UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Closed Thread
 
Thread Tools
Old 19th Sep 2018, 5:37 pm   #1
Nanozeugma
Heptode
 
Nanozeugma's Avatar
 
Join Date: Nov 2017
Location: London, UK. Bury, Lancashire quite regularly :)
Posts: 609
Default FM Stereo Multiplex Decoder.

Seeking some insight from those more knowledgeable than I...

I've had it in mind for some time that I would like to build a *valve* FM stereo multiplex decoder as an adjunct to a classic valve mono FM tuner.
The question is - is this a realistic proposition nowadays?

The stumbling block has always been the wound components which, when described at all in old articles - refer to parts long since unobtainable - Mullard Vinkor pot cores for example.
Said obstacle has so far prevented the desire becoming reality.

I think if I could source (or wind) the relevant coils (given availability of premade components or cores / formers / enamelled copper wire etc), I'd like to have a go.

Would anyone care to offer guidance regarding a design that could be built with parts (all of which) may still be sourced?

Many thanks in advance
__________________
Thermionic Emission, warms the cockles of your tubes.
Nanozeugma is offline  
Old 19th Sep 2018, 7:12 pm   #2
Restoration73
Nonode
 
Restoration73's Avatar
 
Join Date: Jun 2013
Location: Surbiton, SW London, UK.
Posts: 2,801
Default Re: FM Stereo Multiplex Decoder.

I do not see the benefit of designing and building a valve switching decoder that can only
offer a mediocre performance, bearing in mind it is easy to interface a pll IC type.
A popular commercially made decoder to use with old Leak tuners used the IC type.
"Hi Fi World Stereo Tube FM Decoder" by Tim de Paravincini is the one I mean.
Does not contain any odd inductors. I added a MC1310P IC to an old Heathkit valve
tuner about 40 years back which worked well.
Restoration73 is offline  
Old 19th Sep 2018, 8:02 pm   #3
Nuvistor
Dekatron
 
Nuvistor's Avatar
 
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,427
Default Re: FM Stereo Multiplex Decoder.

I have never seen a valve stereo decoder, by the time Radio 3 started Stereo transmissions from Holme Moss the decoders were discrete transistor ones. I never had a call from customers to retrofit their valve stereo radiogram with a decoder. Perhaps because it was just R3 that was in stereo.

I can see it as an interesting exercise but an IC type would work much better and be much easier to get going.
__________________
Frank
Nuvistor is offline  
Old 19th Sep 2018, 8:17 pm   #4
Philips210
Nonode
 
Join Date: May 2007
Location: Redruth, Cornwall, UK.
Posts: 2,562
Default Re: FM Stereo Multiplex Decoder.

Hi.

About twenty years ago, I had a Grundig 5490U radio which had a valve multiplex stereo decoder fitted to the inside of the back cover. ISTR it had just one valve, ECC81 and a few germanium diodes. The complete decoder was in its own box and it didn't have that many peripheral components. I never did manage to find a circuit diagram for that model but there is some mention on Radio Museum.

If I was building a stereo decoder and wanted a vintage theme then I'd probably build a discrete version else the MC1310P IC option is a good one. Practical Wireless magazine did a discrete decoder project in December 1969 which is a Mullard/Philips circuit and can be used with either a transistor or valve discriminator.

Regards
Symon.

Last edited by Philips210; 19th Sep 2018 at 8:33 pm. Reason: Additional information
Philips210 is offline  
Old 19th Sep 2018, 8:47 pm   #5
Philips210
Nonode
 
Join Date: May 2007
Location: Redruth, Cornwall, UK.
Posts: 2,562
Default Re: FM Stereo Multiplex Decoder.

Quote:
Originally Posted by Nanozeugma View Post

The stumbling block has always been the wound components which, when described at all in old articles - refer to parts long since unobtainable - Mullard Vinkor pot cores for example.
Hi.

It should be possible to work around this by using the Ferroxcube RM series of pot cores. I have done this a few times on old projects where Vinkor types were specified.

Regards
Symon
Philips210 is offline  
Old 19th Sep 2018, 8:53 pm   #6
Ted Kendall
Dekatron
 
Join Date: Jul 2008
Location: Kington, Herefordshire, UK.
Posts: 3,658
Default Re: FM Stereo Multiplex Decoder.

There were more valve decoders in the US than Europe, largely because they were transmitting stereo in the early 1960s and Europe didn't agree on a standard until 1966, by which time transistors were the way to go. The Quad, Leak and Rogers multiplex decoders were all transistor designs.

To decode these days with valves is rather doing things the hard way - even the Paravicini design uses valves in the sum signal path, not the difference. With no PLL to keep things locked and thermal drift, it could be quite a tedious thing to use, too.
Ted Kendall is offline  
Old 19th Sep 2018, 9:36 pm   #7
Sideband
Dekatron
 
Sideband's Avatar
 
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,549
Default Re: FM Stereo Multiplex Decoder.

