(10-11-2015, 04:44 PM)Nowhere-Man Wrote: Been having a few issues with the grid resistors around some of these designs. Fortunately there's a few diagrams of all the various Mixer Frequency Changer circuits, including the Triode Hexode affairs you will find in most home based tube radios. I sort of winded my way back to the grid leak "plate detector" as American designers call it. Any tube that uses a grid as a detector is called "Plate Detector" in old American texts. I do have such a radio that uses grid leak - namely my late thirties TRF.
I'm surprised grid leak was so popular in the USA. Another point made is Triode Mixers are said to be less noisy than enveloped, all inclusive Mixer tubes (where the triode and Hexode are enveloped in one glass tube. Some American sets I think actually used 2 different tubes - one as mixer and one as I.F. Amp.
It wouldn't surprise me if I don't soon have a bash at the Simplex Super X American HAM Receiver. Many others online have built this 3 tube radio. It would be nice to power it up a bit more so you don't need to rely on phones as in the actual design.
Still, in the real world I still have my HMV to string, reassemble and see through to the finished job.
You could drop the Xtal and go the Regenerodyne route, simpler, fun to build, fun to operate and gives very good results on the Ham bands and shortwave in general, it will give a constant tuning rate on any band, to make things simple you could forget building the oscillator section for the mixer and use a standard bench sig gen fed into what would be the oscillator grid in a pentagrid mixer (eg: 6SA7) That's what I did and it worked first class, the bandwidth tuning of the regen detector was 500 khz which is nice and easy to tune with a low value band spread tuning capacitor incorporated, I used an IF center frequency of 3.25 mhz, eg: regen detector tunes from 3 to 3.5 mhz, the regen control can be the same set up as in the Simplex X Super, eg: screen grid control.
The Regenerodyne can also do the dual band tuning for one oscillator setting trick, eg: RF tuned to Osc + IF or RF tuned to Osc - IF.
No separate BFO is required, the "saved" valve can be used as an RF amp in a Regenerodyne which is wot I did.
The broadband tunable regen detector "IF transformer" can be air cored and is a doddle to make if you adopt the Hartley topology, eg: tuned winding is cathode tapped.
All food for thought....
Lawrence.
