Steve
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Post by Steve on May 22, 2022 18:38:14 GMT 12
I am hoping to get some input from people smarter than me so that I can have a more rounded technical description of the model in the Radio Corp book (I've started laying it out - its a long way from finished but at least its begun!). Obviously this model, being the genesis of the brand, is important. But its also very odd. And it predates my area of knowledge so I'm hoping others will chime in with some info. I've written up a reasonable amount on it - but I just want to get a better feel for the technical quirks It was released in early 1930 by W. Marks Ltd (before he changed the company name to Radio Corporation (N.Z.) Ltd) and sold through the master agents, Stewart Hardware. There is no factory schematic. Bob Hatton did a deep dive into his one in 1994, and this is the schematic he drew up And here is the power supply module So, things I want to understand better. 1. The speaker output - its straight off the output valve. My understanding is that sets like this got built with a choke-fed plate on the output and a series capacitor with the speaker. I know this is a budget set but shouldn't it have that? We've got 100V B+ on one of the speaker terminals the way I see it... 2. 12A's in 1930 - out of date? Budget valve option perhaps? Or were they still valid and current in 1930? I'm guessing he got a bulk deal. 3. 12A as a RECTIFIER?! Genius or madness? The option of using a proper rectifier would have required a significant amount more current, and a bigger and more expensive transformer. The 12A only draws 0.25A, but I can't imagine it delivers much current when used this way... is it a case of 'just enough'? 4. The elephant in the room - the diode. Its a crystal set with a 2-valve amp. Here is my take, having listened to anode-bend detected early 30's sets. This would (should?) sound better than a valve only model? A lot of the advertising plays on this notion. I believe this is a genius entry-level set for the joe average public to get their first radio, at a time when most people still didn't own one. It was 10 guineas and cheaper than most other sets. Thoughts? Questions? Concerns?
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Post by Peter Walsham on May 23, 2022 9:32:14 GMT 12
Good morning Steve I do have some thoughts about this, but they might not be all that accurate. The 12A valves were released in the 1920's as being suitable for audio use (as in this set) as low mu valves, I am guessing that William got these at a 'knock down' price, so used them. He may well have specifically designed this set around those valves. William was a electricity revenue meter manufacturer, and may not have had a lot of radio design skills or experience to design a more complicated set?. I think his use of a 12A as a rectifier was sheer genius. Only one valve type was needed to be bought when the 'radio' stopped working. His set was very simple to operate - only having one control. My thoughts regarding the direct connection of the speaker to the output stage go along the lines that the radio was intended to be connected to a high impedance speaker - possibly not much more complicated than a moving diaphragm headphone coupled to a horn. These (probably) had some sort of a vernier type of adjustment to keep the diaphragm away from the pole pieces so 'loud' signals wouldn't make the diaphragm hit the pole pieces & distort the sound. This 'vernier' control may have also been quite effective as a volume control. As we know, by using an output choke & coupling capacitor to a speaker would prevent DC passing through the speaker coil thus removing any bias on the diaphragm, but William may have wanted to keep the cost of the receiver down - or he was unaware of the desire to keep DC out of the speaker. One of the disadvantages of passing DC through a moving diaphragm (or reed) type of speaker is the DC tends to de-magnetise the magnet if the speaker is connected around the wrong way. A reference suggests that moving coil speakers became available in the early 1930's. Somehow, this suggests that William was either trying to market a cheap radio (as you say this set was priced at 10 guineas), or he lacked knowledge or technical skills to develop a more advanced design? Thank goodness his later radios were well built, unlike some of his contempories! (References: 12A Valves: en.wikipedia.org/wiki/List_of_vacuum_tubes, Speakers: en.wikipedia.org/wiki/Moving_iron_speaker)
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Post by Richard on May 23, 2022 9:51:20 GMT 12
I wonder what the diode was made of physically? a cats whisker ?
