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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 13th Jan 2017, 7:40 pm   #41
turretslug
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Default Re: Valves in commercial applications for which they were not specifically designed.

Cg1-a will be low (the valve data may claim hundredths of a pF) but not always negligible in a high-gain tuned amplifier, and compromised by layout. Cg3-a will be somewhat higher- hence the mere single turn on the IFT, as mentioned it would be interesting to know if the phasing is degenerative or regenerative in action. Even the actual value of g2's decoupling capacitor can sometimes enter into the situation of IF strip stabilisation- some circuits used a specified value as part of a neutralising bridge, though seemingly not here.

Last edited by turretslug; 13th Jan 2017 at 7:48 pm. Reason: Rephrasing.
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Old 13th Jan 2017, 8:31 pm   #42
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Default Re: Valves in commercial applications for which they were not specifically designed.

Could it be this? When the grid tank is at resonance then that just leaves the R component of that tank circuit, Cg3 to Ca couples the anodes tank all be it weakly to the grid tank via that extra winding, if the anode tank is detuned upwards from resonance slightly the anodes tank will become inductive, thus a certain amount of regeneration will occur.

Lawrence.
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Old 13th Jan 2017, 11:04 pm   #43
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Default Re: Valves in commercial applications for which they were not specifically designed.

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as mentioned it would be interesting to know if the phasing is degenerative or regenerative in action
I acquired this receiver in 1974 as a teenager, but unfortunately when I got it this coil had been disconnected and both the wires were black and the same length near where they connected, so I did not know what the correct polarity was. Also I did not have a sweep generator to sweep the IF while trying the coil both ways around to see how it affected the band-pass response. But at that stage I would not have known how to do that properly anyway.

I took the chassis to John Stokes (Author of 70 Years of Radio Tubes and Valves) because it had another problem I couldn't fix. He had been mentoring me and helping me learn to repair valve radios & TV's in the early '70's. His shop was in Dominion Rd Auckland. He was the valve radio authority in New Zealand at the time. He remarked that he had not seen this arrangement before in any sets other than Philco, but he was not sure why they did it and he wondered about avoiding some issues of licencing & patents. So this coil and its purpose had always remained a mystery.
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Old 14th Jan 2017, 9:13 pm   #44
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Default Re: Valves in commercial applications for which they were not specifically designed.

Probably if the alignment details were available, some idea of the purpose of that winding could be gleaned. Tony
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Old 14th Jan 2017, 9:19 pm   #45
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Default Re: Valves in commercial applications for which they were not specifically designed.

I looked in the full Rider's sheets earlier on today, what I saw just said tweek all IF's for max at 470kHz, no full circuit description was given.

Lawrence
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Old 14th Jan 2017, 9:52 pm   #46
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Default Re: Valves in commercial applications for which they were not specifically designed.

From memory, this was a fudge to allow the manufacturer to describe it as a "triple-grid amplifier".

Given the way radio patents were litigated-over in the 1930s I suspect it was also a 'dodge' to try and avoid needing to pay royalties to the likes of Hazeltine Corporation!
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Old 16th Jan 2017, 2:00 am   #47
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Default Re: Valves in commercial applications for which they were not specifically designed.

Another example may be the use of the ECC189 as a cascode (1st) RF amplifier in HF receivers.

The ECC189 data sheet describes it thus:

“Double triode with variable transconductance intended for use as V.H.F. cascode amplifier in television receivers.”

Thus its use at HF, and indeed down towards LF, was, strictly speaking, outside of this stated intention. On the other hand, that use was a relatively mild extrapolation, and not a step sideways.

Eddystone used the ECC189 as a cascode 1st RF amplifier in the 880/2 (but not I think the original 880), then in the 830 series and the 940. And as far as I know, Racal updated the RA17 RF amplifier to use an ECC189 at some stage in its production run.

Interesting is that it was the ECC189 iteration of the cascode double triode evolution that was used at HF, and not the earlier editions such as the ECC84. Possibly the HF receiver designers did not think of using a cascode until later on, by which time the ECC189 had arrived. Or perhaps (by happenstance?) the ECC189 was the first VHF cascode valve that offered worthwhile benefits as the 1st RF amplifier in an HF receiver.


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Old 16th Jan 2017, 10:25 am   #48
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Default Re: Valves in commercial applications for which they were not specifically designed.

Would the variable transconductance have been a feature that encouraged its use at HF over other cascode-oriented types? The HF bands could be both very crowded and with a wide range of signal strengths, maybe something that at least could be subject to the AGC circuit and/or manual gain control appealed to HF receiver designers.

