The ARP-3 Pentode
It is interesting how we sometimes miss little details when we plunge into trying to understand bigger issues. It also doesn’t help if we find ourselves juggling several bigger issues at the same time.
Since the Wireless Set No. 52 evolved out of a British design, and the British based valves were retained in that process for the wireless set, a means to test those valves becomes important, and hence the existence of this Stark designed British Valve Adapter which could be used with North American tube testers.
The ARP-3 pentode valve is by far the commenest in the 52-Set and the interesting thing about the manufacture of these valves in Canada during the war is that all of them are marked “ARP-3 EQUIVALENT”. The wording does not seem that significant at first glance, but it screams volumes when you take the time to think about it.
The original British Military ARP-3 is nothing more than a direct copy of the prewar commercial 9D2 pentode valve. There was nothing preventing a company in Canada from getting a licence to produce said valve in Canada. In which case, all those Canadian valves would have simply been marked “ARP-3”. However, tooling up for production would have taken time and money to accomplish. Time being the important factor at the start of WW2.
So something in the list of available pentode valves already in existence in North America must have been a close enough match to work as an ‘equivalent’ to the ARP-3, thereby saving a lot of time getting production up to speed.
As this adapter project came closer to completion, I started looking to see what the history of the ARP-3 was, which was tricky. Lots of data is on the web about the ARP-3 but this data and photos are nearly always of the Canadian Marconi ‘equivalents’. This last weekend, I finally ran across an early wartime Brimar publication and in it found the technical details for the commercial 9D2 valve the ARP-3 was based on. These specs matched the Canadian Marconi ARP-3 equivalent perfectly. In very small print at the end of the article it said. “For further description see the 6K7G.” That was an eye opener! The 6K7G is part of the tube set in the Wireless Set No.19.
When I pulled out my RCA tube guide for North American tubes, the data for the 6K7G was indeed a match for the 9D2, except for the Heater/Filament Voltage. The 6K7G was rated for only 6.3 Volts. The 9D2 and Marconi ARP-3 Equivalent were both 12.6 Volts. So then I checked the 12K7G pentode and there it was. Same specs AND the 12.6 Volt Heater/Filament rating. I then carefully traced out the octal pin arrangement for the 12K7G tube through the British Valve Adapter wiring diagram I had just worked on and arrived at the correct pin arrangement for the B7 British Valve Base. So now i know when the adapter is connected to my tube tester with the Marconi ARP-3 valve in place, I just have to set the tube tester to read a 12K7G tube and I am in business.
I was so pleased with this discovery, I sent an email to a friend in Quebec to tell him about it. His reply was, “Did I not tell you that last Summer in an email?”
After several minutes review of old emails on the computer, there it was. He had! But I had lost that little tidbit while wrapping my head around a bigger problem we had been working on.
Silly me.
David
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