Simple Valve Rx from 1966

[Bazz4CQJ]

Originally Posted by 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.

Yes, I suspected that would be the case. I'll see what I can hear on 80m AM. My IC-718 does "AM", but my inverted V for 80 got blown down, so I'm lacking in the transmit antennas at the moment. If that 6 valver works in any decent sort of way, I could tie it to one of the Chinese inverters (i've got two of them identical to yours sitting in the drawer) and try some /P working?

Any suggestions for a /P 80m antenna?

B

[Aub]

Some of the VMARS members go on 5.317 AM at various times of the daytime. Care should be taken to limit bandwidth to 6Khz.

Cheers

Aub

[G6Tanuki]

Be prepared to put some effort into quietening the Chinese inverter; though my attempt seems to have worked out OK, in retrospect I would have made more of an effort for close _ferromagnetic_ screening [a Colman's Mustard tin cut down?] as well as the feedthroughs and chokes. Remember that my inverter is also feeding into some of the original R209 vibrator-PSU components as well, which must provide some extra hash-reduction.

All my low-band HF operation is done using end-fed antennas because the house is at the end of the land and it's mechanically easier that way. Just put up the longest piece of wire you can! And be innovative with the earth; here, I have a series of the X-shaped galvanised-steel fence-post-sockets linked together using 6mm green-and-yellow-sheathed earth-bonding cable, but in the past when out /P I have even used an 'octopus' - half a dozen lengths of cable with big alligator-clips on the end, all joined back to one big tag, and used to link to the wires of a barbed-wire-and-galvanized-sheep-netting fence.

Be innovative! Someday I want to try a balloon-supported half-wave vertical.

[Bazz4CQJ]

@ aub, I could put a woolly sock over the microphone

@6Tanuki; my home QTH was bought at a time when I was totally inactive. I've got large trees in all the wrong places (like Eric Morecambe) and overhead phone and power cables in similarly bad spots. The inverted V I had made use of a well-placed tree, but it means I need to get someone to climb that tree, which I'm not willing to do! Last time, there were 2 guys, both wearing full PPE for mountain climbing. When does bob-a-job week come around?

Thanks for the tips about the inverter.

B

[Bazz4CQJ]

Something that I don’t much like about the design of this receiver is the fact that one half of V1 is the RF amp and the other half is the AF amp. I think I would prefer to have the RF amp free to have the layout around it fully optimised for the RF components.

So, if we used a separate valve as the AF amp, what might we then do about the RF amp?

i) just leave the pentode of the ECF82, or
ii) trawl the junk box for something that might be a better valve for that position?

If the latter, then what valves might be worth a look? The fourth Ed of the RSGB Handbook (1968) chooses an EF183 for the double conversion superhet it describes; certainly a valve with a higher gain. It was also popular for use in Preselectors of the time (the Codar one and a design in Radio Constructor).

My junk box offers just a single 183, which looks like it’s used and I will have to test it, but they are cheap to buy. Would anyone like to suggest other alternatives for single valve for V1? The junk box offers EF94, E180F, EF184 and EF186 which might be of interest.

Let’s worry about the audio circuit when we get to it.

Thanks

B

[G6Tanuki]

If you are using a pentode then my go-to would be something like a 6AK5 or if you are looking for something a bit older what about an EF50 or it's later evolution the EF54??

ECL80 as audio output??

What are your plans for regulation of the HT?? Drifty oscillators are nobody's friends

[Aub]

Could you use the humble EF91 ?

Aub

[Bazz4CQJ]

The junk box has a single EF95 in it which I think is a 6AK5; don't have much in the way of older valves. I have quite a lot of US 6xyz series valves

Certainly it has some hot EF91 as I used to use them has standard valves for the VCM.

The thing here, is that I'd like some guidance as to what makes a good 1st RF valve. Intuitively, you'd think it would just be the valve with the highest Gm with capacitances useable at the frequency, but I'm sure there is more than my poorly informed brain knows.

When National progressed from the HRO 5 they replaced the 6D6 with the 6BA6, and I'm sure they knew what they were doing, but there's a valve with quite a low Gm.

Re the audio, could be an ECL80, but I had an EF91 in mind; there are precedents; lots to choose from.

Re stability; yes, have been sorting out polystyrene caps and the junk box has lots of voltage stabilisers.

So, the choice of the RF amp....? Is there some reason why not to use the EF183?

B

[G6Tanuki]

The EF183 is quite a gainy valve, in some TV IF strips it needed neutralising by way of a bridge network and some not too critical capacitors. Not sure how it would play on 455KHz though.

For choosing a RF amp valve as well as the slope you need to consider the Equivalent Noise Resistance, this is more relevant at the upper part of the HF band though, on 80 and 40M the antenna noise will probably be much more than valve noise, there may be a case for considering the RF amp stage more as an impedance matcher and provider of selectivity rather than as a source of gain! Grounded Grid may seem counter intuitive but could be worth looking at. Pretty much any triode will be OK for that.

