11-09-2021, 08:51 AM
(This post was last modified: 11-09-2021, 08:59 AM by Murphyv310.)
Hi.
Interesting posts both from Tracy and Peter. I have also noted variations in specimens of the same type. The 1j42a can be quite wide. I also agree that a potential divider is the best route for the G2. In this little project I deliberately didn't use them as I've left the HT unswitched.
I often wonder if the performance of the original 2 valve TRF was dictated by a 1j42a that was bang on for the project and when it failed due to my mistake a replacement didn't cut the mustard.
I suppose that the differences between valves just make the whole aspect of these valves more interesting or time consuming which way you want to look at it.
No matter what, experimentation seems to be really the only way.
The other thing I've come across is like a "running in" effect. I noticed this early on in 2016 when I got my first batch of 1j18b valves, after a few hours of running characteristics changed and for the better. Also in this set the HT current was a total of 4.8ma (with the higher voltage valves in the audio) now its settled at 5.5ma with an increase in audio output before clipping. Also seen this on the 1j42a with an increase in reaction level. Values may need to be adjusted to reflect this and of course filament volts as this effects bias. I believe bias is derived in the filaments too of course and that's why gain can change with filament volts, not directly proportional to heating effect IMO.
Interesting posts both from Tracy and Peter. I have also noted variations in specimens of the same type. The 1j42a can be quite wide. I also agree that a potential divider is the best route for the G2. In this little project I deliberately didn't use them as I've left the HT unswitched.
I often wonder if the performance of the original 2 valve TRF was dictated by a 1j42a that was bang on for the project and when it failed due to my mistake a replacement didn't cut the mustard.
I suppose that the differences between valves just make the whole aspect of these valves more interesting or time consuming which way you want to look at it.
No matter what, experimentation seems to be really the only way.
The other thing I've come across is like a "running in" effect. I noticed this early on in 2016 when I got my first batch of 1j18b valves, after a few hours of running characteristics changed and for the better. Also in this set the HT current was a total of 4.8ma (with the higher voltage valves in the audio) now its settled at 5.5ma with an increase in audio output before clipping. Also seen this on the 1j42a with an increase in reaction level. Values may need to be adjusted to reflect this and of course filament volts as this effects bias. I believe bias is derived in the filaments too of course and that's why gain can change with filament volts, not directly proportional to heating effect IMO.
