(23-05-2012, 10:02 PM)pwdrive Wrote:(23-05-2012, 09:09 PM)Skywave Wrote: Quote, pw_drive: "it is probably due to direct pickup by the regen det tuning coil itself".
Certainly sounds like that's the reason. After all it only takes a few pF of 'hidden' coupling to produce that symptom.
Incidentally, have you retained that R.F. choke in the anode of the R.F.Amp., or did you implement my suggestion of replacing it with an R.F. transformer with a variable-capacity tuned-secondary?
Al.
Hello Al, yes I'm pretty sure it's just stray pickup, it does not make to much of a problem, I feel sure when fully screened that it will be reduced by a lot or maybe even eliminated. I would be to embarrassed to post a picture of this mess but I gues you can imagine what form it takes.... coils hanging on wires everywhere...!
I still have the psuedo RF choke in the anode of the RF amp this is then capacitor coupled to the primary winding of the tuned circuit feeding G3 of the mixer valve.
The reason for this at the moment is that the coils that I found and that I am using all have a common pri/sec cold end ie: 3 connections therefore I cannot feed the primary with HT. I have a spare former for one of these coils so I might have a go at winding one with an isolated pri/sec ie: four connections and then connect the primary as the anode load in the RF amp. Eventually I will wind all new coils as the ones I am using (RF grid tuning & Mixer grid tuning differ a bit in their inductance values, I have managed to get them tracking "reasonable" but you have to remember that this whole test receiver is built from junk bits and I only have a limited number of resistor and capacitor values. I have the full coil winding details for all the B40 and also all the HRO coils which I can use as a guide when designing my own coil set as well as numerouse web based tuned circuit calculators (remember log/antilog tables?)
In all that I am pleased with the results so far as I am trying to design this thing so the controls do what they are supposed to do with the right operating feel to them which as you know is important for SWL especially on the Amature Bands.
Interestingly the Murphy B40D I have just found out uses a similar coupling arrangement from it's 2nd RF amp to the Mixer grid tuned circuit, they do not use a choke however but two carbon resistors in parallel, I think the parallel arrangement is just for dissipation purposes and I am guessing that a resistive load is better than a choke considering that the B40D has to tune from MW up to 30 Mhz. Interstingly the HRO uses high inductance wave wound primaries designed to self resonate just outside the low end of the band being tuned in order to provide gain compensation at that end of the selected band, later on Hallicrafters hit on the same idea, the primaries in the B40 are only a few turns though.
Lawrence.
Hello again Al, when the power came back on this afternoon I rewound the primary of the coil as has been mentioned so that the primary and secondary windings were fully isolated ie: 4 connections as opposed to the original 3. pri. connected as anode load, sec tuned and connected to the mixer grid. The test receiver works fine with this arrangement, the gain would now appear to be a bit higher at 14 mhz than it was with the choke capacitor coupling (the coil tunes from 5 to 15 Mhz) I suspect that if I had a matching set of coils for freqencies higher than 15 Mhz then the gain would increase proportionatly compared to the choke and capacitor method.
Lawrence.
