The 50B5 vacuum tube

The 50C5 beam-power tetrode tube was preceded by
functionally equivalent type: 50B5 (different pinout)
The reason for the issue of the 50C5 and the
"forced obsolescence" of the 50B5 is explained below.



Reference
https://antiqueradios.com/forums/viewtopic.php?f=19&t;=311352

A bit of trivia, the 50B5 shares same base connection as the 6AQ5.
Since RCA already had jigs for forming leads to elements from button why not??
UL said "woah hold on here" as there was an issue that a chassis could
become hot if a tube was plugged into wrong socket!

RCA responded with the revised base connection pinout 50C5,
so the 50B5 only saw widespread useage for approx 2 years!

It would be interesting to know where you saw or heard that,
as there are other versions of the story of "why UL had the 50B5 replaced by the 50C5."
The version I got from somebody who did UL inspections for a radio retailer
and had first hand experience with this was that if you look at the 50B5 base diagram,
you'll see that pin 4 (heater) is next to pin 5 (plate).
So you could have as much as +140 VDC on pin 5 and -140 peak AC volts on pin 4
during the negative half excursion of the AC power cycle.
The concern was that the thin phenolic wafer sockets then in vogue
could not withstand 280 volts from pin to pin,
and it looks like UL was right because sets with a 50B5
still turn up with carbonized sockets to this day.

The fix in the 50C5 was to put the first grid on pin 5.
Since the grid is in series with several hundred thousand ohms of bias resistor in most applications,
there's effectively no leakage path for carbon trails to start.
Don't know why this was not a problem for the 6AQ5,
except that tube almost never appeared in low cost
series heater radios with thin phenolic wafer sockets.
There was no problem with the 50B5 either, as long as
it was used in a molded plastic or ceramic socket.

The 6AQ5 is rated for maximum of 275 volts on the plate,
but at that point they start warning you about keeping
the bulb temperature from becoming excessive,
so presumably you'd be using a better tube socket than a wafer type.

Personally I like the 50B5 better than the 50C5;
by keeping the grid connection remote from the heater pins, reduces the chance of hum pickup.
Though not as common as 50C5's, 50B5's are still readily available and inexpensive.
When I have to repair a radio with a 50B5, and the socket is blackened or falling apart,
I simply refit a new molded or ceramic socket which takes care of the problem for good,
and therby avoid confusing future owners with whether the set should have a 50B5 or a 50C5 fitted.

That may well be what somebody said, but it doesn't make a lot of sense.
If a chassis is already hot, putting the wrong tube in isn't going to change that fact.
And if a chassis is isolated, none of the pins of a 7-pin miniature socket
should be connected to it (unlike metal octals,
where pin 1 may be connected to chassis for shielding purposes
even if everything else in the circuit is isolated).
Therefore, putting a 50B5 or 50C5 in won't make a chassis hot.

Going by that logic, they should have kept the 50B5 and discouraged the 50C5 !

Pin out of 12BA6	G1	G3	H	H	P	G2	K
Pin out of 50B5	G1	KG3	H	H	P	G2	G1
Pin out of 50C5	K	G1	H	H	G1	G2	P
Pin out of 12BE6	G1	KG5	H	H	P	G2G4	G3
Pin out of 12AT6	G	K	H	H	D2	D1	P
Pin out of 35W4	NC	NC	H	H	P	Ht	K

In other words, it looks like RCA thought of the possibility that
somebody might inadvertently plug a tube into the wrong socket
when they designed the 50B5, as its pins are more or less aligned
with the other tubes and no damage will occur
if it is plugged into one of the other tube sockets.

The G1 jumper is between pins 1 and 7 in the 50B5.
In the 12BA6, pin 1 is G1 and pin 7 is the cathode,
so no source of B+ to couple into the tuning cap.
Same with the 12BE6.
You do get a grid to plate short if a 50B5 is plugged into a 12AT6/12AV6 socket,
but in most cases there would be a coupling cap in series,
so no shock hazard to speak of.

The G1 jumper is between pins 2 and 5 in the 50C5.
If plugged into other sockets, it will short the plate voltage into the cathode or suppressor grid
circuits of the other tube sockets (except for the 35W4).
No shock hazard likely, but it sure could burn up some resistors or an IF transformer primary.

© 1Q 2020