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Old 6th Jan 2017, 5:57 pm   #1
ukcol
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Default Valves in commercial applications for which they were not specifically designed.

In this thread David (FERNSEH) pointed out that the Sobell T143 and the Pye FV1 series used an ECL80 for the (Band 1) frequency changer.
The Philips and Brimar data sheets for the ECL80 suggest a number of applications. The triode section can be used for an audio pre-amplifier or an oscillator for field or line time bases. The pentode section being suitable for field output, audio output or sync separator stages. Its use as at Band 1 frequencies was probably not in the mind of the designer(s).

I own a Murphy A4 table radio which uses an AC/Pen as the frequency changer. The A4 is an early superhet and certain aspects of its design contain “carry over” techniques from the days of the TRF; I’m thinking particularly of the volume control which uses a method common in TRF sets.

The Mazda data for the AC/Pen describes it as an “output pentode for A/C receivers”. Similarly the designer probably didn’t intend it to be used as a Medium and Long Wave frequency changer.

In the case of the Pye FV1 a second ECL80 is used for the audio amplifier and in the Murphy A4 a second AC/Pen is used as the audio output valve. No doubt this reduced manufacturing costs by reducing the number of valve types used in the respective designs.

Don't misunderstand me, I'm not suggesting this is in anyway "wrong", just interesting.

How common was it to use a given valve in an unusual application and can anyone point to other examples.
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Old 6th Jan 2017, 6:08 pm   #2
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Default Re: Valves in commercial applications for which they were not specifically designed.

One of the Philips portable radios used a DM70 tuning indicator as a phase inverter for the push-pull output stage.
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Old 6th Jan 2017, 6:23 pm   #3
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Default Re: Valves in commercial applications for which they were not specifically designed.

That is interesting. I wonder what informc that sort of decision. Perhaps Philips had a surplus of DM70s.
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Old 6th Jan 2017, 6:31 pm   #4
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Default Re: Valves in commercial applications for which they were not specifically designed.

An EL84 used as crystal calibrator (with 300k anode resistor ) in a Telefunken HF receiver and an EL360 used as focus voltage adjustment/shunt stabiliser (i.e. so that a conventional off-the-shelf LV pot could be used as a front panel control) in a Pye CTV monitor. Also an EF86 used as EHT series regulator in the Hartley CT436.

All done for sound reasons (the latter two I can rationalise, if anyone can come up with a good reason for the EL84 placement, I'd be interested! Maybe when operated this way, it was a good harmonic generator, though surely there were candidates with lower heater power....) no doubt, but attention-getting for being outside the marketed remit.
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Old 6th Jan 2017, 6:55 pm   #5
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Default Re: Valves in commercial applications for which they were not specifically designed.

Another oddity - in the WWII-era Collins "TCS" communications receiver the local-oscillator is a 12A6 beam tetrode - originally specified for output-stage use (kinda like a 6V6...)

Why such a beefy valve was used as a LO I can't imagine.


Equally, the use of TV line-output valves as RF amplifiers in a whole range of radio gear (both amateur transceivers and CB 'burners') from the 1950s onwards. The US and Japanese manufacturers were heavily into this, as was the UK's "KW Electronics" who used a PL81 as the RF PA stage in a small 160-metre transmitter.

I also recall seeing a QQV06/40A VHF double-tetrode used as the modulator-valve in a marine band "fish-phone". I guess it was more compact than a pair of 6L6 ??

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Old 6th Jan 2017, 7:47 pm   #6
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Default Re: Valves in commercial applications for which they were not specifically designed.

The Airmec 304 RF oscillator I have has an EL34 as an RF amplifier and another EL34 as an HT regulator...

Not only that, it used a pair of 5763s as the oscillator proper before their output is amplified by the aforementioned EL34...

It's the only application I've seen of an EL34 as an RF amplifier and up to 100MHz to boot!
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Old 6th Jan 2017, 7:51 pm   #7
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Default Re: Valves in commercial applications for which they were not specifically designed.

One possible rationale for the EL84-as-crystal-calibrator I'd come up with (and it really is clutching at straws.....) was that operating at very low anode volts but high screen volts made for high distortion and good harmonic spread, but that that made screen dissipation high and an EL84 made a common, off-the-shelf candidate with higher screen dissipation than the EF91, 92, 93, 94-type gang.

