ECF82 frequency changer

[radioman]

Hi,
I was having a look through my copy of the RSGB handbook (Fourth edition) and came across this circuit - however, I'm not sure how the oscillator is coupled to the pentode section.
Is there an error with this circuit or do you think it relies on coupling from the RFC to the input tuned circuit ?

Andy

[Peter.N.]

I don't see any coupling either, couldn't be internal valve capacity could it? The ECF/PCF82 was used in TV tuners, perhaps you could get some reference from a circuit.

Peter

[G8HQP Dave]

I think it is just an error. Given the circuit as it stands, you would need to add a low value capacitor from the triode cathode to the pentode grid (or possibly the pentode cathode).

[Radio Wrangler]

The pentode cathode is hard decoupled to ground so it would be hard to drive. The pentode grid has a signal freq tuned circuit to ground, so it could be rather low-z at the LO frequency, so also hard to drive.

Stray coupling isn't nice to rely on.

I think there's more missing than just a coupling component.

David

[turretslug]

I wondered if the 10nF (sorry, 0.01uF...) connected to the pentode cathode ought to be connected to the triode cathode (rather than ground) for low-impedance injection from a low-impedance point in the oscillator circuit, but I also can't help feeling that that would be excessively damping. Additionally, is it wise to involve an unspecified "RFC" in an oscillator tuned circuit? I could accept the presence of such a component if it was isolated by active device action, e.g. loading the triode anode but I would still be wary of it encouraging unwanted modes, aperiodic resistive loading seems less likely to cause puzzlement.

It won't be the first time I've looked at circuits in this kind of publication and thought, "Has anyone actually tried it to see if it works, and even if it does, is it anywhere near optimal for the sake of a little simple reappraisal?"....

[radioman]

Thanks guys, I thought it didn't look right..
Will probably just use a ECH81 instead.

Andy

[Alf]

This circuit relies on capacitive coupling between pins 1 and 2.

[kalee20]

Originally Posted by Alf

This circuit relies on capacitive coupling between pins 1 and 2.

And that circuit, very helpfully includes the stray capacitances which the operation relies on, marked in.

The OP's circuit doesn't, so yes I'd be confused similarly. Unless the text makes it clear... but while it could work at SW, it definitely would not work well for LW unless stray capacitances were augmented by a deliberately-added extra component.

As the RFC in shunted by a capacitor (part of the Colpitts capacitive tap) then it should not be overly critical.

[ms660]

The triodes anode decoupler might be better connected to the pentodes cathode if it's for lower SW or MW, LW.

Lawrence.

[radioman]

The circuit that I posted above is from a section talking in general terms about mixers. There is very little circuit explanation given although it's mentioned that the ECF82 has good conversion gain at high frequencies.
Another example circuit from the previous page is shown below.
(as proposed by Lawrence !)

Andy

[ms660]

The book:

https://archive.org/details/RSGBRadi...book/page/n103

Lawrence.

[Radio Wrangler]

Note that in the circuit Andy's posted, the coupling is scaled down by thecoupling winding on the oscillator coil and that the coupling is applied to an undecoupled cathode resistor in the pentode.

In the first circuit everywhere stray coupling could have gone to was shunted to low impedance with either an off-tune tuned circuit or else a decoupler.

David

[ms660]

The ECF82/6U8 in a version of the KB MR10 had the triode's anode connected to the pentode's screen grid and to the pentodes cathode, both couplings by 0.01uF capacitors, pentode's cathode not decoupled to ground, screen grid decoupled to ground with a 0.001uF capacitor, triode osc uses tuned grid with cathode feedback, triode's anode resistor shown as 15k in the schematic.

Lawrence.

[G6Tanuki]

The circuit with the anode-cathode capacitor makes more sense.

I also remember seeing a 50MHz-converter circuit in a 1950s US handbook which used the pentode of the ECF82 as an electron-coupled Colpitts xtal oscillator, extracting the 3rd harmonic of the xtal from a tuned-circuit in the anode of the pentode, then feeding this at high-level to the grid of the triode; RF input (preamplified by a common-cathode-connected 6AK5) was fed to the triode cathode in a circuit that looked at first rather like the classic cascode RF-amp, the oscillator drive to the triode was said to be so high that it basically operated as a switch rather than a traditional mixer so it had good mixing-linearity when faced with high adjacent signal-levels (US 50MHz hams have big issues with crossmodulation due to loads of TV stations in their equivalent of "Band 1").

[Aub]

I don't understand the bit about independent oscillator tuning. Frequency changers normal require ganged tuning caps for the oscillator and the RF stage, otherwise it won't maintain the sum or difference required by the i.f transformer, or am I missing something?

[turretslug]

I was a little puzzled by that statement, too, my assumption was that it was a roundabout (that's less rude than "evasive"....) way of saying, "because of the high residual capacitance of the Colpitts, it's less easy to get close tracking over a wide frequency ratio". Perhaps.

Presumably the author was thinking of a receiver with oscillator tuning control driving the calibrated frequency scale, and a mechanically separate RF preselecter control? It does seem lumpen though, with the near-certainty of hitting the image instead!

[Herald1360]

Even with a 465kHz IF at 30MHz, 1MHz difference between the wanted and image signals would allow a roughly calibrated separate RF tune to pick the right signal.


The idea would be more attractive for a homebrewed receiver where getting the RF/Oscillator tracking right would be more problematic than the slight operational difficulty of separate controls.

[Radio Wrangler]

I think the point is that the oscillator circuit employed has other capacitors around the variable capacitor that it becomes very difficult to arrange adequately good tracking with normally tuned circuits in the RF stages.

Having to have separate manual tuning to track it yourself is tedious, but that's what you had to do with a lot of professional Racal receivers.

David

[turretslug]

It struck me as a little ironic that radio designers had put a lot of maths, headscratching and mechanical design into getting away from the embryonic 1920s superhets with an array of individual variable capacitors and with arbitrary scaling to the sophisticated, civilised ganged-and-tracked superhet with a single tuning knob and tuning dial that anyone could go to and easily find a station with. Then the RA17 came along.... and both hands needed to dart around again. I wonder what US oppoes who were accustomed to the contemporaneous R390 with its complex but thorough ganging arrangements thought of it.

[G8HQP Dave]

Maybe what happened is that the author of that handbook chapter had said that the ECF82 made a good frequency changer and his editor insisted on showing a circuit, so he just made one up in a hurry and never tested it or even thought about it very much.