Mystery Leak Troughline III with internal stereo decoder

[Arkless Electronics]

The valves at RF/IF probably don't matter much beyond, perhaps if the LO valve isn't steady. However the details of the IF filtering do matter as that can alter the FM pattern, and hence cause distortion, etc. The demodulator can also affect this.

For the above reason, the performance might also vary with signal level if that means the source impedances of the valves alters, in turn altering the matching to the staged filters.

Devils in many details.

Very much so yes and these details will matter vastly more than whether valves or SS are used at RF or IF. Microphonics in the LO valve would cause FM and be demodulated yes and could add a tiny amount of colouration... there again microphonics in any part of a LO whether SS or valve will do this.

My main point was that any thoughts of "oh it's a valved tuner so will have 'the valve sound'" are probably incorrect other than any "valve sound" added by an output amplifier or cathode follower, where used, and that in the case of a TL2 with decoder there are no valves in the audio signal path.

FWIW I once designed and built an FM tuner using wideband minimum phase ceramic IF filters and an unusually wideband Foster Seeley Discriminator (and 74LS TTL devices in the limiter) and it was the best sounding tuner I've ever heard! I had an inkling that maybe a good FSD would give better subjective sound quality than a quadrature detector, hence trying this out.
A McIntosh MR67 was about the best sounding commercial tuner I tried.

 

[Jim Audiomisc]

Yes. In an ideal world we could use FM tuners with a wideband IF and demodulator. The snag is the way the FM band has become more congested over the decades. In theory an SDR can deal with this by varying according to conditions and having an ultra-flat response in-band. But in reality SDRs tend to bring their own problems to the party. :-/

When I first 'emigrated' to Scotland I found the CT7000 invaluable. The signals here were weak and R4 FM wasn't broadcast above the border, so even in mono was difficult to get decently. Now the signals are much better and a simple dipole lets something like a 626 or FM4 work nicely. But if I lived in a big city they might have problems with the congestion of the band, pirate stations, etc, etc. So the reality comes down to depending where you are, reception conditions, etc, when choosing what's 'best' as a tuner.

 

[Jim Audiomisc]

Actually, the use of 74LS made me think of pulse counters as linear detectors. IIRC Uncle Clive tried that but I've no idea how well his implimentation worked given the devices of the period (pun alert!). Might work with modern faster devices, though.

 

[Woodface]

Well there’s not much love for the poor old Troughline here is there! I’d posted this hoping for a little help in identifying what I thought was a fun little curiosity, but thanks for reminding me that this hobby isn’t meant to be fun. Foolishly, in my time away from all things audiophile, I’d forgotten that the real purpose of this interest is to quash any positivity or enthusiasm, while simultaneously demonstrating how much more you know about an individual’s personal possessions than they do. Silly me, my memory really isn’t what it used to be y’know.

Probably the best post ever, so true.

 

[Arkless Electronics]

Actually, the use of 74LS made me think of pulse counters as linear detectors. IIRC Uncle Clive tried that but I've no idea how well his implimentation worked given the devices of the period (pun alert!). Might work with modern faster devices, though.

There were many designs for valved pulse count tuners in the late 50's early 60's in such as Practical Wireless magazine and excellent sound quality was claimed for them.

 

[Paul R]

I have a couple of old 'Radio and Electronics' (I think) magazines featuring a DIY FM tuner that uses a low IF so that a CD4046 PLL chip can be used as the detector. It would be fun to try and build nowadays, the only issue is that it can't tune above about 100MHz. But then, back in the day, that was where the police lived.

And I have a broken Quad FM1 in the loft that looks to be a more comprehensive design than the TL, other than the lack of output buffer and need for a separate PSU.

Anyway if the OP's tuner pleases him then awesome, otherwise it might benefit from some sensitive return towards original spec work. Perhaps even make the decoder a separately powered distinct device?

 

[Andrew Leitch]

I've got a Leak Troughline Stereo, which had been badly modded. Got it back to spec now, ready to run through the alignment.

The OP mystery unit has the EZ80 rectifier tube replaced with silicon rectifiers. This isn't a good idea unless substantial changes are made elsewhere. The EZ80 is designed to warm up more slowly than the other tubes. The other tubes are therefore already conducting when HT arrives. That means you get a voltage drop over the pass resistors. With silicon you will over voltage parts of the circuit while the tubes warm up, because they aren't conducting so there are no volt drops over the resistors.

Also, the EZ80 will drop some 10s of Volts itself, so without making matching resistor mods along with the change to silicon, biasing will be off.
Not sure why anyone would exchange a tube rectifier for silicon - is there a benefit somewhere?