I've only had experience with one valve stereo decoder. It used a single ECC81 and to be honest, it wasn't very good. To be fair, it worked but it needed a top-notch signal and I had all the alignment details so went ahead but it really made no difference.

An interesting project perhaps but you would be far better using one of the many IC designs using a dedicated decoder chip. The Americans were far ahead in the design of valve decoders and some special valves were developed as well. As stated above, by the time we had stereo, transistors were the obvious choice.
__________________
There are lots of brilliant keyboard players and then there is Rick Wakeman.....
Sideband is offline  
Old 19th Sep 2018, 9:57 pm   #8
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
Default Re: FM Stereo Multiplex Decoder.

I built an MC1310 decoder for a valve mono tuner and it works really well.

If the old mono tuner was not designed for stereo (and even if it claims it was because it had probably never been tested), the chances are it will need some circuit changes:

1. Some mono tuners had 'multiplex' outputs for stereo decoders. Chances are it is taken from the discriminator and needs a buffer if you want to drive an IC decoder. I used a JFET and took power from the heater circuit using a voltage doubler.

2. The de-emphasis filter after the discriminator will need to be removed as it will strip out the higher frequencies you need for stereo. I have seen various claims but I reckon you need to be able to cope with at least 50kHz audio.

3. The IF bandwidth may need widening for the same reason. I certainly spent time re-aligning the IF on mine but I never tried it without.

With those changes I ended up with a decoder that locks very well even on weak signals. After much tweaking and waving of scope probes, I finally came to the conclusion that FM broadcasting is now so heavily compressed the output is far from hi-fi with stereo breaking up on loud passages on the worst culprits.

Last edited by PJL; 19th Sep 2018 at 10:10 pm.
PJL is offline  
Old 19th Sep 2018, 10:14 pm   #9
wd40addict
Octode
 
wd40addict's Avatar
 
Join Date: May 2009
Location: Stevenage, Herts. UK.
Posts: 1,515
Default Re: FM Stereo Multiplex Decoder.

I can recommend the Heathkit AC-11, this is an American valve based unit. The UK equivalent SD-1 used transistors.

Obviously it will be ready built, but will probably require some servicing. Sound quality in conjunction with my Heathkit FM-4U is excellent.

Cynics might say the de-emphasis is wrong: 75us (US) Vs 50us (Europe), but testing showed it to be almost exactly 50us (3dB down at 3.2kHz)!!

Last edited by wd40addict; 19th Sep 2018 at 10:21 pm.
wd40addict is offline  
Old 19th Sep 2018, 10:54 pm   #10
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: FM Stereo Multiplex Decoder.

Let’s assume that the objective, however much it is the long way around, is to build a valved FM stereo decoder, and not simply to build a stereo decoder suitable for a valved tuner. In that case it may be worth looking at several sources for inspiration.

The Mullard circuit described in Wireless World 1962 October p.487ff does include coil-winding details, albeit that Vinkor cores are specified. But it appears that the latter might be substitutable.

Audio magazine in 1961 had several articles on valved stereo decoders,, including one by Norman Crowhurst about filter design. These articles were reprinted in Audio Anthology #6, but would also be available at: https://www.americanradiohistory.com/Audio-Magazine.htm.

The first edition of the book “FM Multiplexing for Stereo” by Leonard Feldman contained quite a bit of information about valved stereo decoders, including analysis of commercial examples.

For an in-depth treatment, have a look at this paper: https://archive.org/details/multiplexadapter00pete.


Cheers,
Synchrodyne is offline  
Old 19th Sep 2018, 11:57 pm   #11
Sideband
Dekatron
 
Sideband's Avatar
 
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,549
Default Re: FM Stereo Multiplex Decoder.

Quote:
Originally Posted by Synchrodyne View Post
The Mullard circuit described in Wireless World 1962 October p.487ff does include coil-winding details, albeit that Vinkor cores are specified. But it appears that the latter might be substitutable.
I knew I'd seen a circuit somewhere that also gave coil winding details. Thanks for finding that. If the OP is interested, the publication is here https://www.americanradiohistory.com...ld-1962-10.pdf

A couple of variations are given one of which uses two ECH84's.
__________________
There are lots of brilliant keyboard players and then there is Rick Wakeman.....
Sideband is offline  
Old 20th Sep 2018, 2:57 pm   #12
dominicbeesley
Octode
 
dominicbeesley's Avatar
 
Join Date: Nov 2004
Location: Hebden Bridge, West Yorkshire, UK.
Posts: 1,885
Default Re: FM Stereo Multiplex Decoder.

This is something I'd be interested in following, if not actively joining in while my workshop is U/S.

I get the idea completely - I've built colour decoders for NTSC/Secam etc and while they were not much cop they were fun!

I've been interested in building an FM decoder too. I'm sure with a bit of reverse engineering from their intended purpose any wound components can be sussed out with a bit of thought.

D
dominicbeesley is offline  
Old 20th Sep 2018, 3:08 pm   #13
Nuvistor
Dekatron
 
Nuvistor's Avatar
 
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,427
Default Re: FM Stereo Multiplex Decoder.