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Steve
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Post by Steve on May 23, 2022 13:44:55 GMT 12
Richard, I think they are a spring-loaded point onto a piece of carborundum by that stage - a 'point-contact' fixed diode. These were a lot more robust than a catswhisker.
Peter - thanks for your thoughts - I'm glad I'm not the only one who thought the rectifier 12A was a genius move for a small set with minimal current requirements.
Just a slight correction on William Marks background - he was trained as an electrical engineer and passed with honours from a facility that specialised in electrical and radio. In fact, first the physics dept, then the entire institute was run by Alexander Popov, who (depending who you ask) was the first or one of the first physicists in the world to demonstrate working radio. The meter repairs he did as part of his job at the Wellington City Council, because he could.
Now that doesn't mean he was an accomplished radio engineer - I believe he probably wasn't. But he was smart enough to realise his skills lay in management and he employed some very smart cookies to do the design work as soon as he could afford it.
Regarding the speaker - were they high impedance units in 1929/30? This is completely outside my knowledge zone at the moment... and I haven't found anything useful about impedances for them so far. I definitely agree it was a budget set - probably to maximise profit as well as minimise sales cost.
My best guess is that he never intended it to be their only model - there were models advertised by November of 1930 of up to nearly 73 pounds in value - I think it was the Bell Colt of its day, a loss-leader in sales parlance. Get them in the door - get the brand known. Talk them up to the fancy one. Even if you bought the cheap one, the brand name was large and bold and front-and-centre on the top of the unit - you couldn't miss it.
Cheers, Steve
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Post by Peter Walsham on May 23, 2022 15:31:20 GMT 12
I probably should have researched William's background more, before I wrote that comment. I was relying on my memory, more than anything, in my haste to write that up, as I was having a morning coffee. I suspect that he may have been working on a limited/tight budget, and built up capital from sales of the cheap 'Model No: 1' radio to enable improvements to later designs to be implemented. I wonder whether he had to 'undercut' competitors in order to convince Stewart Hardware to sell his radios against those competitors?
I can only assume that the early speakers were high impedance. I don't have any examples to make tests from.
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Post by DHockey on May 23, 2022 15:44:14 GMT 12
Hi Steve High impedance horn and cone speakers were still widely available in the late 20's and early 30's. Here is a page from the April 1930 John's catalogue on the NZVRS website, they claim that horn speakers are "still the best" for smaller radios: The Wellmayde Ace has a similar audio stage to the Courtenay, the speaker terminals are "hot" with B+ I guess the attitude back then was "don't touch them". In the same John's catalogue the Ace is supplied new with a N&K horn or cone speaker.
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Post by Peter Walsham on May 23, 2022 16:07:38 GMT 12
It's very likely that the cone speakers that were available at the time were high impedance 'reed' types. Moving coil speakers appear to have been available in the late 20's/early 30's, but they had electro magnets (to get sufficient magnetic 'power', as permanent magnets of the day were not efficient enough). However, it seems like a horseshoe type of magnet appeared to be OK to use in a reed speaker. Electro magnets were too power hungry to energise using batteries. Horn speakers 'loaded' the earpiece quite well, and amplified the volume enough to be used in a typical lounge.
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Steve
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Post by Steve on May 23, 2022 19:28:33 GMT 12
I just found another Stewart Hardware advert for the model 1 from late 1931, so it was still being sold after probably 18 months on the market although at just over 6 pounds, with a speaker... So it was obviously being cleared at that point. It was being sold with an RCA 103 speaker - which I think has a 230-ohm voice coil, so not exactly high impedance. It says in the RCA 103 service manual to use a choke and cap, or a 1:1 isolating transformer when the plate current is over 10mA or damage to the speaker could occur. At that price I'd buy a couple of them...
