FET TRF.

[Mark Hennessy]

Nice job Joe - you're really starting to enjoy these lovely transistor things now. Remember though - bipolar transistors are the best for most things - much more gm and cheaper to boot!

Re. picture 2, is there a nice original panel door trying to escape?

[Joe]

Yes, it is. They've always been something of a dilemma to me as the hardboard is now more a 'Period Feature' than the panels would be. They also have quite nice art deco corners.

   

My grandfather made them and the radii are the same size as an old penny.

- Joe

[Skywave]

Ah yes! Earthing! Can sometimes make a big difference . . . as you appear to have found out. I still don't like the cct. in the drain lead of that F.E.T. though . . . .

Al.

[Mark Hennessy]

My grandfather made them and the radii are the same size as an old penny.

I see the dilemma. If I'd inherited them in a house I'd just bought, then it would be an easy decision. However, undoing the work of my grandfather is a much tougher call - as I lost mine just a few months back, I'm not the best position to judge.

Oh well - a decision to defer, and perhaps one for whoever looks after the house after you

[Skywave]

Mark - F.E.T.s versus bi-polar transistors. One advantage that a F.E.T. does have over bi-polars in an application like this is that the input impedance of a F.E.T. is much higher that what can (usually) be obtained with a bi-polar, leading to a higher loaded Q of the input tuned cct. - a desirable feature in such an elementary receiver.
As for variations in gm, in the past I've always been fortunate enough to have a generous quantity of '3819s to play with - and thus measure and select the one with the highest figure (if the gm is critical to the application).

Al.

[Joe]

I've been doing a bit of poking about re FETs. I knew how they differ from bi-polars in terms of construction but not application. I've found several circuits for high-power amplifiers using high - c50V - supplies but so far nothing suitable just to play with.

I never knew my maternal grandfather as he died seven years before I was born. He was an engineer and I often wonder if there's some sort of genetic disposition to that sort of thing as I certainly didn't get it off my Dad - as he would readily admit.

- Joe

[pwdrive]

Al is right about the input imp. of the fet so little loading on a standard homebrew coil/capacitor parallel tuned circuit at resonance...much better, traditionaly a low gain device is better for regen. (smoother in/out operation of regen.) as already said if tickler coil is coupled to closely it will squark like a good 'un and can really hurt your ears if using the old style tin diaphragm 'phones, traditionaly a 2.5 mh choke would be fitted in the collector or anode circuit (I guess it would work also with the FET, the RF end of that would go to the tickler coil, this results in less turns required in that coil, the cold end of the coil could then be throttled to earth (-ve)_by using a pot via a blocking capacitor (RF frequency) or variable capacitor instead of a pot, either way the shafts can be earthed (-ve) thus reducing any proximity effect caused by your hands.

Hope this is of some use.

Lawrence.

[Skywave]

I have for long suspected that the desire and the ability to be analytical runs in the genes. My father was a professional gardener, and considering the abysmal education that he had (left school at 14 years of age - such was 'schooling' in those days), it used to amaze me how he mastered all the subtle sciences and arts involved in the many aspects of horticulture, especially as he could recite and refer to whole types of plants by their Latin names . If anyone here has ever visited Folkestone in Kent, the ornamental gardens (Kingsnorth Gardens) that is near the main railway station in that town is 99% of his creation. And his French was just about adequate, too. So clearly he had a flair for languages as well, which he seems to have passed on to me. He was also a bit of a dab hand at mending things and was no slouch when it came to basic mathematics & logical reasoning. He also taught me how to play chess: we spent many a challenging hour battling it out over the chess board!

Thanks Dad - now long since passed away.

Al.

[Skywave]

Indeed Lawrence: something like the arrangements you suggest is more-or-less what I had in mind for that F.E.T. cct. And on the related topic of valves suitable for regen. ccts., I've always favoured the 6BR7: it has a remarkably low gm - in the order of 1.25 mA/V. I built such a regen. RX many, many years ago using one: a cct. published in Practical Wireless by F.G. Rayer (G3OGR) - and it worked very well.

Al.

[Skywave]

Joe: applications for F.E.T.s. A good way to regard them - and their applications - is like that of a very low-voltage triode.

Al.