The wound components will be resonant at 19 and 38khz, so should be possible to make inductors for those frequencies, bandwidth of the mixing transformer will need to be about 30khz around the 38khz subcarrier.
May not even require a 19khz one.

I suppose it’s like why climb the mountain? because it there.

It would be an interesting thread but the IC type appeals more to me.
__________________
Frank
Nuvistor is offline  
Old 20th Sep 2018, 7:10 pm   #14
Jon_G4MDC
Nonode
 
Jon_G4MDC's Avatar
 
Join Date: Nov 2015
Location: Welwyn Garden City, Hertfordshire, UK.
Posts: 2,013
Default Re: FM Stereo Multiplex Decoder.

Each to his own choice.
I very much like the idea of doing this with Valves.

If I wanted some semiconductor or IC decoder I could just buy or copy some well respected antique - could I not?
Jon_G4MDC is offline  
Old 20th Sep 2018, 9:19 pm   #15
MotorBikeLes
Nonode
 
Join Date: Dec 2012
Location: Kirk Michael, Isle of Man
Posts: 2,346
Default Re: FM Stereo Multiplex Decoder.

Some of the replies surprise me. The OP wants to build a valve stereo decoder, and all the collectors of 1920's crystal sets and beyond advise the hated solid state approach.
As I mentioned only a few minutes ago in another thread, I have a radio upstairs with such a decoder, I retrofitted one some time between 1967 and 1971, (probably 1969) and only a couple of weeks ago was doing a repair on a set I sold 8 years ago, and to which I added a stereo decoder, but which needed a minor modification for the stereo indicator. These were all NordMende sets from between 1963 and 1968, and I have circuits for the decoder-ready radios, the integrated ones and the one supplied for retro fitting.
I could scan all or part circuits, if the mods say that is OK. I also have a type 384 stereo signal generator, though it has never been used "in anger" for alignment.
I recall about 40 years ago I acquired a couple of Italian made "Rosita" stereo chassis. One was BER, but it had a little solid state (IC) decoder and I have the distinct memory that I fitted that to a NordMende set without a decoder. I MAY have notes somewhere, but not sure I could find them. I have seen the decoder circuit of a valved Grundig set, but fairly sure I don't have it myself.
Les.
MotorBikeLes is offline  
Old 21st Sep 2018, 11:11 am   #16
G8HQP Dave
Rest in Peace
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
Default Re: FM Stereo Multiplex Decoder.

The few real 'valve' decoders which exist mainly use germanium diodes for the actual multiplex demodulation. The valves act as buffers, frequency doublers and stereo pilot light switches.

Then there are the 'valve' decoders which use a silicon chip to do almost everything, apart from a final (and, it coud be said, unnecessary) bit of matrixing.
G8HQP Dave is offline  
Old 21st Sep 2018, 11:33 am   #17
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: FM Stereo Multiplex Decoder.

Quote:
Originally Posted by G8HQP Dave View Post
The few real 'valve' decoders which exist mainly use germanium diodes for the actual multiplex demodulation. The valves act as buffers, frequency doublers and stereo pilot light switches.
True, although it may also be noted that by the time stereo arrived, many if not most valved FM tuners were using germanium diodes for FM demodulation, so doing the same for multiplex demodulation was consistemt with that, and perhaps not a major departure from "valviness".

There were exceptions though. One Mullard design used a pair of heptodes (ECH84 or EH90) for multiplex demodulation, and I recall seeing an earlier circuit that used the 6BU8 double pentode. One Zenith design used the 6AR8A beam deflection tube, and another used the diode pair in a 6BN8 double diode-triode.


Cheers,
Synchrodyne is offline  
Old 21st Sep 2018, 5:46 pm   #18
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
Default Re: FM Stereo Multiplex Decoder.

I've always intended to build a valve decoder too!

Wound components don't bother me - I would use RM ferrite cores (or possibly toroids if inductance isn't critical as long as it is high). Yes, silicon diodes for synchronous switching would be technically better, but larger signals and 6AL5 diodes (of which I have several) or even ECC82 triodes (of which I have even more) ought to be possible.

Look forward to developments!
kalee20 is offline  
Old 21st Sep 2018, 8:03 pm   #19
Nuvistor
Dekatron
 
Nuvistor's Avatar
 
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,427
Default Re: FM Stereo Multiplex Decoder.

Circuit of the Heathkit AC11 stereo decoder, uses 3 12AU7’s and 2 1N60 germanium diodes plus a PSU.
https://www.schematicsunlimited.com/h/heathkit/
__________________
Frank
Nuvistor is offline  
Old 22nd Sep 2018, 1:37 pm   #20
G8HQP Dave
Rest in Peace
 
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
Default Re: FM Stereo Multiplex Decoder.

The thing you have to watch with a true valve decoder is that the gain of the DSB mixer must be stable, as that affects stereo separation. Hence in most cases a switching mixer is used.
G8HQP Dave is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 8:26 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.