03-09-2021, 05:42 PM
(03-09-2021, 12:53 PM)Murphyv310 Wrote: As an aside.
The low voltage 1J42A is greatly effected by external magnetism the others to a lesser extent.
Put a coil close to the valve with an AF signal and low and behold you get audio out, a magnet when it gets close results in lower gain, we of course know the reason, perhaps a crude AGC with a coil strategically placed .......... OK where's my coat
I have tinkered with this before, I forget for what reason but it was not sucessful. The Russians always seemed to put screens on these valves. I generally follow the same practice, but individually make them from tinfoil as the only Russian examples I have seen are in old equipment which tends to be expensive.
(03-09-2021, 03:04 PM)Kalee20 Wrote: Look forward to the results Amie, wacky ideas which work , result in huge amounts of lab cred
Tracy, I probably should've started a new thread for AVC rather than hijack Trevor's thread, but were already talking about it before my post. Your suggestion of an op amp is well worth looking at, why don't you get a traditional superhet and put an op-amp in that to drive the AVC line and see!
My circuit has the high-gain DC amplifier. At the moment the gain is limited by the 33 megohm feedback resistor (I could've looked at system stability to avoid AVC motorboating but chickened out and limited the gain!) but that's just the first cut. In principle I'm aiming to take that out and let the gain in there go skyhigh, as high as the 1j24b can provide, which will approximate your op-amp anyway. That should keep the output level absolutely constant until the AVC system runs out of range.
Yes I have fully realised that I am tramping a bit on Trevors thread with some of the things I am putting too although it's me being typically impulsive plus sidetracking, not intentionally and I apologise to Trevor for it. I have an idea which I may go to, still in the embryonic stage.
I've not seriously got to the op amp stage yet. It's just an option I thought of. I need to consider the detector and derivation of a basic AGC signal first. I only have two simple superhets and don't really want to risk messing them up as one is a bit cramped and the other works well and is a nice radio at present which I use.
