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Old 26th Nov 2011, 3:29 pm   #121
G8HQP Dave
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Default Re: Valve Questions

Once Radios 2, 3 and 4 were on FM the 'serious' UK listener had little use for AM apart from the cricket on long wave (which was a minority interest).
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Old 26th Nov 2011, 7:43 pm   #122
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Once Radios 2, 3 and 4 were on FM...
Ahem.

The Light, Third, and Home.

There were several years before nationwide FM became reality, so respectable AM performance was important well into the 1960's. And then, just as it did, Radio 1 was launched. though I suppose the target audience was youngsters with little AM-only portable trannies.

The EABC80 does seem to appear in 90% of AM/FM designs - while having limitations, it does allow the minimum of IF switching as all the diodes can stay permanently connected to their respective transformers.

A separate diode for AM AGC would have been nice - this could have shared the triode/ two diode cathode, though even then an extra pin would have been needed. Was the B10B base (as used for the PFL200) in existence then? This would have accommodated it.

I've always been a bit curious as to the numbering of the EABC80, with regard to the diodes that is. The 'B' indicates a double-diode, so would this have been the two diodes intended for FM (one of which has an isolated cathode), with the 'A' denoting the single, higher-impedance AM diode (sharing the cathode with the triode and one of the FM diodes? Or did Mullard consider the 'A' as being the isolated FM diode, with the 'B' being the two common-cathode diodes, ignoring the fact that they are different?

(I know, I should get out more...)
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Old 26th Nov 2011, 10:36 pm   #123
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Originally Posted by kalee20
Ahem.

The Light, Third, and Home.
Sorry, I couldn't remember exactly when it was that the new imaginative names were adopted.
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Old 3rd Dec 2011, 3:44 am   #124
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Certainly as the UK FM coverage spread, the need for good local AM reception diminished, at least until, as kalee20 has noted, Radio 1 arrived to complicate the scene. And that I think happened before the BBC started using bandwidth restriction for its MF services (early 1970s) and probably when compression was still done on a judicious basis. In any event, domestic AM receivers with provision for high-quality (wide bandwidth) local reception had probably become quite scarce even before FM arrived. (The Murphy A186 and A188C would have been late examples.) Even so, once FM became established for local station reception, it was reasonable for the setmakers to assume that AM would have been used primarily for distant station reception, and for that purpose delayed AGC would have been advantageous (an RF stage would have helped, too, but that would have meant another valve.)

The Mullard “standard” FM-AM valve line-up of ECC85, ECH81, EF85, EABC80, EL84 and EZ80 (“Valves, Tubes & Circuits” #16, WW 1954-06) did seem to place limits as to what the setmakers could do reasonably economically. Agreed that an EBF89 in place of the EF85 would have allowed delayed AGC on the AM side without increasing the valve count, but the EBF89 (and EF89) were in the future when the initial series of B9A radio valves were announced. Unlike the TV and audio cases, I have never seen these referred to as a “World Series”, but they were tantamount to being such. The EBF80 was available, but I suspect that it might have had too low a slope for use as an FM 2nd IF amplifier following the ECH81. Also, the EABC80 precluded the use of a balanced FM ratio demodulator, whereas the 6FM8 would have allowed this. The EF89 and EBF89 arrived with the second wave of B9A radio valves, which I think also included the EBC81. Per “Valves Tubes & Circuits” #29, WW 1955-05, the EF89 was presented as the replacement for the EF41, and for use in FM-AM receivers, with no mention of the EF85. The two-stage replacement of the previous B8A types may have been why some export AM-only models had mixed valve lines-up, such as ECH81, EF41, EBC41 and EL84.

1955 also seems to have been the time that Mullard started pushing germanium diodes, perhaps because by then they had become acceptably economic relative to diode-containing valves. The last two issues of “Valves, Tubes & Circuits”, #30 and 31, each refer to the OA70 and OA71. Using these for FM demodulation would have obviated the need for the EABC80, and if it were still desired to use an EL84 output stage, then an EBC81 could have been used to provide the voltage amplifier triode and at the same time the AM demodulation and AGC rectifier functions.