Andy

 

[Big John]

I have a Troughline Stereo that came with the original leak decoder, Len Gregory persuaded Tim de Paravicini to make me one of his stereo decoders and fit it to the Troughline this was years after he stopped making/selling them. I was and still am very happy with it, I also have a Meridian 104 tuner which unlike the leak will get a signal from a piece of wet string.
No aerials are allowed on the outside of our flats. I have a John Linsley Hood designed Quad FM aerial that I made from 1mm thick mild steel sheet it is very large. After building into the loft my son fitted it in the eaves in a space created for it it has 25m of WF100 cable with a BNC connection to the leak. unlike my previous Antiference 5 element aerial the JLH quad allows full limiting ie the EM84 "magic eye" top and bottom virtually touch, good signal is vital with a Troughline.
Like the Tuner shown various other components are not as it left Leak's factory thank you TdP, as Arkless states normally there are no valves in the signal path being an external decoder I have 1 on each channel, I use 1980's Beijing square foil getter ECC83's in the TdP decoder. The result is far different to a factory standard Troughline.

 

[davidsrsb]

I used to have a Sansui pulse counter tuner. It was not very good.
My Sony STS-S361 PLL is much better. I have tweaked it up with better RF decoupling than originally fitted. Single layer and jumper links is a horrible way to start designing a radio receiver.
I remember JLHs transistor array PLL detector. That sort of thing works well in the lab on a signal generator, but struggles with the crowded FM band

 

[TheDecameron]

I’m now down to just two tuners now. A Magnum Dynalab MD102 and a Troughline + TdeP decoder.

 

[Big John]

I’m now down to just two tuners now. A Magnum Dynalab MD102 and a Troughline + TdeP decoder.

Do you have a preference?

 

[TheDecameron]

Do you have a preference?

Yes, the MD most certainly! Nice thought the Troughline is.

 

[davidsrsb]

Back to the OP.
I have never seen the Troughline using a Philips decoder. The usual after market mod kit uses the similar and more common MC1310

 

[Kyriacos]

I've got a Leak Troughline Stereo, which had been badly modded. Got it back to spec now, ready to run through the alignment.

The OP mystery unit has the EZ80 rectifier tube replaced with silicon rectifiers. This isn't a good idea unless substantial changes are made elsewhere. The EZ80 is designed to warm up more slowly than the other tubes. The other tubes are therefore already conducting when HT arrives. That means you get a voltage drop over the pass resistors. With silicon you will over voltage parts of the circuit while the tubes warm up, because they aren't conducting so there are no volt drops over the resistors.

Also, the EZ80 will drop some 10s of Volts itself, so without making matching resistor mods along with the change to silicon, biasing will be off.
Not sure why anyone would exchange a tube rectifier for silicon - is there a benefit somewhere?

Andy

Dear Andy

I need to correct you and others on several points!

1. The 3 was NEVER meant to be used with a stereo decoder. The "front end" is poor for stereo. Thus, the Leak stereo was made. It has a much better front end in terms of sensitivity and selectivity and noise, and uses an ECC88 instead of the ECC84.

2. YES the EZ80 must be removed and good quality diodes used in the valve base. OTHERWISE, the oscillator will drift and thus intoduce distortion. Do not worry about current rush etc. This is not an amplifier with hungry output valves.

3. The aerial must be able to provide full signal to get proper quieting. Otherwise do not bother. Also, the screen resistor on the tuning indicator needs to be replaced/adjusted so that a 1.5 mm gap means full signal. You need a proper FM alighment generator eg Sound technology ST1000A for that.

4. The decoder needs to be aligned again using the ST1000A.

Then you get a really nice tuner. Now we need the BBC to re-introduce proper analogue transmissions and not re-fabrigated digital signals sent from London to the rest of the country.

Kyriacos

 

[GT]

Dear Andy

I need to correct you and others on several points!

1. The 3 was NEVER meant to be used with a stereo decoder. The "front end" is poor for stereo. Thus, the Leak stereo was made. It has a much better front end in terms of sensitivity and selectivity and noise, and uses an ECC88 instead of the ECC84.

2. YES the EZ80 must be removed and good quality diodes used in the valve base. OTHERWISE, the oscillator will drift and thus intoduce distortion. Do not worry about current rush etc. This is not an amplifier with hungry output valves.

If the Troughline has a factory fitted multiplex output socket then it can be used with an external stereo decoder, or an internal one for that matter. Don't fit Silicone diodes!!! Just buy a new valve rectifier and keep the unit standard but sympathetically replace the necessary parts. It worked well in factory trim and has done for the last 60 years so don't bugger about trying to mod it as it usually ends up as scrap. Take it to someone who knows what they are doing and will keep it in factory spec for you.

