's now at 4v with a slightly higher gain, lowering the anode load resistor although giving more HT its negated by damping the RF, a choke works but the reaction is harder to get working smoothly I may try a choke in series with a lower value resitor, a bit of trial and error.
Overall pleased with the result.