Re the EABC80 and which letter refers to which diode, I suspect that on the one hand the Philips/Mullard folks saw the ambiguity, but on the other were not concerned as there was no way out of it. “C” could not be used for a triple diode because it was already taken for triodes. “A” was used for single diodes in multiple valves whether or not they were independent (as in the EAC91) or shared a cathode (as in the EAF42). “B” was used for double diodes regardless of their associations. So the EB91 had two independent diodes, whereas in the EBC41 the two diodes not only shared their cathodes between themselves but also with the triode. The EAA91 designation was used in place of EB91 by some valve manufacturers, and one may argue that it is more accurate, conveying the impression of two independent diodes. But evidently Mullard was not swayed, and perhaps took the Occam ’s razor approach that EB91 was simpler than EAA91.

Against that background, one could argue for assignment of the “A” in EABC80 to the independent diode, basis the EAA91 precedent, or to the higher impedance diode, simply because it is different. My take on that is simply that I should get out more....

But before I step out, I wonder if, had Mullard decide to introduce the 6FM8 or something like it, whether it would have named it as an EBC8x, or an EAAC8x, the latter to differentiate it from the established EBC types.

Cheers,
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Old 3rd Dec 2011, 11:29 pm   #125
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Well, I did actually step out, but to come back in by a different doorway, I recently came across this website, which has some useful and interesting information that speaks to some of the themes in this thread:

http://www.tubebooks.org/tube_data.htm

The Philips publication "1953 Data and Circuits of TV Receiver Valves" pretty much covers the original Mullard World Series TV valves and their applications.

The triode vs. pentode issue comes up in that the ECC81, which was described as being suitable for both RF amplifier and frequency changer applications, was stated to have a noise advantage over the EF80 for RF amplification at Band III frequencies, although the EF80 was said to be usable where high sensitivity was not required. There were grounded-cathode, grounded-grid and (shunt) cascode circuits shown for the ECC81. So this material evidently predated the arrival of the ECC84/PCC84, purpose-designed for series cascode RF amplifier applications.

The use of the ECC81 as a frequency changer was also stated to be advantageous from the noise viewpoint when a low gain RF stage was used. (Presumably this applied to the grounded-cathode and grounded-grid ECC81 RF stage options.) But in those days IFs were still relatively low, with 18.25 MHz (sound), 23.75 MHz (vision) quoted for the complete receiver example provided. So a triode mixer would not have posed any complications for the Band I frequencies.

The EF80 was presented as RF amplifier, IF amplifier and video amplifier, and also as a self-oscillating mixer for Band I reception only. It was also used with agc. So this book must have predated the EF85, which was the variable mu counterpart to the EF80. 1953 refers to the date of the English translation, so the Dutch original may have been written circa 1952. Actually that leaves a bit of a mystery as to just when the EF85 was released, and as to whether it started life as an addition to the World Series TV valves, and was later adopted into the (notional) World Series Radio valves.

The AT 7501 channel selector (tuner) described had an EF80 RF amplifier (with agc) and an ECC81 frequency changer, so it wasn’t a low-noise design. Interestingly it included channel E1 (see: http://www.pembers.freeserve.co.uk/W...s.html#Missing).

Against that background wherein pentodes were used as RF amplifiers at Band III, I suppose it is not so surprising that pentodes were also found as RF amplifiers in early FM receivers. On the other hand, TV front-end design seemed to have swung decisively in favour of cascode RF amplifiers once the ECC84/PCC84 was available. The case for lower noise at Band III was probably quite compelling, as was the the case for the highest gain reasonably possible ahead of the (relatively noisy) pentode mixers that became desirable once IFs moved up to the point that they were nudging the lower end of Band I. It is somewhat paradoxical that a triode mixer could have been used for Band III even with the IFs around 40 MHz, and might have been better, but that the use of a pentode (ECF80/PCF80) was driven by Band I considerations.

