14-05-2012, 11:41 PM
(14-05-2012, 10:06 PM)Skywave Wrote: O.K., a few thoughts.
I see that the input tuning uses a 300 pF variable cap. and that the regenerator tuning uses a 300 pF variable cap. too (albeit padded down: C6, 100 pF). Hence, question: are these two caps. ganged - or does that not matter, on account of the fact that the R.F. tuning is broad enough so that, in effect, it becomes a simple 'peak' control?
I see that you use a 6AC7 from the freq. converter and that you say that it is prone to overload on strong signals. Since the 6AC7 is a sharp cut-off R.F. pentode, did you consider installing a remote cut-off, vari-µ valve instead - like a 6K7G? That way, a simple manual R.F. gain control could be incorporated.
Overall though, an interesting cct. We need to think about how we can get a relatively simple osc. design going to replace the ext. sig. gen.: I'm thinking about that.
Al.
Thanks for the reply Al, the RF tuning and the tunable detector are not ganged as in the circuit as posted the tunable det only has a range of 600 kcs ( I have since done another det coil that has a range of 1.1 mhz) that would mean a lot of switchable or plug in coils for the RF...to much I think.
The RF coils I am using are from a scrap set, each has its own aerial winding bottom coupled to a tuned winding, with four of these coils I can go from MW BCB up to about 20 mhz so the short answer is yes the RF tuning capacitor does just that ie: tune/peak, it's not fiddly to use, it does not need a reduction drive, it will have a find your way around scale fitted in the final design.
It performs well considering what it is and it is stable, listening to a couple of VK's this morning on 20, no JA's yet though....
I take your point about the valves, I am a bit limited for choice at the moment, the 6AC7 is the only valve I have that will take cathode injection mixing, I tried a 6K7 originaly for the mixer but it is a poor performer in that role.
The 6K7 works well as the regen det.
The 6AC7 does not appear to overload from a signal point of view but the maximum osc injection voltage is quite critical.
I have a couple of pentagrid mixer valves but as you know they need a lot of osc volts in order for them to mix properly.
So far as an on board osc goes for the final design there are not many choices.
Don't bother and use the signal generator....easy option.
Usual triode hexode/pentagrid.....don't really want to go that route, I can forsee drift problems over 10 mhz I need it to be really stable for SSB.
Switched or plug in Xtals.....good solution but restricts coverage to probably just the amature bands......possible.
Cheat...make the receiver a hybrid...DDS VFO with knob and shaft encoder....tempted.
Harmonic generator using a 1 mhz xtal...just select the harmonic required to set the band then tune away....this was my original idea, I like the thought of this but can it be done......on the positive side the mixer in the receiver only needs a low level input (currently running it with 25 mv injection)
Lawrence.
