The old 'large pin' 7 contact tube base came in two flavours with a very
small difference in pitch diamerter, something like 1/16" difference. Who
knows why that was done. Cinch or Amphenol [or both] came up with a socket
for tube testers that had the already rectangular holes in the moulded
Bakelite body elongated radially to allow pins of both pitch diameters to
fit. If you look at Garry's photo of the Eico 615 adaptors, you can clearly
see the difference in the pin slots of the upper and lower version of the
adaptor 7 pin socket [notice the inner edge is closer to the centre by about
1/32"]. The 'universal' one also has the pilot lamp test socket in the
centre.

Neil S.

"Radio Waves" wrote in message
...

I'm scratching my head on this one. What would be a 7 pin large tube?
Those 2 sockets in the original post look the same to me. I have a I-177
and there is only 1 7 pin old style socket.