The PL82 was stated to be suitable for audio output as well as field output purposes. In fact it looks suspiciously similar to the UL41 in characteristics. The PL81 line output valve was also stated to be suitable for AF purposes, and curves for the push-pull case were provided. So the later use of PL509 and PL519 line output valves in audio amplifiers was not a new idea! An interesting item was the use of the ECH42 from the radio valve series as a line timebase coincidence detector that was intended to reduce the effects of interference. Whilst functionally different, in a way this pointed to the later use of the ECH84 in a noise-gating role.

For FM sound demodulation the EQ80 nonode was featured. This evidently faded from the scene, but quadrature FM demodulation returned to some extent in the 1960s with the EH90, this time in self-oscillating fully synchronous form whereas the EQ80 was quasi-synchronous. In the solid state era, there was a return to the quasi-synchronous quadrature form once suitable ICs became available.

The Mullard publication "Valves and Electron Tubes", although apparently undated, appears to be from the same era as the Philips book, covering the B9A television valves but being ahead of the B9A radio and audio valves, and also ahead of the 25 mA filament battery valves. The listing starts with the pentodes – purely accidental, perhaps? Under “pentodes with diode(s)”, the EBF80/UBF80 is shown as a preferred type, but not the EAF42/UAF42. So Mullard expect that the EBF80/UBF80 would be used for new designs, regardless as to whether one or two diodes were required. (Eddystone must have missed this message, as its 1960s 670C and 840C models both used the UAF42, by which time all B8A types were surely obsolete for new designs.) The EB91 was a preferred type, but not the EB41, even though it was the more recent of the two. Possibly what happened was that once the EB91 was made a member of the TV World Series, it was logical to abandon the previous distinction in which the EB41 was preferred for domestic equipment and the EB91 preferred for industrial/commercial equipment.

The ECC81 was listed as a frequency changer and RF amplifier, having been an original member of the TV World Series. It’s curious then that in later 1953, Mullard was advertising the ECC81 as being one of three new noval-based triodes, along with the ECC82 and ECC83. These three were in the Audio World Series, although usually with the ECC81 and ECC82 presented as being subsidiary to the ECC83. The ECC82 also became an important TV valves, although maybe that happened later on.

Amongst the output pentodes, the EL37 was still listed as a preferred type. But oddly the EL34 was included, although not as a preferred type. Had this been lurking quietly for a couple of years or so before it was announced as a new member of the Audio World Series? And so did it predate, not postdate the EL84? The EQ80 was still a preferred type, as was the GZ32, the GZ34 not yet listed.

"Amperex 1956 Hi-Fi and Audio Tubes" basically covers the Mullard Audio World Series Valves. However, in respect of the EL84 it includes the following comment that I do not recall seeing in any Mullard publications:

“The true pentode characteristics of this tube reduce distortion at low instantaneous plate voltages which allow larger A.C. swings and increased undistorted output as compared with beam power tubes in the same power class.”

It looks as though it could have been a somewhat contrived argument in favour of pentodes in the pentode vs. beam tetrode debate.

Cheers,
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Old 4th Dec 2011, 1:27 am   #126
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Yes, it's interesting about the ECC81 and its association (or otherwise) with the ECC82 and 83.

Mullard listed all three types together in a Valves, Tubes and Circuits, mainly written with audio applications in mind, while gently steering the reader away from the ECC81 towards the ECC83 (better linearity; less microphonic I seem to remember), without highlighting its virtues at RF.

Quote:
Originally Posted by Synchrodyne View Post
The PL81 line output valve was also stated to be suitable for AF purposes, and curves for the push-pull case were provided.
Yes - and Philips made a small public-address amplifier using two push-pull EL81's, which (I think) are the same as PL81's except for the heaters. It beats me why they used these and not EL84's, which need no top cap connectors.
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Old 4th Dec 2011, 3:25 am   #127
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It could have been that once the PCC84/ECC84 and PCF80/ECF80 were in view, Mullard thought that the ECC81 would pretty much be out of a job as a TV valve, but saw a place for it in the Audio World Series. Insofar as the ECC83 (µ = 100) was replacing the ECC40 (µ = 32), having a couple of supplementary lower µ double triodes in the series may have been seen as prudent. In any event some designers may have elected to standardize on the ECC83 for all audio triode functions, but with options available it would not have been seen as a forced choice for those applications where its properties and benefits were of minimal consequence.