 

[Kyriacos]

If the Troughline has a factory fitted multiplex output socket then it can be used with an external stereo decoder, or an internal one for that matter. Don't fit Silicone diodes!!! Just buy a new valve rectifier and keep the unit standard but sympathetically replace the necessary parts. It worked well in factory trim and has done for the last 60 years so don't bugger about trying to mod it as it usually ends up as scrap. Take it to someone who knows what they are doing and will keep it in factory spec for you.

If the Troughline has a factory fitted multiplex output socket then it can be used with an external stereo decoder, or an internal one for that matter. Don't fit Silicone diodes!!! Just buy a new valve rectifier and keep the unit standard but sympathetically replace the necessary parts. It worked well in factory trim and has done for the last 60 years so don't bugger about trying to mod it as it usually ends up as scrap. Take it to someone who knows what they are doing and will keep it in factory spec for you.

Dear Graham, the mods and comments I made above ALL come from the late Tim de Paraviccini THE expert in this field ! I am sure you will agree he knew what he was doing. I have two of the Leak stereo tuners he modified and the accompanying tube decoders he designed. He told me NOT to mix up the decoders as they are "tuned" to their tuner. I was present in his workshop for one of them and I saw exactly what he did. I then bought my own ST1000A to play around. BTW he also showed me how to modify (needed) the Leak TL12+ to be better and to be used without the Leak pre amp, because ordinarilty the TL12+ MUST be used with its own pre amp as some of the amplification takes place in the pre amp, so the input impedance is custom. and more ......

 

[GT]

Dear Graham, the mods and comments I made above ALL come from the late Tim de Paraviccini THE expert in this field ! I am sure you will agree he knew what he was doing. I have two of the Leak stereo tuners he modified and the accompanying tube decoders he designed. He told me NOT to mix up the decoders as they are "tuned" to their tuner. I was present in his workshop for one of them and I saw exactly what he did. I then bought my own ST1000A to play around. BTW he also showed me how to modify (needed) the Leak TL12+ to be better and to be used without the Leak pre amp, because ordinarilty the TL12+ MUST be used with its own pre amp as some of the amplification takes place in the pre amp, so the input impedance is custom. and more ......

I used to fix and put back to standard many Troughlines that Tim worked on. Most of them were butchered, as in their chassis's were cut and drilled and were very poorly done and the components used were poor. Whether Tim actually did the work I couldn't say, maybe it was done by some of his YTS staff, either way I would debate about Tim being the expert in this field as it was not my experience having seen, fixed, repaired and put right some of what he did on the Troughlines, but I always respected and more recently recommended what Tim designed for EAR. Anyway, have you not seen my HiFI World magazine articles on the Troughline and the specially developed decoder my colleague and I produced for the Troughline and other older tuners, These got the very best sound out of the Trougline and we restored many hundreds over the years as well as adding our decoder. The Troughline and decoder were reviewed in both HiFi News and HiFi World magazine. It was also the one Noel Keywood used to refer too when he did tuner reviews as it was his reference tuner.

Regarding the TL12: I have seen and had just about every modded Leak through my workshop over the last 40 years and I can tell you the best performing ones were the ones that were sympathetically restored and kept to the standard circuit design. Regarding a preamp for any of the Leak power amp range: A good quality passive is all that is required, however there are a couple of good active preamps that can be used to obtain very good results with the Leak range, namely the Croft Micro and any other Croft preamp which had a zero gain/buffered output. Also, J.C.Verdier's Control B preamp which has selectable gain. With J.C.Verdier's Control B you can select the line stage gain to 0dB which is perfect for amplifiers of that period. Leak power amps are very sensitive, 125mV for full output, as Leak put most of their amplification gain in their power amp section. That means you have to use a unity gain preamp or one with very low gain. Check out some of my restoration articles (Leak, Radford, Quad) in Hi-Fi World magazine back in the early 1990s. Component selection is also critical for getting the best audio performance out of these old amplifiers.

 

[eksiil]

...the specially developed decoder my colleague and I produced for the Troughline and other older tuners...

I think my TL3 is one of those. I was told at the time (bought it on AOS maybe 10 years ago) the internal decoder was by GT Audio. given a good signal it rivals CD replay in quality. very pleased with it.

 

[Kyriacos]

I used to fix and put back to standard many Troughlines that Tim worked on. Most of them were butchered, as in their chassis's were cut and drilled and were very poorly done and the components used were poor. Whether Tim actually did the work I couldn't say, maybe it was done by some of his YTS staff, either way I would debate about Tim being the expert in this field as it was not my experience having seen, fixed, repaired and put right some of what he did on the Troughlines, but I always respected and more recently recommended what Tim designed for EAR. Anyway, have you not seen my HiFI World magazine articles on the Troughline and the specially developed decoder my colleague and I produced for the Troughline and other older tuners, These got the very best sound out of the Trougline and we restored many hundreds over the years as well as adding our decoder. The Troughline and decoder were reviewed in both HiFi News and HiFi World magazine. It was also the one Noel Keywood used to refer too when he did tuner reviews as it was his reference tuner.

