Valve FM Radio - UK Vintage Radio Repair and Restoration Discussion Forum

Neil Purling · 9th Dec 2015, 12:02 pm

Is it possible to make an FM radio that works by slope-detection and has as few parts as possible? Can you use an EF91 as I have plenty needing employment.

Mike Phelan · 9th Dec 2015, 12:22 pm

Don't see why not, Neil. The hybrid Pye colour TV used this method and it worked well.

Bazz4CQJ · 9th Dec 2015, 12:52 pm

"Few parts" tends to imply limited selectivity, so the slope may be somewhat flat and the audio low. Recalling the days when people were using old RT gear on 2m, it was still possible to copy narrow-band FM on a AM Rx, but then the latter was probably quite selective.

B

G6Tanuki · 9th Dec 2015, 1:42 pm

It's actually possible to make a "FM Crystal set" - you use slope-detection but you need a *seriously* high-Q tuned-circuit at the broadcast-station's frequency to get a steep-enough 'slope'. This is usually a copper-tube 'organ-pipe' resonant cavity or a 'Lecher-line' like approach.

kalee20 · 9th Dec 2015, 2:14 pm

Why not go for a super-regen design? Several single-valve circuits available with a google search.

milaiosifsorin · 9th Dec 2015, 8:18 pm

Single valve FM superregen with PCL200.
https://www.youtube.com/watch?v=zFm-YD2n8-k

ParcGwyn · 9th Dec 2015, 9:17 pm

What about a pulse counting design, EF91s would probably ideal. This site http://members.iinet.net.au/~cool386/ has some nice designs for FM receivers including a single valve loudspeaker regent set.

Trevor · 24th Dec 2015, 8:09 pm

There is an EF91 pulse counting set in the archive I did it
Trev

Wellington · 24th Dec 2015, 9:49 pm

I think the set to which Trevor refers is the one he built here.

G6Tanuki · 25th Dec 2015, 6:48 pm

I remember playing around with similar circuits in the early-1970s.

The big problem I found was that the local-oscillator was so close to the signal-frequency, and coupled with the lack of any sane RF selectivity a really strong signal within a few MHz of the signal you wanted to receive could result in an unwanted pulling or even a 'PLL lock' action and the LO would 'latch' to the strong local signal. Annoying when you're trying to listen to weak continential stations and the BBC are pushing out fat signals a few hundred KHz either side.

A bit like the similar issue with superregens.

My answer was to operate the LO on half the required frequency and follow it with a cathode-follower/frequency-doubler [a 12AT7 will do both functions in one 'bottle'] which did away entirely with the pulling issue.

Synchrodyne · 30th Dec 2015, 3:22 am

For information on very simple FM receivers with slope demodulators, etc., and comment upon their actual performance (or lack thereof), perhaps look at BBC Research Report 1951/7, “Test Report on Three Simple V.H.F. Adaptors made in Germany for the Reception of Frequency Modulation Transmissions in the 90 Mc/s Band”, available here: http://downloads.bbc.co.uk/rd/pubs/reports/1951-07.pdf.

Some schematics were included in the report, and I imagine that the circuit using a pair of EF41 might be replicable using the Z77 (EF91) instead.

Cheers,

9th Dec 2015, 12:02 pm	#1
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Valve FM Radio Is it possible to make an FM radio that works by slope-detection and has as few parts as possible? Can you use an EF91 as I have plenty needing employment.

9th Dec 2015, 12:22 pm	#2
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: Valve FM Radio Don't see why not, Neil. The hybrid Pye colour TV used this method and it worked well. __________________ Mike.

9th Dec 2015, 12:52 pm	#3
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,934	Re: Valve FM Radio "Few parts" tends to imply limited selectivity, so the slope may be somewhat flat and the audio low. Recalling the days when people were using old RT gear on 2m, it was still possible to copy narrow-band FM on a AM Rx, but then the latter was probably quite selective. B

9th Dec 2015, 1:42 pm	#4
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: Valve FM Radio It's actually possible to make a "FM Crystal set" - you use slope-detection but you need a seriously high-Q tuned-circuit at the broadcast-station's frequency to get a steep-enough 'slope'. This is usually a copper-tube 'organ-pipe' resonant cavity or a 'Lecher-line' like approach.

9th Dec 2015, 2:14 pm	#5
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: Valve FM Radio Why not go for a super-regen design? Several single-valve circuits available with a google search.

9th Dec 2015, 8:18 pm	#6
milaiosifsorin Diode Join Date: Nov 2012 Location: Sibiu, Romania Posts: 1	Re: Valve FM Radio Single valve FM superregen with PCL200. https://www.youtube.com/watch?v=zFm-YD2n8-k

9th Dec 2015, 9:17 pm	#7
ParcGwyn Hexode Join Date: Jun 2005 Location: Aberystwyth, Wales, UK. Posts: 358	Re: Valve FM Radio What about a pulse counting design, EF91s would probably ideal. This site http://members.iinet.net.au/~cool386/ has some nice designs for FM receivers including a single valve loudspeaker regent set.

24th Dec 2015, 8:09 pm	#8
Trevor Octode Join Date: Aug 2006 Location: Stoke-on-Trent, Staffordshire, UK. Posts: 1,082	Re: Valve FM Radio There is an EF91 pulse counting set in the archive I did it Trev

24th Dec 2015, 9:49 pm	#9
Wellington Hexode Join Date: Jun 2009 Location: Cheshire, UK. Posts: 363	Re: Valve FM Radio I think the set to which Trevor refers is the one he built here.

25th Dec 2015, 6:48 pm	#10
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: Valve FM Radio I remember playing around with similar circuits in the early-1970s. The big problem I found was that the local-oscillator was so close to the signal-frequency, and coupled with the lack of any sane RF selectivity a really strong signal within a few MHz of the signal you wanted to receive could result in an unwanted pulling or even a 'PLL lock' action and the LO would 'latch' to the strong local signal. Annoying when you're trying to listen to weak continential stations and the BBC are pushing out fat signals a few hundred KHz either side. A bit like the similar issue with superregens. My answer was to operate the LO on half the required frequency and follow it with a cathode-follower/frequency-doubler [a 12AT7 will do both functions in one 'bottle'] which did away entirely with the pulling issue.

30th Dec 2015, 3:22 am	#11
Synchrodyne Nonode Join Date: Jan 2009 Location: Papamoa Beach, Bay of Plenty, New Zealand Posts: 2,944	Re: Valve FM Radio For information on very simple FM receivers with slope demodulators, etc., and comment upon their actual performance (or lack thereof), perhaps look at BBC Research Report 1951/7, “Test Report on Three Simple V.H.F. Adaptors made in Germany for the Reception of Frequency Modulation Transmissions in the 90 Mc/s Band”, available here: http://downloads.bbc.co.uk/rd/pubs/reports/1951-07.pdf. Some schematics were included in the report, and I imagine that the circuit using a pair of EF41 might be replicable using the Z77 (EF91) instead. Cheers,