It’s interesting that the first two Cyldon TV tuners (as advertised in WW during 1953) followed both the “old” and “new” Philips/Mullard precepts. The TV.5, with user-accessible 5-channel Band I tuning and lower IFs, had an EF80 RF amplifier and an ECC81 frequency changer. The TV.12, of the turret type with 12-channel Bands I & III tuning and higher IF, had a PCC84 RF amplifier (with agc) and PCF80 frequency changer. I suspect that the TV.5 was not in fact much used, whereas the TV.12 and its successors were apparently quite widely used. Cyldon may well have been one of the very early UK users of the PCC84 and PCF80 (and probably their E-series counterparts for the export market).

I am not sure when the EF86 and ECC83 audio valves first appeared in commercial equipment, but the Quad II/QCII, announced in September 1953, appears to have been an early application. (Although one of the Trader articles suggests that very early QCII production may have used the ECC81. Possibly ECC83 production had not started in earnest at that time.)

The earliest UK use of the ECC85 FM front-end double triode that I know of is the Chapman FM81 tuner, which was advertised in WW from November 1953. The FM81 also included an EF85, the earliest I know of in a UK radio application. But one might well find one or both of these in other early FM tuners; I think that Goodsell and Lowther were offering models in the same time period.

As far as I know the EL81 and PL81 are similar apart from their heaters, as are the EL82 and PL82. But I do know that some care is needed with these “L” valves. The PL84 and UL84 do not align with the EL84, but rather with the EL86. And the UL41 differs from the EL41 in other than heater voltage and current. Why use an EL81 instead of an EL84 in an audio amplifier seems to be one of those unanswerable questions. But if something of around the EL81 anode dissipation capacity was needed and the EL84 was seen as "too big", why not the EL82?

Cheers,

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Old 4th Dec 2011, 12:56 pm   #128
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Amongst the output pentodes, the EL37 was still listed as a preferred type. But oddly the EL34 was included, although not as a preferred type. Had this been lurking quietly for a couple of years or so before it was announced as a new member of the Audio World Series? And so did it predate, not postdate the EL84?
The Philips book "Valves for Audio Frequency Amplifiers" by E. Rodenhuis includes a number of characteristic curve graphs on the valves it covers. These graphs show a date in one corner and at least one of the EL34 graphs shows a date some time in 1948. This leads me to believe that the EL34 was in development for a number of years and that EL34 development most likely started before EL84 development.

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Old 4th Dec 2011, 1:40 pm   #129
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Yes - and Philips made a small public-address amplifier using two push-pull EL81's, which (I think) are the same as PL81's except for the heaters. It beats me why they used these and not EL84's, which need no top cap connectors.
I don't know if it is the same one as your thinking of, but Philips made a HiFi amplifier 'AG 9006' using 2 - EL81's, but this was a transformerless design to use with special speakers, a photo of the amp' with circuit appear in the book, 'From Microphone to Ear' by G. Slot, published by Philips Technical Library, on the previous page there is the circuit of another HiFi amp' using EL84's. it doesn't give any reason for using two different types of valve, but just reading through you would get the impression that the EL81 was more suited to a transformerless output stage than a EL84.

John
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Old 11th Dec 2011, 1:21 am   #130
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Looking again at the PL81 data in Philips Book IIIC, it could have been that its attractiveness as an AF amplifier, at least for PA use, lay in the fact that in Class B push-pull, it could deliver reasonable outputs at low anode voltages, namely 13.5 W at 170 V and 20 W at 200 V. Book IIIC mentioned in passing that most of the P-series valves mentioned therein had E-series counterparts, specifically including the PL81 and PL83. So I imagine that the EL81 and EL83 were available as adjuncts to the TV World Series from the start. The EL82 seems to have been a later addition though, perhaps because the EL41 could have filled the role of a 9 W (anode dissipation) field and AF output valve.

