regenfreak Wrote: Ed Dinning has introduced me to this forum
Welcome! I've seen your posts on UKVRR
regenfreak Wrote:If you want to build an AM superhet without searching in the dark, study existing proven designs; either using either European heptode or American pentagrid convertors. You dont need to re-invent the wheel. It does not matter if it is Hartley or Armstrong oscillator; the circuit is very simple.
The 'standard' 4+1 valve superhet has a lot going for it, moderate gain at IF; moderate gain at AF; high gain overall but without being hugely prone to instability. Easy to make it work.
That said, it has room for improvement! The British / European triode-hexode is generally reckoned to be better than the American pentagrid or octode (less 'pulling' of frequency with AVC; less oscillator radiation - which user won't notice but everyone else does
Amie Wrote:I've noticed that a lot of commercial designs are poor both in amplitude stability and waveform purity, both degrade performance in the mixer).
I'd second that - and also that lots have rather crummy tracking, meaning sensitivity is poor; the AVC whacks up the gain, and circuit noise gets obtrusive...
A good oscillator, providing sufficient but not excessive output, will maximise sensitivity without giving rise to spurious responses.
A good IF response helps reject adjacent-channel signals without incurring treble loss. Again, sweeping the IF response of a 'standard' superhet is rather alarming...
The all-American 5 is a good wheel and it is almost guaranteed to go round and round, as-built. But it can be improved: better static and dynamic balance; precision bearings; pneumatic tyres; correct lubrication. There's lots to reinvent or improve.
It's turned out that I'm carrying on similar activities to Amie - my aim is to make a no-compromise AM superhet, not make a working superhet with the least amount of trouble. I've just finished developing push-pull output stage based on Russian rod pentodes (see another thread); I've got an infinite-impedance detector lashed up (working but not optimised) using a directly-heated valve; and I've just started on the IF amplifier, and Amie's well ahead with oscillator that I've barely thought about. My IFT's are currently based on ferrite RM cores. Why? Because I want to, and I can... if they are better than the traditional IFT I'll put up results; if not, I'll put up results anyway
Finally (regenfreak), I'm interested in your own FM IF work, I hope you keep us informed