Told you I was clutching at straws!
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Old 6th Jan 2017, 8:02 pm   #8
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Default Re: Valves in commercial applications for which they were not specifically designed.

I would think the 12A6 in the TCS was used for at least two reasons

1 to reduce the spares holding required

2, to reduce the number of stages needed in the transmitter

It is of course cheaper to build equipment when you can source valves all of the same type to reduce the unit cost
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Old 6th Jan 2017, 8:10 pm   #9
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Default Re: Valves in commercial applications for which they were not specifically designed.

I can't remember model numbers now but there was a TV that used an EF80 as sound output and another (a Derwent) that used an ECC82 as sound output. I always thought it was a little unusual for Thorn to use a PCF80 as sound output in some of their portable TV's.

A continental Philips 'Bi Ampli' radio uses a PL81 to drive the bass channel.
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Old 6th Jan 2017, 8:18 pm   #10
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Default Re: Valves in commercial applications for which they were not specifically designed.

Lots of interesting examples, keep 'em coming folks.
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Old 6th Jan 2017, 8:34 pm   #11
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Default Re: Valves in commercial applications for which they were not specifically designed.

Quite a few TVs used EF80s and EF91s as audio output valves. They must have been working very hard. Maybe the manufacturers had surplus stocks, or the setmakers negotiated very good quantity discounts.
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Old 6th Jan 2017, 10:07 pm   #12
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Default Re: Valves in commercial applications for which they were not specifically designed.

Quote:
Originally Posted by ukcol View Post

I own a Murphy A4 table radio which uses an AC/Pen as the frequency changer. The A4 is an early superhet and certain aspects of its design contain “carry over” techniques from the days of the TRF; I’m thinking particularly of the volume control which uses a method common in TRF sets.

The Mazda data for the AC/Pen describes it as an “output pentode for A/C receivers”. Similarly the designer probably didn’t intend it to be used as a Medium and Long Wave frequency changer.
That's an interesting circuit- I find early superhet mixer/oscillator techniques (before the refinements of the so-called "multi-grid" and triode-hexode dominated the scene) to be intriguing and ingenious in their diversity of ways to use existing valves. I'm scratching at the very limits of my understanding here, but is that self-oscillating AC/Pen circuit using transitron technique between anode and screen grid? Again, I find myself reaching for the screen grid dissipation limitation excuse....
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Old 6th Jan 2017, 10:32 pm   #13
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Default Re: Valves in commercial applications for which they were not specifically designed.

Yes I agree! It could be a 'triode' oscillator, tuned anode, with the anode as anode and screen-grid as grid - don't non what C7 is doing then, unless it's some form of neutralisation (very low value). Or it could be a 'triode' oscillator, with screen-grid as anode and control grid as grid, with the tuned circuit actually in the anode but heavily coupled to the grid by C7... Values would help!

Full marks to the designer for getting it to work, almost certainly without the benefit of an oscilloscope too!
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Old 6th Jan 2017, 10:37 pm   #14
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Default Re: Valves in commercial applications for which they were not specifically designed.

Quote:
Originally Posted by ukcol View Post
That is interesting. I wonder what informc that sort of decision. Perhaps Philips had a surplus of DM70s.
I think it was a cock-up, they intended the DM70 to work as a tuning indicator, and had the IF amplifier reflexed as AF phase inverter, but late in the design process found they couldn't make it work repeatable enough... So dumped the idea of reflexing the IF amplifier, and as there wasn't room for another valve had to use the DM70 as a triode amplifier instead of a tuning indicator! At least it doubled as a 'power on' indicator...
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Old 6th Jan 2017, 10:59 pm   #15
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Default Re: Valves in commercial applications for which they were not specifically designed.

I suppose it only needs a triode of miniscule gain for the application, but I'm guessing that gm and mu weren't explicit in the DM70 data sheet.... Reminded me of the tiny phase-inverter triode slipped into the ECLL800.

The various non-scripted ways that the ECH21 and ECH81 with their separated triode control and heptode modulator grids got used presumably fit into the thread heading also.
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Old 7th Jan 2017, 12:07 am   #16
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Default Re: Valves in commercial applications for which they were not specifically designed.