Philips/Mullard interest in output transformerless valve amplifiers seems to have been ongoing. Mullard Technical Communications #30 of April, 1958 was devoted to this topic. Nominated suitable output valves were the EL86 and UL84. The EL86 was described at the time as being new. It appears to have been the 6.3 V heater counterpart to the UL84 and PL84, and so somewhat different to the EL84. As the PL84 was not mentioned in this article, perhaps it had not been released at the time? Did the need for 12 W field output valves not materialize until the advent of 110° picture tubes?

Looking back at the Philips/Mullard pathway to the EL84, one may wonder if there may not have been an element of misjudgement in the choice of 9 W as the primary output valve (EL41, UL41) size in the Rimlock series, and which parameter was a determinant of envelope size. In the octal series, the 12 W 6V6GT had been widely used, and wisely or otherwise, its key features had been carried over into the B7G miniature series as the 6AQ5/EL90. In that light, it could be said that Philips was playing catch-up with the EL84. And thus its published EL84 raison d’être: “Recent developments in the art of recording music on disc and tape, together with improvements in the quality of sound broadcasting provided by V.H.F. transmissions such as the television service, are leading to increased demands for high fidelity in the amplifier or receiver. To meet this requirement it is desirable to increase the available audio output power and hence decrease the distortion preset a given output”, tends to look somewhat like a post facto rationalization.

Still on output valves, the UL44 and UL46 seem to have escaped mention in either Philips Book IIIA (which covered the Rimlock radio and audio and adjuncts to this series) or Philips Book IIIC (which covered the TV World Series valves.) Presumably they were out-of-scope for IIIA, and obsolescent by the time IIIC was compiled. However, Kerkhof and Werner include an example UK 405-line TV receiver schematic that includes the UL44 and UL46 types.

Cheers,
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Old 11th Dec 2011, 8:14 pm   #131
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Looking back at the Philips/Mullard pathway to the EL84, one may wonder if there may not have been an element of misjudgement in the choice of 9 W as the primary output valve (EL41, UL41) size in the Rimlock series, and which parameter was a determinant of envelope size.
Perhaps. But then the EL41 / UL41, at 9W in a B8A envelope, is a fair feat. My guess is that Mullard/Philips introduced these valves, let the dust settle, let tha valves amass a few years in service, before embarking on the even more ambitious task of squeezing 12W of anode dissipation into the B9A envelope.

Don't forget, the UL41 does have reliability problems, even at 9W, with internal deposits causing pin-pin leakage. I'll bet the Philips engineers were glad, in retrospect, that they rated it at 9W and not 12W. Otherwise, they could have had a lot of disgruntled customers...

Incidentally, this does raise the question as to how they overcame this particular problem. I don't know - does anyone?
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Old 12th Dec 2011, 11:59 am   #132
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I think part of the problem is the combination of high power and high gain in B9A. The 6BW6 is also high power, but lower gain so element spacings can be wider.
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Old 12th Dec 2011, 1:45 pm   #133
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Not to mention half the heater power due to the increased efficiency of a beam tetrode over a pentode.

The 6BW6 and the (even better) 6CH6 are far more reliable than the best of EL84s. With a minor base re-wire and a resistor to shunt the existing bias resistor to bring the value to 100 Ohms, the 6CH6 makes an excellent substitute for an EL84 in most applications.

It's also a lot cheaper...

Leon.
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Old 18th Dec 2011, 4:36 am   #134
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Thanks; that prompted the looking up of the 6CH6 data.

The 6CH6 was evidently intended to be a video output pentode. It had a 4.7 W heater, 12 W anode dissipation, a slope of 11 mA/V, and 275 maximum anode voltage.

These numbers were close to those of the EL84, which had a 4.8 W heater, 12 W anode dissipation, a slope of 11.3 mA/V, and 300 maximum anode voltage.

So the inference is that Brimar, by whatever combination of design, materials and manufacturing tolerances, had worked out how to produce a reliable 12 W output pentode in the B9A envelope, at least more reliable than the EL84. I wonder if the 6CH6 was costlier than the EL84 when both were in normal production for original equipment use.

