New find McMichael Mains Dimic Three

[RadioHist]

So what are the chances Some years ago I wrote an article for the BVWS Bulletin on restoration of a McMichael Screened Dimic 3 (battery powered) found here in the USA maybe 20 years ago. It was a heroic restoration project for me that brought me great satisfaction..... AND just two days ago at the vintage radio and phonograph 2 1/2 day meeting in Charlotte, NC, USA a dealer came looking for me in the flea market..... "I have something I think you will find interesting...." Yes indeed! The AC mains version of the Screened Dimic 3! I might just be the best informed person here in the USA to restore and conserve this specific radio! What are the chances? First Question... My battery version has a back wooden cover. Does anyone have a photo of the back cover for this Mains version? Seems like there should be one although it is not obvious to me how it would fit in place.
Know of any vintage reports/pictures/adverts on the mains version? All I have found is a brief mention in WW for 19 Sept 1928

Thanks, Robert

[cathoderay57]

Hi Robert, looks like a lucky find. I had a look through my McMichael data but only found an advert for the battery version. I've attached it for interest. You might want to try contacting David Cochrane; he used to be a Forum member but is now listed as "retired - dormant" so I don't know if he is still around. His contact details are still on the McMichael Museum website here:-

https://www.mcmichael.org.uk/contact.htm.

The website dates the Mains Screened Dimic Three to 1929. Jerry

[RadioHist]

I've brought the Mains Dimic 3 into the shop and pulled the valves... The Mullard PM 12 is something I have in my Battery Dimic 3.... But Oh Nellie! the two Cosmos Short-Path A C Red Dot valves are truly bizarre to my American eyes... And fascinating....

This receiver has the proper valve holders on the chassis. I.e. It does not use the adapter wafer to make filament connections...

I note that the PM 12 has crinkled silver getter on the bulb.... Does that indicate it has 'gone to air'?

It will probably be a few more days before I can pull the power supply box and have a look. We never used the Westinghouse copper oxide HT rectifiers in broadcast receivers here. Will be interesting to see. My McMichael Twin Supervox uses something similar.

Seems like I have a rare bird here eh?
Robert

[RadioHist]

Originally Posted by cathoderay57

Hi Robert, looks like a lucky find. I had a look through my McMichael data but only found an advert for the battery version. I've attached it for interest. You might want to try contacting David Cochrane; he used to be a Forum member but is now listed as "retired - dormant" so I don't know if he is still around. His contact details are still on the McMichael Museum website here https://www.mcmichael.org.uk/contact.htm. The website dates the Mains Screened Dimic Three to 1929. Jerry

I'll send a query. Some years ago I had no luck in getting info on my Twin Supervox. I did find this evening the Feb 19th, 1930 issue of Wireless World with a two page description of my set.... No clues as to what the back of this wireless cabinet may have looked like.

[cathoderay57]

Info on Twin Supervox here, in case you didn't find it previously: https://www.service-data.com/search....&search=Search Jerry

[Sideband]

Some sets didn't have back covers. Would this be one of them?

[HamishBoxer]

I think the PM12 will be fine, normally white when gone to air.

[Gridiron]

Interesting to find out how the filament of the PM12 was powered, as it is a 2 volt accumulator type. It would need a well smoothed DC supply to prevent mains hum. I think that large value low voltage electrolytic caps were becoming available around this time, as used in some Ekco eliminators.

[RadioHist]

Originally Posted by cathoderay57

Info on Twin Supervox here, in case you didn't find it previously: https://www.service-data.com/search....&search=Search Jerry

Yes, thanks for the offer... I did collect the Supervox schematic from the BVWS CD of some years ago.... I'll have to order the rest won't I

I cannot believe I failed to include the schematic in my extensive write-ups on my restoration...... I guess I can blame it on not having folks to proof read my work....

(It's Hell not being Superman sometimes, eh? )
Robert

[RadioHist]

Originally Posted by Sideband

Some sets didn't have back covers. Would this be one of them?

Ah, I have part of an answer from the Wireless World of 19 Feb 1930....
On review, it is showing us that there is a wood framed panel behind the receiver chassis. So was there a same style panel behind the Mains power supply? Maybe not...

The tiny mains cable is missing a plug..... Now I see why.... I think I see that the original was a mains plug that mimics the British bayonet lamp base.

Are such plugs showing up in collector junk boxes? It would be great to have a proper example. (I have no intention of ever powering this set.)

[RadioHist]

Some few more comments on my new find...

Trying to find more info on the Cosmos Short-Path Red Dot AC valves...
This valve seems to be originally for converting a battery powered receiver to mains operation.... But this Dimic 3 does indeed have the proper sockets to receive the two short filament pins without resorting to an accessory connecting wafer.

On the Dutch site I found an on-line copy of Keith Thrower's detailed history of British valves and found that he has extensive coverage of Cossor but I found no discussion or even mention about these valves.

What a curious construction!

[RadioHist]

New Years Eve 1928.....