Though connection of valves with "straight" characteristics and varying degrees of variable mu-ness to external control influence is a subject in itself....
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Old 16th Jan 2017, 12:03 pm   #49
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Default Re: Valves in commercial applications for which they were not specifically designed.

For an HF receiver in an RF stage, I would most definitely want as linear an amplifier as I could get. Variable transconductance as a feature would make my search for a device quickly move onwards.

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Old 16th Jan 2017, 12:46 pm   #50
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Default Re: Valves in commercial applications for which they were not specifically designed.

That's an interesting perspective- at one point post-war, there was a move among some designers to use a "straight" pentode as the first stage of an HF receiver, Murphy and Dynatron (and surely others) produced sets that followed this idea. I thought that the conventional wisdom was that a straight type offered a better noise performance than a variable-mu type, though I was never very convinced by this argument as it seemed to me that connecting even a fairly modest aerial would result in a noticeable noise increase all the way to 30MHz over a resistor-terminated aerial input in any set I tried. Perhaps with a top-coupled whip aerial, the first-stage noise would be significant but a half-decent radio deserves better anyway.

Adding to the "valves in unaccustomed places" list- the EL91 used as LO in the GEC BRT400 (originally touted as a small audio PA) and the 5763 used as AF output in the Eddystone 910 (originally touted as small RF PA....). I quite like to see this sort of thing, at least it shows that the designer looked for what could do the job based on technical data, rather than marketing headline. In the case of the 910, this set was evidently derived from the 750, which used an N78 as AF output- perhaps this B7g type had already acquired a negative reputation for hot-running and the designer wished to steer away from both it and the similarly-afflicted EL84. The 6CH6, supposedly a video amplifier-orientated valve, also found itself used as AF output in the Pye CAT- perhaps a similar desire to avoid the EL84.
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Old 16th Jan 2017, 1:11 pm   #51
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Default Re: Valves in commercial applications for which they were not specifically designed.

There was an early KB FM receiver which used the 6CH6 as an audio output valve. I assume that this was a means of obtaining "EL84 performance" without purchasing anything from Philips. The 6CH6 would have been obtainable in-house.

Looking at a Brimar price list dated "post October budget 1955" the 6CH6 weighs in at 20/- plus 7/10 purchase tax, whilst the EL84 is 13/- + 5/1. Of course, the parts prices bear no relation to OE cost.

I find the 6CH6 to be superior to the EL84 in all respects, particularly reliability, and have often used it to good effect in EL84 applications. A minor base re-wire and a bias change (shunt the existing resistor to give 100 Ohms) are all that is needed.

Leon.
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Old 16th Jan 2017, 1:32 pm   #52
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Default Re: Valves in commercial applications for which they were not specifically designed.

"Variable transconductance" is just another way of saying "non linear". Not what you want anywhere there may be strong unwanted signals about causing intermodulation sprogs.
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Old 16th Jan 2017, 1:40 pm   #53
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Default Re: Valves in commercial applications for which they were not specifically designed.

It could be argued that an ECC189 cascode has far too high performance (in gain terms) for an HF receiver RF stage. It won't improve noise figure by much, because that is largely set by the first tuned circuit. An ECC189 will have poor strong signal performance. Maybe OK for a receiver intended for poor antennas. I suspect specmanship.

If you want a variable-mu cascode for HF, how about a triode-connected EF92 (or EF89) feeding into almost any reasonable triode above it? You get the good signal handling of the pentode, but with less noise.

To get a rough idea of the grid signal which a valve can handle without too much distortion, simply divide the transconductance into the anode current.
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Old 16th Jan 2017, 2:42 pm   #54
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Default Re: Valves in commercial applications for which they were not specifically designed.

Reading around various sources, there did seem to be something of a race-to-the-asylum as far as fitting/retrofitting allegedly high performance/low noise valves to HF front ends was concerned. Could it be that valve development by the end of WW2, say, had more or less optimised the noise performance achievable in practical/real-world environment terms but marketing pressure and the urge to "improve" meant that the new and enticing VHF-orientated valves got used in the urge to best the competition, at least in advertising terms? It makes me think of the marketing race, a few decades later, to fit GaAsFETs to FM tuners and shout the fact on the front panel and packing carton- even if this was actually detrimental to performance compared to established technology which had similarly ceased to offer much in the way of revolutionary year-on-year progress to excite the reviewers.
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Old 16th Jan 2017, 3:56 pm   #55
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Default Re: Valves in commercial applications for which they were not specifically designed.

