To accommodate different tube configurations, you may
need to modify your supply of sockets. This crafty veteran shows you
how.
By C. Wolf
One of the real pleasures of producing your own audio gear is setting it up the way you choose. Too many over-the counter sound systems look great from the outside, but are crammed full of cheap hardware and plastic. It makes you wonder what other sacrifices they made to lower the price of the units.
Custom-built equipment often includes a variety of parts that may be overkill for their actual application, but so what? Take bayonet sockets, for example. These were originally used to keep tubes in close contact with their mounts, and to make them stay put.
Transmitting cabinets or panel mounting also come to mind as practical uses for bayonet sockets. Another application is with a tube that's going to spend a bunch of time bumping and crunching around in a cockpit or Jeep.
There's no other way of mounting a 211, but bayonet sockets are usually overkill for typical 811, 45, and 300B type tubes. Like I said, some folks dig overkill parts. When you're building your own system, why not use them?
PHOTO 1: WE 300 B-style socket after finishing and plating.
OVERKILL BLUES
Well, I had the overkill socket blues the other day. Let me preface this sad story by confirming the fact that not all 300Bs out there are pretty. I have a couple with downright scuffed logos, which would look just nifty hidden in a nice bayonet socket.
My collection of bayonet sockets work just fine when I am using them on virtually any 4D-based triode in the Known Universe. Unfortunately, Western Electric threw us a curve when they modified the 300A by rotating the pin 45°, and thus created the 300B. Bernard Mager's book [1] says this was to accommodate the 300B in old 205D-equipped amplifiers. These are great triodes-but it's impossible to find matching bayonet sockets.
"Not so," said the hired help at a couple of tube houses I contacted. "The 300Bs that we have fit the old E.F. John sons and the new Chinese versions just fine." I wondered whether they were using an original 300B, but my buddy Alex at Darcell Electronics straightened me out by pulling out a set of new-production WE 300Bs for inspection. "The Chinese ones work OK, but man, I see what you mean," said Alex, "the real ones don't fit." Well folks, here's the skinny. All those thousands of repop300Bs floating around out there using 811-style bases are actually 300As, the way I see it. The 300B base has a different bayonet pin location, and there is no possible method to use the standard E.F. Johnson bayonet socket on the original. I'm sure Western Electric made a few, but call me (in about ten years) after you find a set.
VENERABLE JOHNSONS
It is said that Henry Ford's Model T could be fixed by anyone with just about anything at hand. The same is true for the venerable Johnson bayonet socket. Since the base pinout is the same on the 300B and others in the 4D family, all you need to make a usable bayonet sleeve is a bit of 0.0625” wall steel pipe (1½” outside diameter, 1 3/8 ohm in side diameter), some tools, a work bench, and-if you're really picky like I am-a vertical mill or drill press milling bed (Photos 1-4).
PHOTO 2: Initial cut. Use vertical milling bit to make vertical guide
slot and right degree bend.
Start out by hack-sawing the pipe into rough dimensions. Use some masking tape to transfer the dimensions of the original bayonet collar onto the steel pipe. Leave enough metal to produce the mounting tabs later. A single-cut file will take care of the burrs.
Jig a supporting piece into the pipe and bend the tabs. I used a welding torch to soften the tabs during the bend, but if you're a real animal, tap them over cold. Form the tabs with the file to fit the original ceramic base and you're in business.
Trial-fit the new bayonet sleeve and file it for a nice, snug fit. Mark the mounting hole location from the underside, drill, and thread the sleeves with a 4-40 tap. The fun part is setting up the milling table to cut the bayonet slot.
Clamp the piece securely and squarely in the vise. Go out and spend six dollars to purchase a ³/32” end mill to cut the slot. Transfer the dimensions from the original sleeve, only rotate the pin location 45° for the proper engagement of the real 300B. Slowly cut the slot with lots of lubricant and then deburr it.
PHOTO 3: Re-adjust sleeve in table vise and mill remaining horizontal
section of slot.
PHOTO 4: Rear view of modified sleeve readily shows difference in pin locations. At left, 812A triode; at right, 300B triode with rotated bayonet pin.
DRESSED TO KILL
I always try to dress up my pieces so they look the part. Carefully sand the inside and outside rims of the sleeve. I hate cutting a groove onto a tube base with a sharp bayonet sleeve, so if you have a rotating sanding drum attachment for your hand drill, a quick clean up is in order.
I prefer to cut a slight taper or bevel at the inside top of any bayonet socket to avoid any potential marring of the tube. File all cut edges true, and sand them smooth with emery paper. Just to bother my friends, I punched W.E.CO. into the sleeves prior to applying 24K gold plating.
Sonic benefits of bayonet sockets over ceramic plate types are nil, but I'm sure some readers will disagree. It is sure nice having bayonet sockets for my beat-up 300Bs, however.
SOURCES
Darcell
4910 Santa Anita Ave.
El Monte, CA 91731 (818) 443-5801, FAX (818) 443-0800
Enco Machinery Tools & Supplie
5000 W. Bloomingdale Chicago, IL 60639; (800) 873-3626, FAX (800) 965-5857
REFERENCE
1. 75 Years of Western Electric Tube Manufacturing, available from Old Colony Sound Lab, 888-924-9465, audioXpress.com.
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