On examining the mains supply unit prior to cleaning, I found this bit of paper intentionally placed 94+ years ago. The paper bit just peeking out from under the two mains filter condensers. It is NOT a note that just fell into place. The only way to extract it was to remove the fixing screws for the condensers and steer around one of the mounting feet for the transformer.

The address of Mr. G B Tabor had to be within walking distance of the factory...
What do you make of this

Robert

[RadioHist]

Originally Posted by Gridiron

Interesting to find out how the filament of the PM12 was powered, as it is a 2 volt accumulator type. It would need a well smoothed DC supply to prevent mains hum. I think that large value low voltage electrolytic caps were becoming available around this time, as used in some Ekco eliminators.

Just have a center-tap resistor and a bit of filtering. See schematic.

[Mr Moose]

Hello,

The mention of Cossor is simply wrong.

The valves are Cosmos valves made by Metro-Vic (Metropolitan Vickers Co), hence the MV logo on the valves.

Some details here. The Red Spot is an AC/R and the Green Spot is an AC/G.

http://www.r-type.org/adverts/adv-0170.htm
http://www.r-type.org/adverts/adv-0726.htm
http://www.r-type.org/exhib/aby0081.htm
http://www.r-type.org/exhib/aaj0297.htm
http://www.r-type.org/exhib/aaj0176.htm

(In the above data the values of ra and gm are given as the same for the AC/G and the AC/R. The figures for the AC/R should be ra=2.5K and gm=4mA/V.)

(You say you have two AC/R valves but the AC/R is an output triode wheras the AC/G is a LF/Detector triode so I wonder if originally it had an AC/G and an AC/R.)

Yours, Richard

[Mr Moose]

Hello,
Apparently "The Doughboys" was a nickname for American soldiers in the First World War.

Perhaps it suggests that the builders of the power supply knew the set was going to America.

Yours, Richard

[Graham G3ZVT]

Horlicks!

I'm not cursing Robert, honest. Slough has been significantly redeveloped, but Stoke Gardens leads to the back of the Horlicks factory on Stoke Poges Lane.
The original factory was built on the same site in 1909.

Horlicks is a malted milk drink. It even had an "MRE" form in the Great War



Of course I could be off the mark, but I really want this time capsule you’ve discovered to be connected to this iconic British brand.

[Graham G3ZVT]

Originally Posted by RadioHist

The tiny mains cable is missing a plug..... Now I see why.... I think I see that the original was a mains plug that mimics the British bayonet lamp base.

Are such plugs showing up in collector junk boxes? It would be great to have a proper example. (I have no intention of ever powering this set.)

Oh yes plugging a radio or even an electric steam-iron into a lighting pendent is part of our heritage!

They are still made.

https://www.amazon.co.uk/Pack-Socket...92&sr=8-7&th=1

The last time I used one in anger was in the '70s for my photographic equipment. My darkroom was in the utility meter cupboard which ironically didn’t have an outlet, only a light.

[Nickthedentist]

I love little bits of secret history like that!

Possibly George Burt Tabor, 1895-1970? (data from the FreeBMD website)

[RadioHist]

Originally Posted by Mr Moose

Hello,

The mention of Cossor is simply wrong.

The valves are Cosmos valves made by Metro-Vic (Metropolitan Vickers Co), hence the MV logo on the valves.

Some details here. The Red Spot is an AC/R and the Green Spot is an AC/G.

http://www.r-type.org/adverts/adv-0170.htm
http://www.r-type.org/adverts/adv-0726.htm
http://www.r-type.org/exhib/aby0081.htm
http://www.r-type.org/exhib/aaj0297.htm
http://www.r-type.org/exhib/aaj0176.htm

(In the above data the values of ra and gm are given as the same for the AC/G and the AC/R. The figures for the AC/R should be ra=2.5K and gm=4mA/V.)

(You say you have two AC/R valves but the AC/R is an output triode wheras the AC/G is a LF/Detector triode so I wonder if originally it had an AC/G and an AC/R.)

Yours, Richard

Your response has certainly highlighted my need to a proof reader, eh
Cossor and Cosmos are names I just can't keep straight in my brain. Thanks for the correction....

I note that the two "Cosmos" valves found in my set are obviously from two different production runs since only one has a top evacuation tit. But both are clearly marked with a single Red Spot. I'm guessing that switching the valves did not make much of a difference for moderate volume as long as you selected a proper bias? (Bias is derived from a multi-tap dry cell.) I don't know how much difference substituting The AC/R for the AC/G would make also.... I'm just thrilled I have the valves I have at all.... Talk about pure 'unobtanium' over here!

Robert

[RadioHist]

Originally Posted by rambo1152

Oh yes plugging a radio or even an electric steam-iron into a lighting pendent is part of our heritage!

They are still made.

https://www.amazon.co.uk/Pack-Socket...92&sr=8-7&th=1

The last time I used one in anger was in the '70s for my photographic equipment. My darkroom was in the utility meter cupboard which ironically didn’t have an outlet, only a light.

I'd love to have one of the modern plugs, but your link turns out to an item out of stock. No doubt the 'Safety Police' are out to quash ungrounded mains connections regardless of historical accuracy