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Originally Posted by G8HQP Dave View Post
It could be argued that an ECC189 cascode has far too high performance (in gain terms) for an HF receiver RF stage. It won't improve noise figure by much, because that is largely set by the first tuned circuit. An ECC189 will have poor strong signal performance. Maybe OK for a receiver intended for poor antennas. I suspect specmanship..
I vaguely remember a Pat Hawker article in which he said that the use of cascode front-ends in receivers intended for Navy use was in part due to their better strong-signal characteristics - he pointed out that being on board ship applies some practical constraints to the extent to which you can physically separate transmitter and receiver antennas(!) and that as a result a shipborne receiver front-end may need to contend with the concurrent arrival of several Volts of locally generated RF on different frequencies at its antenna terminals at the same time.

Push-pull neutralised grounded-grid triodes were sometimes used in this sort of application - using relatively high-power valves (6C4) running plenty of milliamps of anode-current. The valves were expected to provide current-gain (not voltage-gain: that was obtained by the step-up ratio of the tuned-circuits). The same idea was continued into the semiconductor era where you'd often find grounded-gate JFETs passing 20 to 50mA to get the strong-signal-handling needed.
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Old 16th Jan 2017, 6:22 pm   #56
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Default Re: Valves in commercial applications for which they were not specifically designed.

Re. the above, I gather that there was an outboard preselector made for RA17s in naval service that had possibly as many as 6(?) tuned sections with a low-gain but relatively high current ECC82 cascode stage in the midst.

Saved from being too off-topic by the ECC82's original presentation as a line-level audio amp.
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Old 16th Jan 2017, 6:36 pm   #57
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Default Re: Valves in commercial applications for which they were not specifically designed.

Yes, the Navy receiver mentioned by Pat Hawker used ECC82 cascode and ECC82 mixer so quite the opposite of using an ECC189. Probably similar anode currents, but ECC189 gm is 12.5mA/V while ECC82 is around 2mA/V. Other things being equal that means 6 times better signal handling for the ECC82. Maybe that Navy receiver qualifies for unusual use of a valve too, although the ECC82 is actually a pair of EC90/6C4 so meant for handling some power.
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Old 16th Jan 2017, 6:48 pm   #58
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Default Re: Valves in commercial applications for which they were not specifically designed.

Pat quoted that navy receiver as meeting linearity requirements with volts of input signal. I remembered that and the ECC82 mixer. The ECC81 was supposed to be the RF beastie in the trio ECC82 general purpose and ECC83 low noise audio.

On the subject of mixers, the RF front end mixer of an HF RX I can't remember the maker of (SSR-1?) was the 7360 beam deflector, which was designed to be a colour TV NTSC decoder demodulator.


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Old 16th Jan 2017, 7:07 pm   #59
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Default Re: Valves in commercial applications for which they were not specifically designed.

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On the subject of mixers, the RF front end mixer of an HF RX I can't remember the maker of (SSR-1?) was the 7360 beam deflector, which was designed to be a colour TV NTSC decoder demodulator.
In the US 7360 was at one time 'the thing' to use [without any pre-mixer gain] in serious amateur-band HF receivers.

Remember that quite a big slice of .US allowed Kilowatt amateur stations and somewhere densely populated with hams like California could mean your receiver was within ground-wave blasting distance of plenty of murderous Kilowatt-plus-8-ele-beam-on-20M stations that came in at S9+60dB - which was a real pain when you were after a S2 DX signal from some guy in Central Africa who was running 20W to a dipole.

Times moved on and solid-state arrived: the "Atlas 210/215" radios switched to double-balanced-diode mixers and lots of LO drive-power to achieve the same result.

http://atlas.wireless.org.uk/

http://www.qsl.net/on6ab/History.html
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Old 16th Jan 2017, 7:17 pm   #60
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Default Re: Valves in commercial applications for which they were not specifically designed.

The British 7360 rx was the G3PDM design, but that doesn't meet the 'commercial' bit in the OP. There was a US one, general coverage, commercial.

It was an interesting time as mankind learned that there were things other than noise floor to work on for HF reception.

Yup, I've experienced California Kilowatts. The 'Bascom Bandit's' shop in Sunnyvale was a good place to try out HF radios you wondered about buying! HRO in Burlinghame was in a slightly quieter location (they've moved since then)

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