Yes the EF91 will work as an audio output o. There were a few TV sets from some surprisingly prestigious manufacturers who used it, so you can feel free to emulate them.
OK

[Bazz4CQJ]

My thinking about valve change is only for the V1a, position; I'm not thinking of a EF183 down at 455kHz.

The 183 (together with EF's 94, 95,180, 184 and 186 are all in the junk box) and just checking out their previous usage, I found the 183 in the Rad Com design and the two pre-selector designs.

I'm sure there could be downsides in increasing the gain at the front (e.g. more image problems?). Again, I think separating the RF amp and AF amp from being on the same valve base is good because (as well as improving layout) it will make it much easier to try other RF valves if desired. But it would be nice to get it "right first time", or close enough. Maybe the pentode of the ECF82 would be fine?

B

[G6Tanuki]

Sorry for the confusion... If you want to use an EF183/184 as the RF amplifier it could be worth looking at the circuit of the Electroniques QP166 front end unit which uses one such valve as its RF stage. The QP166 was the basis of many ham band receiver projects in the early 1960s, and worked rather well.

Circuits and details are available in loads of places online.

[Bazz4CQJ]

Dead right; the design in the 1968 Rad Com Handbook I mentioned above is based on the QP166. I'll see if I can scan it, but usual problem with large, hard-backed books.

B

[Radio Wrangler]

Originally Posted by G6Tanuki

T
Yes the EF91 will work as an audio output o. There were a few TV sets from some surprisingly prestigious manufacturers who used it, so you can feel free to emulate them.
OK

Also used as audio output device in the Racal RA17 HF receiver. The less common RA117 saw an upgrade to EL91.

David

[Bazz4CQJ]

QP166 EF183 front end attached.

Re the AF valve, I do have an EL91 too, but guess that depends on how much HT current is at hand.

B

[G.Castle]

Just a few thoughts:

ECF80's were mixer osc valves with separate cathodes ISTR, I'm sure these could do nicely if you have them. I have also used EF80 and its descendants in low RF, the only caveat there is if there is too much gain! One TRF set up I tried worked nicely on somthing in the order of 30 volts HT, on the front end.

Those commodity rf pentodes can also be used for audio too, the comment there will be if they are pushed for gain they can get quite vocal with microphonics.

The RF and AF in the same bottle may not be as daft as it seems, it can be used as a technique to prevent interaction between successive stages. A friend put together an airband sr set with the pentode from a PCF80 as RF (isolating) amp, a PC97 sr detector and the triode of the PCF80 as first AF. It worked well.

Greg.

[Synchrodyne]

The combination of RF/IF and AF in the same valve envelope actually has a good pedigree as a valid technique for general use, and not just as a possibly performance-compromising economy measure.

Some FM-AM receivers used the ECH81 triode section as an additional AF amplifier on FM and/or for the gramophone input. A notable example was the Pye FenMan II, which was far from being an “economy” model. This use was noted in the original Philips information for this valve. (The triode was specified to function as any of AM oscillator, a self-oscillating FM autodyne frequency changer, and AF amplifier.)

The triode section of the ECF80 was sometimes used as an AF cathode follower in FM tuners, whilst the pentode served as an IF amplifier or a limiter. Leak did exactly that in its Troughline FM tuner (all variants).


Cheers,

[Bazz4CQJ]

Let's distinguish the situations of something that will go through development and is going in to production, and something that's going to be for a hobbyist to experiment with (a toy).

My initial thought was that optimum (RF) arrangement of the coils for the two bands, and the twin gang tuning cap around V1a is a little more tricky when V1b is the AF output. It would be easier to do it, for the simple penalty of adding (say) an EF91 as AF output.

But once you reach that stage, you are free to consider whether the ECF82 pentode could be improved on?

However, I'm bright enough to know that I'm poorly qualified to figure out what a good strategy might be; is the EF183 a hot little bottle that might be a problem? But if it were, with the whole valve base free to play with, other valves could be tried without great difficulty.

The fall-back position would always be to make V1a the ECF82 pentode again

B

[dtvmcdonald]

I do note that it is said that this was an April issue. But still ....

I looked at the schematic. Did you notice? The detector with semiconductor diodes? With no AC return?

Yes, 1N34 diodes are leaky enough to work OK, but I'd not trust silicon ones.

[Bazz4CQJ]

There are comments on the internet by PA3GSV about using a grid detector instead of the two germanium diodes, using the triode section of the first ECF82. He can do that as he made changes, using an ECL82 AF amp) Claims a sensitivity of "-114dBm (0.4 uV) / 10 dB S/N with the FT241 crystals in place". Is that good?

B

[Aub]

The one I always wanted to build is in the attached mag from 1986. Just the receiver bit and modify it for 80.

Cheers

Aub