This was my original prototype design to see how well a simple superhet could be made to function, and is still the "guinea pig" for testing new ideas.
In order to derive the maximum performance, i decided to go for a fairly high IF frequency to eliminate images. The usual 1700kC is low enough to achieve reasonable gain, but selectivity suffers. Both these issues are nicely avoided by using IF regeneration. Its also out of the range of interferring AM broadcast stations.
The best and latest in valve mixer technology was in the 60s when Philips created the ECH81 triode/heptode. This valve had little frequency pulling between mixer and oscillator, and is fairly tolerant of oscillator voltage level into grid 3. By using a variation of the Vackar type oscillator, thus keeping the Oscillator level constant over the tuning range, the effective gain is also kept constant over a wide tuning range. The triode oscillator section is fairly low gM, and has to be driven hard to get into the higher SW bands.. The ECH84 came later, and although designed primarily for synch seperation and gated AGC in TV service, appears to exhibit slightly better performance as a mixer in this application.
Because of the very high impedance of the dynamic load on the mixer anode, a result of the "Q" multiplication effect of the regenerative IF, the conversion gain is increased dramatically, therefore, an RF attenuator is required to limit overloading in the IF. The high conversion gain also allows a relatively lossy passive bottom coupled dual tuned bandpass filter on the input, again improving mixer performance by narrowing the RF bandwidth, thereby reducing out of band noise. This is quite noticeable when comparing this to my simpler RX having just the one RF tuned circuit.
Using the Pentode screen voltage to vary the IF gain, results in very smooth and controllable regeneration with little frequency shifting. I did try a second IF bandpass filter between mixer and IF in an attempt to improve the IF shape factor. This worked to a point, but something I had not envisaged was the "double humping" when adjusting the regeneration. As the "Q" increased, the effective coupling between the two coils increased, causing extreme interaction and effectively splitting the signal into two several kCs apart at the critical threshold.
I would like a 3:1 transformer in the detector anode to gain a free voltage stepup to increase the audio gain, have yet to find anything suitable.
This radio I used 5 pin DIN sockets , and PVC plug in coils, an easy way to change bands and makes superhet tracking design slightly easier. Air cored coils and air spaced tuning capacitors are the ultimate for stable and "drift free" oscillators. The best tuning capacitors are made from brass, these have less thermal expansion than aluminium vanes, good luck finding these. Polyvaricons are abysmal, only slightly better than varicap diodes.
Having no tuning slugs improve the thermal effects on the coils, but also makes adjustment very time consuming, needing to add or remove turns to get it just right, but its worth it to maintain the high "Q". This set of coils covers 5 to 15megs, tuning is touchy. More coils and less tuning range would be the next goal with this one.
Some may find this approach interesting, I know back then i had lots of fun building and experimenting with it.
@dayleedwards About removing the LO signal from the mixer -- how about using a single crystal filter like this one? The variable capacitor at the bottom serves to balance out the crystal holder's parasitic capacitance so that any signal that sneaks its way "around" the crystal through parasitic capacitance will be cancelled by the out-of-phase signal at the bottom capacitor.
Do you think the greater filtering offered by the crystal would help reduce the LO loading on the regenerative IF stage, thus leading to a more consistent regeneration level as the LO is tuned?
"The best tuning capacitors are made from brass, these have less thermal expansion than aluminium vanes, good luck finding these"
Philips made them, and are actually very hard to find. I have found 2 in many years (4 sections and 6 sections), with ceramic shaft.
In certain US surplus material, a 2 sections and 4 sections versions of the famous BC221 /LM/TS variable cap can be found, but they are extremely rare. I also have them, and they feature a pseudo-logarithmic variation of the capacitance, offering a straight linear relation between shaft angular position and tuned frequency.
I suggest you build up a regen using a valve , this is the easy way to visualise whats happening I think. Transistors are low impedance devices and in my experience dont match well with high impedance, high Q coils. Even if the transistor base is tapped down the coil, it may impedance match ok, but the coil is still loaded with the equivalent of a semi shorted turn or two. Valves operate with voltage change, transistors require current changes, and transistors are always forward conducting, always loading the previous circuit. , valves are normally not..
I have built transistorised regens , they have never worked as well as valved equivalents in a direct comparison, The best transistor one I have working uses a dual gate mosfet, but is compromised by the relatively low voltage headroom, limiting the dynamic range. DG Mosfets are getting quite rare now, and harder to find.