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Post by Peter Walsham on May 23, 2022 20:20:29 GMT 12
Steve: I note that it appears as though the DC resistance of the RCA speaker is about 230 Ohms (see: www.antiqueradios.org/gazette/rca103.htm, servicing the loudspeaker, step 4, using a 'VOM (multimeter)). The impedance (AC 'resistance') of the speaker will be much higher. Much like the DC resistance of an output transformer (one I have just tested) measures 340 Ohms DC, but has a rated impedance of 7,000 Ohms. Interesting to note the large price difference between the 3 valve set with RCA speaker & the 3 valve set with a dynamic speaker! I think reinforces the cost of these things 'back in the day'.
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Steve
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Post by Steve on May 23, 2022 20:52:50 GMT 12
Thanks for that link Peter, and the note about resistance vs. impedance... I was thinking more of the DC current when I mentioned it but I'm surprised the impedance was that high. And I note this bit from the page: "With Radiotrons UX-112-A and UX-120, an output device is desirable though not absolutely necessary" - so the Courtenay would have suited this speaker with or without
I find it interesting that they decided to stick with the 10 guinea price for their entry-level set, once they moved on to the table model. I wish there were more examples of these early sets around. I know of one 4-valve set, although I don't know where it is... and one 5-valve set that looks the same, which is in an NZ based collection. Year unknown, no serial number that I could see. I wonder how they tracked those early sets - the serial numbers I have seen seem to start on the 5A, 5B, 5C series of radios which I believe are 1932/33 - and these seem to start at around 2000 - maybe they just estimated 2000 sets sold before that and decided to start about there? Does anyone have any Courtenay set with a lower serial number?
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Post by Peter Walsham on May 23, 2022 21:42:32 GMT 12
The company may have ended up with surplus or unsold stocks of the 3 valve sets perhaps? I don't have enough old sales stuff here to research, however, a 1933 copy of 'The Radio Constructors Guide' I have describes a 3 stage TRF & even an Autodyne SuperHeterodyne set to build, so amplified crystal sets would have been 'really old tech' well by then, by the looks of it. Somehow, my gut feeling suggests that William needed to get rid of those sets, so marketed them at fire sale prices. Unfortunately, I don't have anything as old as that in my collection.
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Steve
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Post by Steve on May 23, 2022 23:12:57 GMT 12
Hi Peter, they actually had a big auction in 1933 and sold a lot of stuff off before they moved from Cornhill St into their new and final home at 80-84 Courtenay Place. There may have been a few in that auction.
When Mac McDonald died (the owner of Moderne) they had a big sale that went for days (weeks?) at his place as he was apparently a packrat and couldn't pass up a bargain over his entire lifetime - there was a lot of stuff there, and some Courtenay buffs found a stack of model 1 bases in amongst everything. I can only assume he attended that 1933 auction and bought them - although I can't be certain of that as I don't know where Marks got them in the first place - or if they were for something else and he re-purposed them.
The NZ Radio Listeners Guide from 1928-29 has a similar-ish design (crystal and 2 valve amp), although for battery use. The Courtenay is more compact and has one less control though.
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Post by Peter Walsham on May 24, 2022 8:39:34 GMT 12
This has turned out to be a very interesting discussion
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6A8G.
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Post by 6A8G. on Jun 19, 2022 8:58:51 GMT 12
Re. point three -a triode as a rectifier... my Heathkit C3 condenser checker uses just such a valve. The rectifier is a 1626 audio triode. A couple of other C3 owners reckon Heathkit got a "job lot" of these, so to keep costs down they were used as the rectifier.
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Steve
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Post by Steve on Jun 25, 2022 11:26:09 GMT 12
Thanks for pointing that out, It's good to know it is not entirely unprecedented to do this - I wonder if anyone had done it before Courtenay on a commercial set? It seems like there were rectifier valves around long before this set. The 213 came out in 1925, and was replaced by the 280 (80) in 1927 - but these had big (and in this set, unrequired) specs, and big current draw. And David - sounds like the voice of experience on that output wiring
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