By way of comparison, the 6BW6 and 6AQ5/EL90 had 2.8 W heaters, 12 W anode dissipation and slope of 4.1 mA/V. The 6BW6 had 315 maximum anode voltage, as per the 6V6G/GT prototype, but the 6AQ5/EL90 was limited to 250 V, presumably in deference to its small B7G envelope.

Back to the EL41, the Philips commentary on the Rimlock series development relative to the choice of envelope size was thus:

“One diameter only has been chosen for the whole of the “A” series – a diameter large enough to accommodate the maximum number of eight pins and one which does not introduce the risk of excessive bulb temperatures being developed at the maximum total dissipation in any valve designed for a normal receiving set – namely the 14 watts dissipated in a 9 W output pentode which, in order to obtain a high mutual conductance, is provided with a long cathode consuming 4½ W. The few special valves which require nine contacts are made in the “B” or “C” series.”

Philips also made an additional comment:

“In the U.S.A. very small all-glass valves known as miniature valves with a diameter of 17 mm. have been developed in recent years. These valves, however, have a maximum of seven contact pins. As a consequence, an indirectly-heated frequency changer of the triode-hexode type cannot be made in that series, and only output pentodes of low mutual conductance or small output, unless a high working temperature of the bulb is accepted.”

I suspect that the closing comment was aimed at the 6AQ5/EL90.

The EL41 had a 4.5 W heater, 9 W anode dissipation, a slope of 10 mA/V, and 300 maximum anode voltage. So it would appear that design, etc., improvements allowed the step to the EL84, with 33% greater anode dissipation, with but a minor increase in heater power.

In respect of the “U” series Rimlock valves, Philips said:

“Apart from their heaters, the pre-amplifying valves in the Rimlock U-series are wholly identical with corresponding E-types. Hence the most important components of the circuit in which the valves are to be used are also identical to those with E-valves. Owing to the differences in the feed, however, the output and rectifying valves of the U and E-series are of completely different design”.

So it could have been that the UL41, which had to work with 170 anode volts, was a more difficult design proposition than the EL41, and maybe in its early years just a bit beyond what the technology of the time could deliver in reliable form. Presumably though the subsequent improvements that were needed for the later 9 W valves, such as the UL46, PL82 and PL83, and then the 12 W valves such as the EL84, UL84 and PL84, were eventually folded back into UL41 production.

I haven’t seen any details as to the constructional changes made in output valves over the period at interest, but I wonder if, for example, the EL84 had closer cathode-grid spacing than the EL41, and even if the UL41 was closer-spaced than the EL41. Some information on the progression in RF pentodes is available, from 150 µm in the EF50, through 110 µm in the EF42 and EF80, to 50 µm in the EF184.

Now turning to kalee20’s earlier question about which letters apply to which diodes in the EABC80, I think that I might have stumbled across the answer. The “A” refers to the “AM” diode with shared cathode. The “B” refers to the “FM” diode pair, one independent and one with shared cathode. Now as to where I found this:

The site: http://frank.pocnet.net/instruments/...V_folders.html contains schematics for some 1950s Philips European TV receivers, including some Belgian four-standard models (Systems B, C, E and F), about which I have long been curious. The earlier models only used split sound for both FM (System B) and AM (Systems C, E and F), with a double conversion IF strip, using an ECH81 to down-convert from the 1st IF (33.4 MHz for Systems B, C and F and 27.75 MHz for System E) to 7.0 MHz. (Shades of Murphy and its UK TV-FM receivers). Demodulation was carried out by a PABC80 valve, with P(A)B(C)80, d1 and d2 designated for the FM ratio demodulator function and PA(B)(C)80, d3 designated for the AGC delay function. The triode, P(A)(B)C80 was the first audio stage, driving a PL82 output stage. The demodulator itself is quite interesting, and might have been an unusual application for the E/PABC80. Bearing in mind that my circuit reading ability has its limitations, it looks to be an unbalanced ratio demodulator, but with two time constants available for the limiting RC network connected across the two diodes. In the FM mode the limiting capacitor bridges the diode with but a small series resistance, and the AF output is taken from the tertiary coil as per usual. In AM mode a large resistor is inserted in series with the limiting capacitor, thus allowing the voltage across the diodes to respond to the modulation. This AF voltage is tapped down and then fed through what appears to be a series noise limiting crystal diode to the AF system switch. The ratio demodulator also provides the AGC voltage, via the delay diode, to the EF80 1st sound IF amplifier, this preceding the ECH81. Not visible to me, though, is any AF de-emphasis switching, bearing in mind that Systems B, C and F had 50 µs pre-emphasis, whereas System E had none. I have attached a snip of the pertinent part of the schematic. Other interesting circuit features are the use of picture tube cathode drive for System B (negative modulation) but grid drive for Systems B, E and F (positive modulation). So the PL83 video output valve handled both positive and negative going video, apparently without any bias switching. The E(C)L80 sync separator was fed from the PL83 anode for System B, but from its cathode via an E(C)C82 grounded grid amplifier for Systems C, E and F. The vision AGC looks to be line-gated (and so probably black-level) on all systems, but I haven’t quite figured out how it worked. All gain-controlled stages in the IF strips were sharp cutoff EF80s, but recently I have seen mention of this approach in a Mullard article on TV AGC systems. Maybe I’m veering off topic here, but there do seem to be some less common application configurations for common valve types, so I would claim non-negligible relevance to the core subject.

