Simple Valve Rx from 1966

[Bazz4CQJ]

I stumbled upon this design for a 6-valve superhet, originally set up to cover 80 and 40m, in the April 1966 Edition of QST.

The "6" valves are all twin valves and all ECF82, so only 3 valves in a way. The 455kHz IF has a crystal filter and the BFO enables SSB reception. It seems that these receivers are still being built in the US today and have a sort of "cult" following, being known as the "Mate of the Mighty Midget", the latter being a small low power Tx published earlier in QST.

It would be quite wrong to expect too much from such a simple design, but I remember being very pleased with the 2-valve TRF I built back in the 1960's. It's easy to see where some improvements might be made; there are no temperature-stabilising caps on the oscillators, which might be nice.

It could easily be run on 12V using one of the cheap Chinese inverters talked abut previously on the forum. I wonder if building it on a PCB might be a consideration?

I admit I've already been looking through the junk box, but found only EFC80's . Any thoughts from anyone as "Winter Project" bit of fun?

B

[turretslug]

9U8/PCF82 with 9.5V heater probably more plentiful and cheaper than ECF82 (Radford etc.). I like the idea of otherwise "Cinderella" dual valves being used for a compact, yet good performance set.

[Bazz4CQJ]

Already checked on the PCF82 on an auction site. There are lots, but not sure about the price being greatly cheaper than an EFF82 and suspect there will be some "pulls" out there. But if you happen to have some in your junk box, way to go.

B

[David G4EBT]

Like so many English designs of the same era which as often as not specified Denco coils, which are no more, the 1966 QST design specified Miller Coils, (and one of two 'wind your own'), so I think that would be one hurdle to overcome. In the words of the song 'Those were the days my friend, we thought they'd never end'...

Quite a lot of data on Miller inductors on WRH if you fancy 'rolling your own':

https://worldradiohistory.com/Archiv...Catalog-59.pdf

[Bazz4CQJ]

There are just the two Miller coils (L5 and L11), which it identifies as 300uH. I suspect that a junk box 455kHz transformer would be the place to find something quite close.

There are other named parts in the parts list; I think many of us would know what to look for in our junk boxes, but a less-experienced member might need pointing at potential sources. I don't think there are any real show-stoppers.

The two crystals have been successfully replaced by some builders by using a cheap ceramic filter, which is how I stumbled on the circuit.

Building anything with valves these days will take a little hunting around for components.

B

[Aub]

That's a nice little circuit. It's got my interest because I need a simple receiver I can just switch on and it's there, rather than booting up the laptop to use my SDRplay SDR receiver, excellent though it is.

I have some new old stock 465 IFTs that I got from Denco when they briefly restarted production in 1998.

Any suggestions as to what ceramic filter to use and where they are sold?

Cheers

Aub

[Bazz4CQJ]

I’ve seen comments that 6kHz filters work well with SSB. There are lots of those 455 kHz filter on auction sites, usually with a part number but no mention of the bandwidth, so needs a bit of homework doing. Many are 3kHz.

B

[Jon_G4MDC]

3kHz or less would be the target for SSB.

As we have been discussing in a parallel thread 6kHz is about the best you can get from a well designed 455kHz filter using just L&C IF transformers. It's good for low-Fi AM reception (speech) but about twice as wide as needed for SSB.

[Bazz4CQJ]

Originally Posted by Jon_G4MDC

3kHz or less would be the target for SSB.

As we have been discussing in a parallel thread 6kHz is about the best you can get from a well designed 455kHz filter using just L&C IF transformers. It's good for low-Fi AM reception (speech) but about twice as wide as needed for SSB.

I was a bit surprised when I heard the comment. The original circuit must be intended purely for SSB and CW (no switch on the BFO), but I think I'd like to be able to listen to AM too (perhaps). Suppose you could have switchable filters; there are some 4kHz ones, wonder whether they might be a compromise solution? As a project, I think that the Mighty Midget could provide hours of cheap and easy fun "messing" with it.

B

[Synchrodyne]

I doubt that the valve choice for this circuit is all that critical. I suspect that the 6U8 was chosen simply because of its ready availability, and not for its specific properties. From Tung Sol, it was one of the first VHF TV triode pentode frequency changer valves in the American series, and also widely used as a multipurpose TV triode pentode (until displaced by later, more purpose-specific, usually non-frequency changer multipurpose triode pentodes). It was also specified (by RCA at least, along with its own 6X8) for use as an AM-FM frequency changer, with, in the AM case, oscillator injection up the cathode, as shown for the circuit at interest. The pentodes in these valves were approximations of the 6AG5. The ubiquitous ECF80 was broadly similar, although I think – don’t know for sure – that Philips made some effort to tailor the pentode electrodes to the VHF mixer job.

