Television Turret Tuner questions

[Argus25]

I restored a 1948 vintage 94C8-1 American turret tuner. It needed a lot of work and some of the biscuits were broken, but I repaired them.I think many of these tuners have been discarded so I'm trying to save some.

From what I can tell the design of the turret tuner concept was pioneered in the USA, though I'm not 100% certain.

The 94C8-1 tuner was used in many sets in the 1948 era in the USA. They used a 6J6 twin triode mixer/osc and a 6AG5 pentode RF amp. The 6J6 has a lead (Pb) metal ring/shield added to the steel shield. Also RCA used the same lead metal shield on the 6J6 in their early 621TS set, so it is not the first time I had seen it.

Question

Why does the 6J6 have a Pb metal shield ? It does mention in Grob's book they helped reduce vibration. Is there any other purpose for it ?


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By the mid 1950's (featured as "offered for the first time" in Practical Television Dec, 1955) turret tuners were being sold to the hobby market in the in the UK.

One popular one being "The Cyldon". See exploded diagram attached. This is in fact exactly the 94C8-1 American tuner from 1948, the metalwork of both tuners is identical. The Pb shield had now disappeared though and the large top mounted converter coil is not there, it was put internally and on an altered frequency to suit the common UK IF frequency of 33-38 MHz back then.

The main difference for the Clydon being that the tubes for the UK version have been changed to European types, but not their USA counterparts.

Instead the RF amp is now a double triode wired in cascode, a PCC84. The other tube is a PCF80, where the triode is the oscillator and the the pentode is now the mixer.

Questions

Is the cascode triode significantly better than a pentode as the RF amplifier for a VHF TV tuner ? Is this why it was changed from the original design of the 1948 tuner, or is there really not much difference with gain & noise?

[ukcol]

My understanding is that the cascode triode arrangement has a much better noise performance because it doesn't suffer the partition noise that is associate with pentode amplifiers.

[Freya]

The Cyldon type C tuner is a development of the American Standard Coil Company turret tuner that appeared about 1948; pictures show the two types

Just found these links to more of the cyldon tuners
https://www.vintage-radio.net/forum/...0&d=1358461109
https://www.vintage-radio.net/forum/...1&d=1358461109

[Nuvistor]

I will take a guess and say the extra shielding on the oscillator valve was to reduce osc radiation.
Looking at the manual for the 621TS the LO could cause problems i.e. Channel 1 uses LO 71Mhz this is used by CH 4, some other channels channels affected 2 and 3 could affect 5 and 6. Interesting that the manual shows shielding around the LO but not the other tuner valves.

http://www.earlytelevision.org/pdf/R...-Rider-TV1.pdf

Perhaps this was not as much of a problem when the IF frequency was changed to 45Mhz and a simple metal shield was used.

The use of a cascode amp for the RF was reduced noise with good gain has noted earlier. When frame grid triodes came out (PC900?) it was used in a neutralised circuit.

Frank

[Argus25]

Originally Posted by ukcol

My understanding is that the cascode triode arrangement has a much better noise performance because it doesn't suffer the partition noise that is associate with pentode amplifiers.

Some references I have found say that the partition noise for a pentode is only 2 to 6dB worse than a triode but others 6 to 14dB. So I think it is obviously significant.

One feature of cascode is that it creates a similar 4 terminal device to a pentode, where a constant voltage K applied screens the output from the input an circumvents the Miller effect. However, Teledyne figured out how to make a 3 terminal "pentode" like device, the Fetron, see attached.

[Argus25]

Originally Posted by Nuvistor

I will take a guess and say the extra shielding on the oscillator valve was to reduce osc radiation.

I think you might be right about that. I was wondering that since triodes are less microphonic than pentodes it was hard to imagine the Pb shield was just to reduce microphonics. When I first saw the Pb, my immediate thought was, are they trying to shield the tube from X-rays. But in the TV sets that used these at the time, most had fairly low range EHT voltages and little chance of X-ray generation.

[Argus25]

Originally Posted by Freya

The Cyldon type C tuner is a development of the American Standard Coil Company turret tuner that appeared about 1948

Thanks for those links to Clydon tuners.

[Argus25]

Also I was surprised with the Mixer coil in the 94C8-1 tuner. I replaced the resistor in the assembly as the original was out of spec. The construction was interesting with a flat annular custom mica capacitor (photo attached). I guess there was more profit margin in making this themselves, than buying standard off the shelf capacitors from another company.

[Nuvistor]

This series of monthly articles, first one in April 1948 covering USA TV sets starting with the tuner.
http://www.americanradiohistory.com/...-1948-04-R.pdf

Those mica capacitors were used in USA AM radio sets and broke down causing crackling in the audio. Some information on Youtube.
Frank

[Synchrodyne]

The cascode double triode was first developed by RCA in 1951 (as the 6BQ7) to provide an improved, lower noise RF amplifier than the triodes and pentodes previously used as TV RF amplifiers. The basic circuit was derived from the Wallman cascode. It was described in an article in RCA Review for 1951 March; see: http://www.americanradiohistory.com/...w-1951-Mar.pdf, page 3ff. It refers back to an earlier article on TV RF amplifiers in RCA Review for 1948 March; see: http://www.americanradiohistory.com/...w-1948-Mar.pdf, page 136ff.

