Re: A question about vacuum tubes

["Dan Strassberg" <dan.strassberg@att.net>]

A 50C5 should be safe in a socket meant for a 35C5 and should not damage the
radio, _provided_ that the tube filaments are not in series. Many radios of
that day had the filaments in series and the series string was connected
directly across the 115V rms ac line. This saved the cost of a power
transformer. I don't think that Zenith table-model AM/FM receivers were
designed in this manner, however. The first number in the tube type
designator (that is, 35 or 50, in this case) is the nominal filament
voltage. I assme (but don't know for sure) that the 35C5 and the 50C5 were
identical except for the filament voltage. If so, the 35C5's filament
resistance is probably about half that of the 50C5's. If you put the 50C5 in
a 35C5 socket in a radio with series filaments, all of the other tubes would
be running at low filament voltage and the radio might not work very well,
though it would probably still play. If the filaments are driven from a
power transformer, the 50C5 will be running with lower filament power than
the 35C5 would have run. Since the tube is, I assume, the final audio
amplifier, there will probably be some distortion in loud passages, but you
may not notice it if you don't play the radio too loudly.

--
Dan Strassberg, dan.strassberg@att.net
617-558-4205, eFax 707-215-6367

----- Original Message -----
From: A. Joseph Ross <lawyer@attorneyross.com>
To: <bri@bostonradio.org>
Sent: Monday, September 02, 2002 2:21 PM
Subject: A question about vacuum tubes


> I'm sure there are some people around here old enough to be familiar with
> how tube sets work.  I have an old Zenith AM-FM radio with a rather nice
> sound, but it's had some problems lately.  I diagnosed the problem as the
> audio output tube, which is 35C5.  I had an old 50C5 tube, which was used
> for the same purpose in AM-only radios, I substituted it, and it works
> well (I seem to remember something about it being sort of OK to do that
> if the substituted tube had a higher voltage, as represented by the
> initial number.).  My question is whether there's any harm to the radio
> in leaving it that way, or whether I really ought to look for a 35C5 from
> somewhere.