One possibility is that one of the later European TV FC triode pentodes with a semi-remote cutoff pentode section might be used if AGC were desired. But these were usually frame grid on the pentode side, so perhaps “too lively” for the application. (As far as I know, there were no such valves in the American series, so the possibility was probably not considered for this circuit.)

Cheers,

[Bazz4CQJ]

Sorry Jon, this may not be you your liking, but I just found the reference to the 6kHz crystal in a youtube video; the radio sounds "fair". Wonder what antenna he has?

https://www.youtube.com/watch?v=6uu2NMsidSU

Seems to me that the osc tuning cap should be as small as it can be and a good slow motion drive is needed.

B

[G6Tanuki]

I wonder if that design could be implemented using the late 1950s 12Volt-HT car radio valves. The ECH83 can be wired as a pentode rather than as a classic multigrid frequency changer.

And there were Nuvistors designed to run on 12V HT too.

[Synchrodyne]

That’s an intriguing idea!

The combined cathode pinout of the ECH83 might require some circuit rearrangements. For the frequency changer, it might be possible to follow RCA’s AM circuit for the 6X8 triode pentode, which also had a common cathode pinout.

Bear in mind though that for its car radio circuit with SW, Mullard used an EF97 rather than a pentode strapped ECH83 heptode section for the RF amplifier, as was used for the MW version. And the ECH83 FC had an external oscillator based upon an EF98, rather than using its own triode. That does suggest that ECH83 performance at HF was somewhat reduced. But then in the car radio SW case, noticeably better sensitivity was probably required than for a fixed receiver, where a long aerial could be used.

Nonetheless, again with the common cathode issue addressed, the ECH81 itself – with normal HT voltage – might be suitable for all three positions in the circuit.


Cheers,

[Aub]

If I built it, I'd be using it mainly to hear 80m AM, so I'd put a switch in the HT feed to the bfo.

Cheers

Aub

[Bazz4CQJ]

Originally Posted by Jon_G4MDC

3kHz or less would be the target for SSB.

As we have been discussing in a parallel thread 6kHz is about the best you can get from a well designed 455kHz filter using just L&C IF transformers. It's good for low-Fi AM reception (speech) but about twice as wide as needed for SSB.

Jon, just to try clarify 6 v 3kHz? Is this just a terminology issue. Are you using 3 meaning +-3 for the bandwidth, while that guy has just expressed the same thing as 6?

B

[Ed_Dinning]

Hi Folks, using P series valve instead of E series it is quite easy to wind an autotransformer to step up from 6.3V or whatever your mains transformer gives.
Being an autotransformer it will be small and compact and not need a mains primary

Ed

[Bazz4CQJ]

Originally Posted by Ed_Dinning

Hi Folks, using P series valve instead of E series it is quite easy to wind an autotransformer to step up from 6.3V or whatever your mains transformer gives. Being an autotransformer it will be small and compact and not need a mains primary

Ed

Not sure if this info is relevant to this, but the ECF82 takes 450mA at 6.3V. The HT required is 100V DC and I guess the current will be 10-20mA? So far, my junk box has yielded transformers which are either just a bit too small, or really somewhat overkill for the requirement. As David pointed out above, the days when you could get just the right component from somebody's catalogue are well gone.

I've just bought a set of four ECF82's for quite a good price (though the P&P dented it a little). I have also found a supply of plastic pill bottles for the coils; low-density polyethylene and just the right size. I think I will set mine to cover 80 and 60m and hope to have it working by Christmas (but I'm not saying which Christmas!).

B

[Jon_G4MDC]

For SSB I want 2.5-3kHz nose bandwidth.
6kHz (+/-3kHz) will pass low Fi AM speech or a wanted SSB channel with the adjacent one as inverted monkey chatter. The HRO does this nicely already in standard form.

[Bazz4CQJ]

I've never sat down and listened to SSB at different selectivity settings. I don't think my TS530 offers me that option, but my IC-718 might and I'll check the manual to see.

Actually, I nearly always find SSB a bit of a strain to listen to, which is one reason why I have pretty much stopped using my licence. It will be interesting to see if this RX might pull in much in the way of AM on 80 or 60m. I'll be using it with my magloop antenna so it might.

B

[G6Tanuki]

There's not very much AM on 60M, most of the bandlets being rather narrow to the extent that double-sideband-AM could be at the limits of spilling outside. There's occasional AM operation around 5320.

80M - there are some of us who still appear there on AM, with radios like the Heath DX40/60/100, Johnson Viking, various commercial/marine radios, an occasional TCS, WS19 or WS62, and several of us with a PRC320 or similar Clansman gear. 3615 is a focus of activity, and there are also a couple of Dutch nets around 3600-3605 that are using WWII-era military gear.

Give it a listen.