24-06-2018, 02:52 PM
(This post was last modified: 24-06-2018, 02:54 PM by Murphyv310.)
Hi.
I wanted to try an Anode bend detector on the frame aerial due to its feature of high input impedance and low loading to the tuned circuits.
I retained the 7:1 toroid to give a direct comparison to the OA47 and ALD10001 chip.
Using a rod pentode (1J29b) with the filaments in series fed via 2 AAA Ni-Mh batteries I connected their centre tap to ground via a 15k & 10uf, G2 was fed to HT via a 47k decoupled by a 1uf. Results were disappointing even on the strongest signals and I tried HT from 18 to 90v with no real difference, I also tried a pot on the "ground" of the filament tap instead of the 15k with no improvement, connecting the valve as a triode made a tiny improvement.
I was about to put it to bed when I had the idea of feeding the screen grid back through the main winding of the frame (See diagram) on the first test there was nothing as the wiring gave negative feedback, reversing the connections and it squealed like a banshee.
I had already made some cuts on the main winding fro a previous experiment and was able to tame the feedback and get very good results indeed. I reinstated the pot and this gives control of the feedback and also brings up the weak signals. The whole circuit works between 18 and 36v above that audio quality gets poor with a lot of distortion no matter what value of resistor you use in the "Cathode" circuitry.
I have to say I didn't think this would work and also if you don't go above 27v HT there is no discernible distortion.
Whether this would work in a "Normal" TRF with simple tuning coils I don’t know but certainly with the Frame aerial and the following Toroid that I have been using for a while now it does, perhaps I'll try it in a conventional set up or even my Music Magnet that actually runs with 4 rod pentodes at the moment.
In addition the filaments require 2 AA or AAA Ni-Mh batteries as I have said no performance change was noted if polarity was reversed.
I wanted to try an Anode bend detector on the frame aerial due to its feature of high input impedance and low loading to the tuned circuits.
I retained the 7:1 toroid to give a direct comparison to the OA47 and ALD10001 chip.
Using a rod pentode (1J29b) with the filaments in series fed via 2 AAA Ni-Mh batteries I connected their centre tap to ground via a 15k & 10uf, G2 was fed to HT via a 47k decoupled by a 1uf. Results were disappointing even on the strongest signals and I tried HT from 18 to 90v with no real difference, I also tried a pot on the "ground" of the filament tap instead of the 15k with no improvement, connecting the valve as a triode made a tiny improvement.
I was about to put it to bed when I had the idea of feeding the screen grid back through the main winding of the frame (See diagram) on the first test there was nothing as the wiring gave negative feedback, reversing the connections and it squealed like a banshee.
I had already made some cuts on the main winding fro a previous experiment and was able to tame the feedback and get very good results indeed. I reinstated the pot and this gives control of the feedback and also brings up the weak signals. The whole circuit works between 18 and 36v above that audio quality gets poor with a lot of distortion no matter what value of resistor you use in the "Cathode" circuitry.
I have to say I didn't think this would work and also if you don't go above 27v HT there is no discernible distortion.
Whether this would work in a "Normal" TRF with simple tuning coils I don’t know but certainly with the Frame aerial and the following Toroid that I have been using for a while now it does, perhaps I'll try it in a conventional set up or even my Music Magnet that actually runs with 4 rod pentodes at the moment.
In addition the filaments require 2 AA or AAA Ni-Mh batteries as I have said no performance change was noted if polarity was reversed.
Cheers.
Trevor
MM0KJJ. Member of, RSGB, GQRP, WACRAL, K&LARC.
Trevor
MM0KJJ. Member of, RSGB, GQRP, WACRAL, K&LARC.