Regarding the TL12: I have seen and had just about every modded Leak through my workshop over the last 40 years and I can tell you the best performing ones were the ones that were sympathetically restored and kept to the standard circuit design. Regarding a preamp for any of the Leak power amp range: A good quality passive is all that is required, however there are a couple of good active preamps that can be used to obtain very good results with the Leak range, namely the Croft Micro and any other Croft preamp which had a zero gain/buffered output. Also, J.C.Verdier's Control B preamp which has selectable gain. With J.C.Verdier's Control B you can select the line stage gain to 0dB which is perfect for amplifiers of that period. Leak power amps are very sensitive, 125mV for full output, as Leak put most of their amplification gain in their power amp section. That means you have to use a unity gain preamp or one with very low gain. Check out some of my restoration articles (Leak, Radford, Quad) in Hi-Fi World magazine back in the early 1990s. Component selection is also critical for getting the best audio performance out of these old amplifiers.

Well I fear you never met or got to know Tim. You see originally Tim offered his valve decoder through the magazine and then shops as a stand alone unit. People used their own Leak tuner. However, he realised people were 1) using the wrong Leak tuner (ie the ones before the model intended for stereo) and 2) butchering the tuners and not followed his suggestions, so results were below average. So he stopped offering the decoder as a stand alone unit but insisted poeple sent in their tuners so that he did all the work. I have seen him work on one of mine, he was anything but butchering them !! My units in an all EAR set up with Tannoys sounds ausome. Also, this was his side activity so absolutely none of his staff were anywhere near the decoder or the Leaks. The staff were exclusively for the high end stereo business items! Other things also , Tim personally redesigned the electronics of other stuff eg my Revox B77, my TEAC AL-700 elcaset, the Revox PR99 mkIII made all valve etc etc with a flat frequency response of sub 20Hz ro 28kHz. He set up numerous studios as you know. This was a serious side of Tim away from the business side of high end stereo. According to Tim the input impedance on the TL12+ needed to be modifed as it was too sensitive and would saturate easily in most peoples hands with "standard" pre amps. Rather than modifying pre amps etc, it makes more sense to bring the TL12+ to standard impedance. Then you dont have to worry what people will do with partnering them. My TL12+ as thus modified and they sound very very good for what they are. Remember that they are 0.1 distiortion at 400Hz not the whole spectrum, or power range.

 

[GT]

Well I fear you never met or got to know Tim. You see originally Tim offered his valve decoder through the magazine and then shops as a stand alone unit. People used their own Leak tuner. However, he realised people were 1) using the wrong Leak tuner (ie the ones before the model intended for stereo) and 2) butchering the tuners and not followed his suggestions, so results were below average. So he stopped offering the decoder as a stand alone unit but insisted poeple sent in their tuners so that he did all the work. I have seen him work on one of mine, he was anything but butchering them !! My units in an all EAR set up with Tannoys sounds ausome. Also, this was his side activity so absolutely none of his staff were anywhere near the decoder or the Leaks. The staff were exclusively for the high end stereo business items! Other things also , Tim personally redesigned the electronics of other stuff eg my Revox B77, my TEAC AL-700 elcaset, the Revox PR99 mkIII made all valve etc etc with a flat frequency response of sub 20Hz ro 28kHz. He set up numerous studios as you know. This was a serious side of Tim away from the business side of high end stereo. According to Tim the input impedance on the TL12+ needed to be modifed as it was too sensitive and would saturate easily in most peoples hands with "standard" pre amps. Rather than modifying pre amps etc, it makes more sense to bring the TL12+ to standard impedance. Then you dont have to worry what people will do with partnering them. My TL12+ as thus modified and they sound very very good for what they are. Remember that they are 0.1 distiortion at 400Hz not the whole spectrum, or power range.

I knew Tim as he exhibited at most of the shows I exhibited at during the early 1990's and up to 2016 when I last exhibited at a show. I knew exactly what Tim did with his specials including the Troughline and Revox G36 tape machine, among other devices. I also offered exactly the same service on the Troughlines. We would never supply or fit a stereo decoder to a 40-60 year old tuner that was fitted with all original parts. For a start you can't offer any sort of guarantee unless the tuner is fully serviced before fitting the decoder. This is nothing new. It's what most companies did if they were offering an "add on" to a vintage device.

Re the Leak TL12+: The problem is not with the input impedance. It is the high sensitivity of the first stage and the amplifiers overall loop gain, so don't confuse input impedance with high sensitivity. They are two different things.