In all my valved superhets, I have not got your problem of regen changes with front end tuning, there are no IF gain changes . The oscillator and RF circuits all tune together. What I do have though is a bloody annoying change of regen setting from say threshold at 4.5 megs, then if I go to 12 megs, I need to change the setting very slightly.... I have tried unsuccessfuly to fix this as it would neatly get rid of the regen control. The cause is oscillator leakage at the mixer anode, the regen IF is more effected as the oscillator frequency gets closer to the IF frequency. Thats another reason I tried a double tuned BPF between mixer and IF, to try and remove as much of the osc signal as possible.. This only is a nuisance with megahertz frequency changes, ... tuning over say a 1 meg bandwidth requires no change in the regen setting at all.
So, I live with that small annoyance , after all, it is a minimalistic radio.
Other very good reasons to try valves is the ease to get them working, and what can be achieved with just a few stages. There is always the "cool" factor as well. And a biggy is the dynamic range, purely down to the higher voltages used.
You explained well why I also noticed an extremely high gain of the mixer+regen chain: i didn't take into account the increase in dynamic impedance of the IF tuned circuit, by the regeneration.
Actually I cannot say if the ECH84 is better than ECH81 regarding the gain, but I made some study by plotting and analyzing with Excel the conversion gain of both tubes, and actually it looks that ECH81 is just a tiny better to this regards. On the other side, being the ECH84 a 'remote cutoff' hexode, instead that being a variable mu like the ECH81, I guess it should be more resistant against intermodulation.
ECH81 was developed (I think ...) mainly for commercial receivers (even though it was used also in professional ones, like the Marconi and Philips, in the 60's), and therefore the AGC was also fed to it, offering a great control over the signals, normally in conjunction to the well known EF89.
I remember my 1st supergainer prototype: no RF preamp and 1 tuned cell only ahead of the mixer. Its S/N performance was much better than that of my Rohde Schwarz EK07 during noisy and stormy days.
Another point you've raised is the regeneration control by varying the screen grid potential, if pentodes are used. And I would like to hear your opinion if, for this application, which among fixed mu or variable mu pentodes are prefereable.
Another interesting variation to this practice, could be to set the regeneration level by varying the negative potential applied to the suppressor grid. To this regards, the ECH84 offers a nicely straight 2nd control grid curve, that might be interesting to exploit.
Last, if vari-mu tubes are used as regenerators, one other thing to try would be to set the negative potential at control grid.
At the end of the day I think all the above methodes could be optimized to perform satisfactorily. But the curiosity of trying each of them is high for me.
77 de Cris
Am I guessing right in saying the variable cap comes from a PA of a ham transceiver, or the like ?
77 de Cris
Let's say you set the regeneration just below critical threshold, for AM reception. Then you tune the LO through its range (also tuning the ganged RF stage). Does the regeneration level get pulled away from the critical threshold when you tune the LO?
This is a problem I encountered with my transistor regenerative superhet (which I hope to write up once I have the bugs worked out). Basically, it seems that the varying LO tuning (and to a lesser extent, the varying RF tank tuning) places a varying load on the regenerative IF stage, and these tiny changes in load are enough to significantly change the regenerated tank losses, pushing it either below threshold (with reduced sensitivity) or pushing it into oscillation. This makes it impossible to keep the regen level right at critical threshold as the set is tuned.
In my tests, I'm intentionally using the worst antenna possible, a 5 cm ferrite rod antenna at shortwave frequencies, which gives very low signal levels and hence requires a lot of IF gain to produce audible signals. I consider this a good "worst case" test for trying to design an IF strip with sufficient gain.
So I was wondering if you had similar problems with your set. Of course, if you use a large antenna, that can help solve the problem, by providing a larger signal, which then requires less IF gain, which then lets you run the IF regeneration somewhat below critical threshold in a more stable condition.
In my transistor set, I could solve this by adding two tuned IF amplifier stages (non-regenerative) between the mixer (which is an unbalanced BJT) and the Q-multiplier. The additional tuned IF stages help isolate the Q-multiplier from the undesired influence of the LO tuning. The additional gain also allows me to run the Q-multiplier somewhat below threshold and still have just about enough volume.
I keep wanting to further simplify my set by removing the 2 IF amplifiers and connecting the regenerated IF stage directly to the mixer, but in practice (when I tried that several years ago) I just couldn't get it to work reliably. Even if you could, for a particular station, set the regeneration right on the knife's edge for plenty of gain, this gain is (1) very narrow-band and (2) highly unstable, so that any changes in LO or RF tuning would then bring you away from critical threshold with corresponding loss of gain. That then requires constant touching-up of the regeneration level as you tune the set. And if you need to do that, you might as well just use a plain regenerative receiver!