Jason FM/TV sound tuner

[Terry]

And 3 digit endings were often used for professional valves (as against those intended for domestic use)

I thought they simply jumbled up the domestic number by putting the number after the first letter. Thus E88CC instead of ECC88, for example.

I note that Philips describe the E88CC as a special quality version of the ECC88 - which one would expect a 'professional' valve to be, surely?

http://www.mif.pg.gda.pl/homepages/frank.../E88CC.pdf

[Mark Hennessy]

The correct link is https://en.wikipedia.org/wiki/Mullard%E2...esignation but when I created it, the forum obligingly stuck an erroneous http:// in front of it!

Fixed link in post #4

Not sure how you entered the link - it had a URL tag around it. I just paste the link - using the non-WYSIWYG editor - and it works fine (the hyperlink A tags are added automatically when the page is generated, so the database just holds the raw link). If the WYSIWYG editor is doing that, then that's yet another reason to avoid using it. I spend most of my life sorting out problems caused by it - especially when folk copy/paste from Word or similar, which results in dozens of spurious font tags and similar.

[Geordie McBoyne]

The attachment shows the Jasonkit JTV2.    This receiver employs a fireball tuner unit.
In 1958 Pye Radio and Television Ltd employed a fireball tuner as an alternative unit in many TV receivers with FM radio facilities.  The TV/FM switching was actuated by a large cam fitted to the rear of the tuner unit.
Pye group brand Pam models 550FM and 551FM.

Geordie McBoyne.

[Synchrodyne]

Here is the schematic for the JTV2:

   

As may be seen, the Fireball turret was substantially modified, with ECF80 and ECC81 valves in place of say ECC84 and ECF80.

The ECF80 triode served as a grounded grid RF amplifier, unusually for that type with AGC.  It fed the ECF80 pentode mixer, its customary role.  But a triode RF amplifier feeding a pentode mixer was generally considered to be an undesirable combination in VHF practice, as the triode would not have enough gain to offset mixer noise.  One ECC81 triode was the oscillator, and the other was the AFC reactance valve.  One may wonder why Jason did not retain the standard turret circuitry with the addition of a varicap diode for AFC.  That was established practice in the USA and Germany for example.

The three-valve IF strip with a Foster-Seeley discriminator was conventional when looked at in FM terms, athough for FM only one might have expected the second stage to be configured as an amplifier/limiter, rather than an amplifier only, the latter being required for its AM function.  AGC bias was taken from the limiter grid back to the 1st IF stage and also to the RF stage without the additional delay typically found in TV receiver practice.  I’d guess that its FM quieting curve was not quite as good as might be obtained in an FM tuner with say an ECC85 front end and a three-stage IF strip, although its limiting sensitivity might not be too much different.

AM was taken from the limiter grid, the diode formed by the grid and cathode acting as a demodulator.  (That same rectifying action produced the AGC bias, after appropriate smoothing.)  And there was a noise limiting diode, as was found in the sound channel of many UK TV receivers.

That though does not give too much joy in trying to derive the JTV circuit, and in particular its two-stage IF strip.  Possibly that was derived from the IF strip for the Prefect switched FM tuner.  That used a pair of EF80s, and quite unusually for a two-stage circuit, had a Foster-Seeley discriminator.  I think though that provided a bigger AFC voltage as compared with a ratio detector, so might have been the reason for its choice.  As far as I know the JTV lacked AFC, so probably could have used a ratio detector to advantage.

Cheers,

Steve

[Geordie McBoyne]

Hi Steve,
thanks for posting up the circuit of the JTV2.
That's a clever trick employing the EF80 limiter valve as the AM demodulator. I'll try out the JTV2 when I return to the shop on Tuesday.

Geordie McBoyne.

[Synchrodyne]

Yes, taking the demodulated AM from the limiter grid was quite a neat idea.  There was prior and significant validation of the technique for high quality purposes in the form of the Ambassador/BBC VHF AM/FM Comparator receiver, used during the UK VHF AM vs. FM trials.  This receiver was briefly described, with mention of its AM detector, in Radio-Electronics for 1951 October, page 51ff.  A generally more detailed description was provided in Wireless World (WW) for 1952 January, page 35ff, although the AM detector was not mentioned.

A little more information on the JTV that I have since found:

It had a ratio detector.  Its sensitivity was quoted as 80 μV for 40 dB quieting.  This same number was also quoted for the Prefect switched FM tuner (although I have also seen 60 μV), which was a local-area model with a two-stage IF strip.  The FMT3 fringe area FM tuner with a three-stage IF strip had a sensitivity of 5 μV for 40 dB quieting, whilst for the JTV2 it was 10 μV.  So, the original JTV fell into Jason’s local area group of models) whereas the JTV2 fell into the fringe area group.