Whilst the improvement brought by the cascode RF amplifier was desirable in and of itself, it was doubly necessary because the change to the new standard 45.75 MHz IF indicated the use of pentode rather than triode mixers to minimize feedback and regeneration on the lowest low-band TV channels, exacerbated by the closeness of the new IF to those channels combined with the necessarily relatively low Q of the IF bandpass circuits. (It could be combated with neutralization of a triode, but that was seen as undesirable for a mass production consumer item.) Because pentode mixers were very noisy at the high-band frequencies (where otherwise a triode would have been a better choice as a mixer, given that the same regeneration problem did not exist at those frequencies), a high-gain, low-noise RF stage was required ahead of it (the mixer), and the cascode fully addressed this need. Thus RCA introduced the first TV oscillator-mixer triode-pentode, the 6X8, at about the same time as the 6BQ7. There had been some early TV tuners with three valves, but evidently two valves was what the setmakers wanted, hence the need for combination valves. The 6X8 was said to be equivalent to a 6AG5 and half of a 6J6.

Thus the TV turret tuner design settled down to use a relatively standard two-valve combination. As well as the 6BQ7 and 6X8 combination, other valves found in early-to-mid 1950s American practice were the 6BQ7 and 6BZ7 cascodes, and the Tung-Sol 6U8 triode-pentode. In Europe, Philips developed the ECC84/PCC84 and ECF80/PCF80 for TV turret tuner use in 1953. Someone else (I am not sure who) adopted the 6U8 as the ECF82 and from there came the PCF82.

Over time, the cascode RF amplifier was to some extent displaced by other forms, such as guided-grid triodes, more I think for economic than outright performance reasons. But the basic tuner form lasted a long time; as far as I know, in 1970, valved VHF tuners were still in the majority in American practice, because the performance of the bipolar would-be replacements was so poor, and the use of mosfets – which matched or bettered valve performance in the application - was only just starting.

There was a detailed description of a Magnavox “two-tube” TV tuner in Tele-Tech magazine for 1951 May and June. See: http://www.americanradiohistory.com/...ch-1951-05.pdf, page 54ff; and: http://www.americanradiohistory.com/...ch-1951-06.pdf, also page 54ff. This used a 6BC5 (improved 6AG5) RF amplifier and a 6J6 oscillator-mixer, with neutralization. Towards the end of the article, the comment was made: “Another version of this tuner, using a 12AT7 double triode instead of the 6J6 is also in production for use especially in split sound receivers where less oscillator microphonism can be tolerated.” From that it seems that the 6J6 was quite microphonic, which might go some way to explain the sometimes use of heavy shielding that would also act as a damping medium.

For additional background the following may be of interest:

Magazine Articles:

Practical Television 1950 December; description of an RCA TV tuner using a 6CB6 and 6J6; see: http://www.americanradiohistory.com/...n-1950-12.pdf; page 295ff.

Tele-Tech 1949 January; description of a GE two-tube TV tuner using a 6AU6 and a 12AT7; see: http://www.americanradiohistory.com/...ch-1949-01.pdf, page 36ff.

JRE Paper:

DJ Fewings, and SL Fife; “A Survey of Tuner Designs for Multi-Channel Television Reception”; 1955.

Book:

DH Fisher; VHF Television Tuners; Heywood, 1956.


Cheers,

[Argus25]

Originally Posted by Synchrodyne

The cascode double triode was first developed by RCA in 1951........

Thanks Synchrodyne for all the detail and the references that make great reading. I had a feeling that early Tuner development was interesting and it is even better than I thought. I think it is worth saving/restoring as many early tuners as possible. Not only for repairs but for the construction of replica sets or custom built TV sets in the future.

[Synchrodyne]

No problem. Here are some more magazine articles that may be of interest:



Radio Craft 1947 December page 32ff: about FM & TV front ends.

TeleVision Engineering 1950 May page 19: “The Printed-Circuit TV Tuner”.

Tele-Tech 1951 August page 30ff: about UHF conversion devices.

Radio-Electronics 1953 January page 57ff: “UHF Circuitry”.

Radio-Electronics 1953 September page 58ff: “Cascode Type Front Ends”.

Tele-Tech 1955 April page 80ff: “Designing TV Tuners for Color Receivers”.

Wireless World 1955 August page 397ff: “The Cascode - And its Advantages for Band-III Reception.

Radio-Electronics 1956 January page 48ff: about Standard Coil’s new “Rainbow” VHF Tuner.

Radio-Electronics 1956 July page 30ff: about Standard Coil’s new “Neutrode” VHF tuner.

Radio-Electronics 1960 January page 43ff: about Standard Coil’s “Guided Grid” VHF tuner.

RCA Engineer 1960 August-September: “A Nuvistor VHF Tuner”/

Wireless World 1960 October page 474: “Permeability Tuners for Television”.

Radio-Electronics 1961 February 56 page 72: brief item on the RCA Nuvistor VHF tuner.


All are available at the excellent “American Radio History Site” at: http://www.americanradiohistory.com.



Cheers,