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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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6th Feb 2020, 5:57 pm | #1 |
Hexode
Join Date: Jun 2005
Location: Buntingford, Hertfordshire, UK.
Posts: 362
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Possibly Front End HF converter.
Hi all,
Long shot, but does anyone recall this unit? believed to be a hf converter It’s a tidy build, possibly scratch built by a professional looking at the finish, hoping based on a published design. Labelled up as using a couple of 6K7’s. PW maybe? Vy Bst Chris
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6th Feb 2020, 6:45 pm | #2 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,009
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Re: Possibly Frontend Hf converter
Yes, though it's built using an interesting range of parts.
The main tuning-capacitor, tuning-drive and coil-pack look to be from the 1930s - Octal valves from the same timescale. I'm thinking that it could have been designed to take something like a 6K7 as the RF amp, and either a 6A8 pentagrid converter, or the 6K8 triode-hexode. |
6th Feb 2020, 7:45 pm | #3 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,400
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Re: Possibly Frontend Hf converter
Intriguing device- my initial thoughts were HF converter but I then thought that what would be noticeably different looking LF-wards oscillator coils all look similar to the others- HF osc coils would always look similar, but LF ones running at different frequency ratios would surely have different-looking windings, especially on the lowest band. Also, no obvious sign of padders, though it's difficult to be sure, or some form of output tuning. (Though there's possibly a mica capacitor at bottom right against the chassis rear apron, though only one visible). Again, all circumstantial. Maybe it's a "tight" preselector, of low-volume production. Post-war, Racal produced a large 19" preselector to give the RA17 a better chance in the ship-board environment with triple-tuning and 2x low-gain ECC82 cascode stages- maybe this is an earlier example of a similar beast?
Surely tempting to put it into action again- the thoroughness even looks to extend to aerial trimming. |
6th Feb 2020, 9:05 pm | #4 |
Hexode
Join Date: Jun 2005
Location: Buntingford, Hertfordshire, UK.
Posts: 362
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Re: Possibly Front End HF converter.
Thanks, appreciate the comments.
Yes, I’ll get some power on it, haven’t studied wiring and not sure of valves yet but the screening partitions both have 6K7 pencilled on them. Working away so can’t take anymore pics until I’m home. But will upload some close ups as soon as get the opportunity. ATB Chris
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BVWS, VMARS, RSGB, DEHS, RAOTA Nothings too big or too heavy, otherwise it wouldn't be where it is! |
6th Feb 2020, 9:38 pm | #5 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,903
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Re: Possibly Front End HF converter.
With the organisation of the coil pack, I wonder how they adjusted slugs to get the stages aligned across each band?
David
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6th Feb 2020, 9:51 pm | #6 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,400
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Re: Possibly Front End HF converter.
I wondered that, too- perhaps they were air-cored without slugs and were made precisely enough to be close without adjusters, as in the BC348? The formers look rather similar to the baked paper roll types in the CR100, with the same arrangement of a small brass bar running through slots and having a tapped hole in the middle for securing.
Talking of CR100s, the tubular capacitors with pitch-seal ends, radial leads and green labels are like the Muirhead ones I found in an early CR100, they were the most dramatically leaky that I've ever come across! |
6th Feb 2020, 10:06 pm | #7 | |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,009
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Re: Possibly Front End HF converter.
Quote:
Others have noticed the apparent absence of padders - but I'm wondering if it could have been designed to front-end an existing receiver, as a preamp for the MW/LW bands and a converter for the amateur HF bands? If it was downconverting to - let's say 1.6MHz - then the difference between the LO and RF frequencies is sufficiently small that, for a small frequency-spread [as would be suggested by the low number of fixed/static vanes on the tuning capacitors] you don't really need to bother with padders. It's seemingly got a separate antenna-trimmer too, which would easily tune out any alignment errors over a half-MHz or so. One thing that puzzles me - the front-panel knob on the bottom left appears to drive a rod that runs through the coil-packs to a switch on the rear of the chassis - perhaps this is some sort of 'straight-through' switch to feed the antenna direct to the 'back-end' receiver ? |
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6th Feb 2020, 10:10 pm | #8 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Possibly Front End HF converter.
I don't think they are any slugs in those type of coils.
All wave pre-selector ? Lawrence. |
6th Feb 2020, 10:20 pm | #9 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,400
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Re: Possibly Front End HF converter.
The apparent simple logging scale would be more appropriate to a preselector, too- select band, tune for peaked noise without necessarily needing precise frequency display. Whereas a logging scale-only converter would be a bit clunky to use. Granted, the HRO does things that way, but with a highly resolving logging arrangement.
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6th Feb 2020, 10:32 pm | #10 |
Nonode
Join Date: May 2007
Location: Preston, Lancashire, UK.
Posts: 2,511
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Re: Possibly Front End HF converter.
They look like Wearite coils?
Andy |
6th Feb 2020, 11:17 pm | #11 |
Heptode
Join Date: Jun 2012
Location: Southeast Norfolk, UK.
Posts: 773
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Re: Possibly Front End HF converter.
Yes, they do and usually there is a type number painted on. There's a thread here on Wearite "P" series coil sets.
https://www.vintage-radio.net/forum/...ad.php?t=64856 Cheers Roger |
7th Feb 2020, 8:33 am | #12 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,903
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Re: Possibly Front End HF converter.
I too think it's a preselector, intended to improve a separate receiver.
The similarity of the coils in each stage fits this. The valves would have been high impedance buffers between the resonator stages to make the bandwidth narrow. In the days of single-conversion low IF receivers, image rejection on the HF bands was very little, and the 40m band had megawatts of broadcasters next door. A preselector was a useful tool. David
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