Mayhem on AmRad 40 m. band

[Jez]

I know what it is. It's also obsolete but I have a few in stock. The MC1496 will probably beat the SA612 and the 7360 is an order of magnitude better. Sa612 was designed for use in analogue mobile phones and although a very useful IC it's not particularly high performance and it's IP3 is nowhere near that of the 7360.

[Mike Watterson]

The IP3 spec of an SA612 is only an issue on receive before an IF. I'd use a quad FET switch mixer for applications in front of IF filter and receive. The best IM performance sets you can buy use the quad switch (a similar idea to a ring diode mixer).  I built many PCBs in 1970s using the MC1496. It's not got the frequency response of an SA612, carrier 300MHz, signal 80MHz @ -3dB Transadmittance.

  mc1496.pdf (Size: 208.59 KB / Downloads: 0)

But feel free to build a clone of a Swan.
Schematic here
16 valves: 6EW6 12BE6 6GK6 6LQ6 6LQ6 12BZ6 12BE6 6EW6 12BA6 12AX7 6BN8 6GK6 7360 12BA6 12AX7 0A2 12BA6

I'm very fond of my portable valve SSB /AM/CW receiver and it does beat cheaper transistor / IC sets. Hallicrafters S72 I never figured why the BFO works fine on the replica battery pack (Alkaline D cells and AA cells) but doesn't oscillate on mains via 230 to 115V transformer. The voltages all check out. Someone had swapped the original audio op transformer and wired the phone socket wrong, which puzzled me for a while. Otherwise it was mostly replacing dodgy caps. My first "serious" SWL set was an R1155 in 1966.

A friend living about 20 minutes drive away has either an RA17 or RA117. He specialised in collecting old valve vintage Commercial sets. Also has a bunch of Eddystones & Collins (some of which I've restored) Replacing paper caps under the switch units on an Eddystone needs special tools, dexterity and patience. Replacing the drive cord on some isn't much fun either.

[Jez]

I have a bag of TO99 MC1496's and I'm familiar with all the Collin Horrabin stuff etc

The G4DTC Ray Howgego "Ultimate" receiver, a hybrid which includes the 7360 as mixer and uses an ECC88 as a push pull Kallirotron local oscillator, seems particularly interesting and would I'm sure give the latest stuff a good run for its money.

[Mike Watterson]

The G4DTC Ray Howgego "Ultimate" receiver

Link?

Edit:
Maybe only in Radcom back issues Dec87 & Jan 88.
It seems to use an SL623, which was a mistake.
A 4.43 MHz IF was cheapskate solution.

I found page 183 of Radcom March 1988 which discusses it, and 37 of Aug 1989 discussing availability & stability of the SL623.

https://islandlabs.eu/plessey-sl623.html

[Mike Watterson]

Here is the front end:
   

It might beat a 1980s FT101ZD, but I'm not sure. Maybe someone built it all and still has it?

[Jez]

Sorry Mike but I don't have one. It is on the interweb but it takes quite some finding! It's one of the receivers in a long article examining some of the receivers claiming SOTA performance around mid 70's ish, the title of which escapes me. That's the form in which I have it...somewhere. It will be in Radcom. IIRC it's in two years of the the RSGB Radio Communication Handbook, poss 74 and 75 but I don't have them and they are amongst one's not available on the internet... After seeing so much discussion on it in "Technical Topics" and debate about specific stages which were shown I was determined to get a gander at the whole thing and IIRC it took me about an hour of googling to find the whole schematic!
It's worth searching out. It uses a mixer as phase detector type PLL, largely discrete, which is interesting in a design from that time in itself.

[Mike Watterson]

Jan 88 has the 4.43 crystals implementing a variable bandwidth IF filter and says more on another month.
Actually the original FT101ZD was 1979. My MkIII was made in 1985.
It's variable IF bandwidth at 8.9 MHz and like the G4DTC has a free running LO. The LO is about 5MHz to 5.5 MHz and the bands via crystals.

You might get marginally better dynamic range with the PCC189 RF preamp and the 7360 mixer than the FT101ZD, but even in 1979 it was an older design updated. Even in 1970s people were replacing triode or pentode RF amps with FETs.

There is little to choose here on IM (dynamic range) or specs generally on the top 20
http://sherweng.com/table.html

[Mike Watterson]

Well, in 1975 or so I built a test chassis using custom PCBs and MC1496 (DIL) demoing AM, DSBSC, SSB without filters, Phase modulation using DSBSC mod and injecting carrier at 90° and MC1496 as phase comparator for a PLL using TTL divider. To demo communications modulation/demodulation systems.

Nearly 10 years later I did PLL synths in Westinghouse for continuous very wide band sweep using PIN diode switches and multiple VCOs.

Then by 2007 was designing 870/921 MHz duplex (to co-exist with GSM gear at same sites) and modifying existing 10.45GHz duplex gear. The 870/921 MHz duplex gear used strip-line filters at RF and 42MHz IFs with 200kHz and 2MHz bandwidths. NatSemi PLL/Synth chips controlled by PC then PIC programmed in JAL.

I think the G4DTC Ray Howgego "Ultimate" receiver is maybe ultimate in terms of ideas he threw at it and complexity. I'd go for something a lot simpler, even then, and now even though I have all the parts, test gear and workshop I'd not bother. I've tested a 1j29b as have others as a really robust RF front end. It's a rod pentode, which means it's not really a pentode at all and very low noise. Very linear sharp cut off, but G2 varied from 20V to 90V will set gain. Used on Russian military gear from man packs, vehicles, MiG and satellite. Ring diodes or quad IC FET switches for first stage mixing.

[Jez]

I'm reasonably sure it also has a PLL and a Kallitron oscillator with ECC88, maybe in other stages.... or I could be getting it mixed up with another contender for ultimate receiver of course! Possibly the G3PDM design.... I'd bet good money on at least the ECC88 oscillator in G4DTC one though.

I do have it on one of my HDD's but a quick trawl through has found nothing .... remembering exactly what you saved something as is a good start when there's 4TB to search!

[Jez]

That's quite some RF CV there Mike! I just dabble with RF here. High end audio equipment is what I do as the day job.

That's very interesting about that valve. As an RF amp yes? It sounds unsuitable at a glance. A low Gm, low power battery pentode, directly heated, as a robust low noise RF amp?...Surely an ENR in the several K Ohm region? There must be more to it than meets the eye then?

Don't they only have a few hours rated life due to being in missiles and proximity fuzes etc? ie they only need to be reliable for a very short time till they get blown into a 1000 pieces!?