Quote:
Originally Posted by kalee20 View Post
Yes I agree! It could be a 'triode' oscillator, tuned anode, with the anode as anode and screen-grid as grid - don't non what C7 is doing then, unless it's some form of neutralisation (very low value). Or it could be a 'triode' oscillator, with screen-grid as anode and control grid as grid, with the tuned circuit actually in the anode but heavily coupled to the grid by C7... Values would help!
I think it is the former. The Trader circuit description is as follows although I have changed the component numbers to match the Murphy ones in the snippet attached to post #1.

"Reaction coupling is established between the oscillator anode circuit coils L11 (MW) and L12 (LW) which are tuned by C1c and the reaction coils L9 (MW) and L10 (LW) in the screen grid circuit."


C7 is not mentioned in the circuit description but the parts list calls it "small neutralising coupling."

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Quote:
Originally Posted by turretslug View Post
That's an interesting circuit- I find early superhet mixer/oscillator techniques (before the refinements of the so-called "multi-grid" and triode-hexode dominated the scene) to be intriguing and ingenious in their diversity of ways to use existing valves.
Hi turretslug,

Yes, I always find these early circuits interesting and enjoy seeing how they evolve over time.

Last edited by ukcol; 7th Jan 2017 at 12:11 am. Reason: Added quote from turretslug
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Old 7th Jan 2017, 12:24 am   #17
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Default Re: Valves in commercial applications for which they were not specifically designed.

This thread reminds me of that late 1960s/early 1970s TV series, Spike Milligan’s “The World Of Beachcomber”, which included a subseries that was approximately “The instruments of the orchestra and the unusual uses to which they are put”.

Re the use of the DM70 as an AF phase-splitter, the Mullard description of this valve included: “Whilst having electrical characteristics similar to a triode, the grid and anode together produce a visual indication of the voltage applied to the grid” (this from Mullard “Valves, Tubes & Circuits (VT&C) #5).

So I suppose if you needed a battery triode, but wanted to pick from the primary battery valve inventory, and not a less common valve, the DM70 would have made sense.

Philips seemed to be quite good at finding additional or secondary uses for its valves, and then documenting them somewhere, although not always in the data sheets or application notes. That said, the Band I oscillator-mixer application for the ECL80 appears to be well outside of even its secondary ambit. Early on, Philips used the PL81 as an AF output valve, and I suppose that this legitimized the use of later line output valves in this role. And the ECF80/PCF80 was particularly noted for having a whole matrix of “authorized” applications beyond its original VHF TV mixer-oscillator role , including using the pentode section as an AF output valve. (See Mullard VT&C #26 for details.) Another Philips case was the use of both the ECH42 and EQ80 as coincidence detectors in TV flywheel line timebases.

Bailey’s use of the ECF82 (which originated as the 6U8 as a VHF TV oscillator-mixer) as an AF phase-splitter I think qualifies for mention here. Although given the diversity of authorized applications for the ECF80/PCF80, it is perhaps not so surprising.

I have seen a schematic for a 1950s Pye industrial TV monitor in which a PL83 rather than a PL82 was used as the field output valve; a PL83 was also “properly” used for the video output. I suppose that one may rationalize this from the precedent of the UL46, which was intended for both field output and vide output applications. I think that the PL82 had characteristics much like those of the UL46, whereas the PL83 was optimized for the video output role. However, it still retained the 9 W anode dissipation capability, probably much more than was needed for most video output applications, so was a candidate for “other uses”

Zenith used the 12AT7 double-triode as both an AM mixer-oscillator and an FM oscillator in at least one of its early AM-FM radio chassis. I doubt that GE had envisaged the use of its 12AT7 in the AM role; rather it was designed for FM and TV applications. A prior Zenith AM-FM chassis had used the 12BE6 heptode as a self-oscillating for both AM and FM, this being within its design ambit. Then for a following FM-only chassis it had used the 12AT7 as mixer-oscillator. Presumably it liked its better performance on FM, and so decided that the FM requirement was going to take precedence in its next AM-FM chassis. Probably it could get away with using a triode mixer on AM because the 6BJ6 RF amplifier was used on AM as well as FM, and so would tend to block an oscillator radiation. Using as many common valves as possible in AM-FM receivers appeared to be a desideratum for Zenith and perhaps other American setmakers. One wonders whether these events had influenced RCA. For FM applications, GE had stolen a march with its 12AT7, when RCA’s approach was a heptode mixer (the 6BA7) with better FM performance. So when RCA designed the 6X8 triode-pentode for VHF TV mixer-oscillator work, as a step-up from the 12AT7 for 40 MHz IF receivers, it also specified it as being suitable for both FM (with the option of triode-strapping the pentode) and AM frequency changer applications, thus making it more suitable for the AM side of AM-FM receivers using common front-end valves than the 12AT7.