Cheers,
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Old 19th Dec 2011, 10:05 am   #135
Chris Parry
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Default Re: Valve Questions

It seems us brits disagreed with the mighty Philips organisation, because type X78 is indeed a triode hexode on the B7G base. The pinout compromises were unpleasant, and this valve saw little commercial application.

Type 6CH6 was a professional valve intended for video line drivers and similar applications. It was correspondingly expensive and durable. The EL84 was more cheaply produced for domestic applications. They were often thrashed and soon developed problems in service.

Chris.
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Old 19th Dec 2011, 11:44 am   #136
murphyv310
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Default Re: Valve Questions

Hi Synchrodyne.
That certainly is an odd way of neg/pos video switching, very cumbersome but really to be expected from that part of the worlds designs, it makes me think that Andre Citroen was on the design team!
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Old 19th Dec 2011, 1:43 pm   #137
Leon Crampin
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Default Re: Valve Questions

The 6CH6 was regarded as a "professional type" and I think, carried a price premium when originally sold.

There is a price list in the back of my Ed 9 Brimar book: 6CH6 20/- + 6/6 tax. EL84 9/6 + 3/1 tax.

No relevance today, of course.

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Old 26th Dec 2011, 8:27 am   #138
Synchrodyne
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Default Re: Valve Questions

Thanks Chris and Leon for the 6CH6 information. Was it considered to be a “special quality” valve, or is that a level above industrial quality, in turn above domestic quality?

It is perhaps strange that the 6CH6 has not been “discovered” by the valve amplifier fraternity. If so, it probably wouldn’t remain cheaper than the EL84 today.

Cheers,
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Old 26th Dec 2011, 11:05 am   #139
Chris Parry
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Default Re: Valve Questions

6CH6: Not automatically "special quality". Brimar was owned by STC and in the nomenclature used by this company, special quality was called "Trustworthy". Those valves had a dagger-shaped yellow triangular paper "T" adhesive, which in later production became part of the white printing. The "T" valves were graded for survival under conditions of shock, bump & vibration. But very often, the cathodes of the "T" valves were no better than commercial grade - because they did not need to be.

Some 6CH6s carried the "T" label if they had been procured for arduous service. However, all STC & Brimar 6CH6s without exception, were suitable for "professional" service. This meant very long and stable continuous operation in benign environment such as telephone exchanges, fixed transmitter sites and broadcasting studios. All of the "professional" valves had excellent cathodes. Thus if you can find a STC or Brimar 6CH6 with the "T" logo on it, there you have a very fine valve indeed which will last for ages.

It's worth pointing out that the 6CH6 was also made by other manufacturers. The Mullard EL821s were also very good indeed, but many of the USA 6CH6s are no better than ordinary commercial grade.

Chris.
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Old 29th Dec 2011, 4:24 am   #140
Synchrodyne
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Thanks for the additional background, Chris.
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