All models in the local area group had two-stage IF strips consisting of an amplifier and a limiter.  The fringe area group all had three-stage IF strips, either two amplifiers and one limiter, or one amplifier and two limiters.  Initially Jason favoured the ratio detector, but later designs used the Foster-Seeley discriminator in order to obtain a larger AFC bias voltage.  The JTV was from the later era in this regard, but as it did not have AFC, the ratio detector could be and was used.

From looking at the information that I have on hand on the various Jason tuner models and their respective circuits, my best guess is that the JTV IF strip looked very much like that of the JTV2 except that the second IF amplifier (EF89) was missing.  Thus, demodulated AM would have been taken from the limiter grid as it was on the JTV2.  Whereas the FM tuners used the Z77 or Z719/EF80 as 1st IF amplifier, the EF89 was probably chosen for the JTV (and JTV2) because whereas with FM (and TV vision), it was not unusual to apply AGC bias to sharp cutoff valves, with AM it was normal to use remote cutoff valves.  Jason FM tuner practice was to take the AGC bias from the limiter (or 1st limiter) and apply it to either the RF amplifier (early designs) or the 1st IF stage (later designs), but not both.  For the JTV2, and I’d guess for the JTV, it was applied to both the RF amplifier and the 1st IF amplifier, perhaps in part because RF stage AGC was normal for TV front ends.

Returning to the Hi-Fi News magazine connection, it would appear from the attached that the article on the Jason JTV was in the 1958 September issue, as may be deduced from the attached page from Hi Fi Year Book 1959.  I imagine that that article contained the full circuit and constructional details.  I also understand that the JTV (probably the built version) was reviewed in Hi-Fi News for 1959 June.

   

Cheers,

Steve

[Geordie McBoyne]

Attachments show the internals of the Jason JTV2.   This example has coils for London TV transmitters, channel 1 and 9.

Geordie McBoyne.

[Synchrodyne]

One gets the impression from the size of the chassis plate cutout for the front end that that was the original JTV chassis, re-used for the JTV2 and so leaving quite a large gap behind the more compact Fireball front end.  I should have more information on the JTV within a moon or so.  I managed to find copies of Hi Fi News 1958 September and 1959 June, both by now on the pigeons’ legs and heading this way.

Re the modified Fireball front end in the JTV2, I wonder how the valve positions compared with the standard UK version.  The JTV2 had V1, the ECF80 (RF and MX) on the right looking from the front, and V2, the ECC81, on the left.  I can’t find the information for the standard version.  My guess is that on a minimum-change basis, Jason had just the mixer swap sides, which would indicate that normally the PCC84 (or ECC84) was on the right and the PCF80 (or ECF80) on the right.

As best I can determine the original Standard Coil Fireball was offered primarily as a neutrode type.  But then Standard Coil normally offered different configurations for its turret and disc-type TV front ends.  There were cascode and neutrode versions as might be expected, but even in the late 1950s, it was still possible to specify the (by then rather old) pentode RF plus double-triode (5J6 or 6J6) mixer-oscillator combination, although quite why anyone might want to do that is hard to fathom.

Cheers,

Steve

[Geordie McBoyne]

The patent for the fireball TV tuner. Filed 1957 by the Standard Coil Corporation:

http://www.google.co.uk/patents/US2955198

Geordie McBoyne.

[Synchrodyne]

Thanks for that – it makes interesting reading.  As expected, much of the patent refers to the mechanical design of the Fireball front end.  An interesting aspect though is that it was also designed for the easy and economical addition of a UHF tuner.  Confirmed was the fact that the basic design used a neutrode RF amplifier, although the possibility of using a pentode, cascode or tetrode was mentioned.


In my previous post I said:

Re the modified Fireball front end in the JTV2, I wonder how the valve positions compared with the standard UK version.  The JTV2 had V1, the ECF80 (RF and MX) on the right looking from the front, and V2, the ECC81, on the left.  I can’t find the information for the standard version.  My guess is that on a minimum-change basis, Jason had just the mixer swap sides, which would indicate that normally the PCC84 (or ECC84) was on the right and the PCF80 (or ECF80) on the right.

That should have read:  My guess is that on a minimum-change basis, Jason had just the mixer swap sides, which would indicate that normally the PCC84 (or ECC84) was on the right and the PCF80 (or ECF80) on the left.

That valve disposition was confirmed in the patent drawings; the RF valve (triode with B7G base) was on the right, and the mixer-oscillator (triode-pentode with B9A base) on the left.

Given that the Jason JTV chassis could evidently accommodate a standard turret tuner, the change to the Fireball type for the JTV2 was apparently not driven by a need for compactness.  I wonder then whether Jason found the Fireball easier to modify to its own requirements than other types.  On the face of it, having all of the channel coils on the same disc, that disc being equidistant from the two valves, would have made doing what Jason wanted easier than with a turret or switched tuner where the various coils were longitudinally separated, and say moving the mixer from the front to the back valve, whilst leaving the oscillator at the front, would have been awkward and would have involved undesirably long wiring connections.

Cheers,

Steve