Hallicrafters had also used a double-triode (7F8) oscillator-mixer on all bands of its 1946 SX-42 receiver, which tuned from 540 kHz to 110 MHz AM and CW, and 27 to 110 MHz wideband FM. So there was a precedent for Zenith’s later choice.

The EL84 was used as a bias oscillator in some domestic tape recorders, but probably only because it was there as an AF output valve and so could do double duty. In that application though purity of output would have been desirable, opposite to that of a calibration oscillator.

An interesting case is the use (originally by Zenith) of the 6BE6 heptode as a noise-gated TV sync separator, an application definitely not mentioned in the data sheets or application notes. Just yesterday I was looking at why this was done for a possible posting in the “Valve Questions” thread. How it came about and what it led to turned out to be quite a complex story that is a bit like something out of that 1980s James Burke “Connections” TV series. Philips also used the ECH81, and then ECH83 for the same purpose in some of its European TV receivers, but I don’t think that it was ever mentioned in their respective data sheets. One outcome of the noise-gated sync separator development pathway was the Westinghouse 6BU8 double dual-control pentode; somewhere I have seen (although I can’t find it now) an early Mullard suggestion that the 6BU8 could be used as an FM stereo multiplex demodulator, although its later papers on the topic favoured the ECH84 and EH90 (both also part of the sync separator story) for this role.

Anyway, it does appear that there are cases where setmakers have used valves for purposes well outside of their original ambits, and that this has inspired additional development work by the valve makers to include those applications.

One can find some valves where the triode part was designed also to be used as a diode, such as the 6AU8 general-purpose TV triode-pentode. But I doubt that the designers of the EABC80 triple-diode-triode had envisaged that its triode would be used as a diode. But I understand that it was so used in the Pye HFT111 AM-FM hi-fi tuner. If so, that could be explained from its commonality with the RF-IF section of the Pye FenMan II radio receiver. The latter had an EABC80 and an EBC41, each contributing an AF triode, neither of which were needed for the HFT111. Two of the EABC80 diodes were used for FM demodulation, and the EBC41 diodes were used for AM demodulation and agc rectification, thus allowing delayed AM agc. (Kudos for Pye for resisting the Mullard-supported trend of abandoning delayed AM agc for AM-FM receivers.) Evidently in the HFT111, the EABC 80 was retained, in this case for FM and AM demodulation, with the triode used as a diode for delayed AM agc rectification.) I think that the usual way to use a triode as a diode was to have the grid as diode anode, and ground the actual anode.


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Old 7th Jan 2017, 3:10 am   #18
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Default Re: Valves in commercial applications for which they were not specifically designed.

My ultrasonic cleaner uses a 6DQ6B line output from the US as a self rectifying power oscillator. The cleaner comes from the US also so they would indeed supply the valve and call it a tube too.
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Old 7th Jan 2017, 1:23 pm   #19
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Default Re: Valves in commercial applications for which they were not specifically designed.

The Pye "FM/AM Piper" (P117) uses an ECC85 as an AM frequency changer when the set is in this mode. It works reasonably well.

This set is a very economically designed 3 band AM/FM receiver - ECC85 EBF89 ECL82 and EZ80 wired as a half-wave rectifier, with Ge diodes for FM demodulation. It has a live chassis and a heater transformer - an excellent arrangement for a cheap set.

Some German sets were designed with similar configurations, but all suffer from "hot switching" in the signal circuits to re-arrange the limited number of active devices. The Pye also uses reflexing in the ECC85.

It must have been a really difficult set to build and align - they never did it again.

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Old 7th Jan 2017, 1:42 pm   #20
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Default Re: Valves in commercial applications for which they were not specifically designed.

I've never looked closely at the P117 design, probably having assumed it was a VHF/FM receiver only (based on the valve line up). It would be interesting to know what sort of performance it gave and how much the AM performance dropped off as the ECC85 aged.
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