[NJARC] tube shields and bucking

[Jim Whartenby]

sgt88?
I doubt that tube shields were designed for electrostatic shielding of post WW2 
tubes.  The early octal tubes were metal shelled with the shell connected to pin 
1.  They were soon replaced by glass bulb versions as a cost reduction but pin 1 
was not reassigned, at least as far as I know.  I believe the tube shield's 
primary purpose is to keep the tube in the socket.

I don't think I have ever seen a tube shield for octal tubes but there are 
several methods of restraining octals so that physical shock doesn't knock them 
out of the tube socket.  7 and 9 pin miniatures on the other hand routinely have 
tube shields in military and high end electronic equipment.  That said, the 
shield connection to the tube socket does not seem to be a low ohmic connection 
that I would think would be needed for proper electrostatic shielding.    Come 
to think of it, Tektronix O'scopes that are tube powered don't have tube shields 
on any tubes; with the only exception of the electromagnetic shield on the CRT.

http://www.pearl-hifi.com/03_Prod_Serv/Coolers/PEARL_Tube_Coolers.pdf
has a chart on page 38 which shows the effect of tube shields on bulb 
temperature.  The standard JAN tube shield (shiny nickle plate brass or steel) 
is much worse then no tube shield at all by a considerable amount.  At 
40C ambient, the bulb is about 100C hotter when compared with the bare bulb. 
 The IERC style shields do lower the bulb temperature a bit below that of a bare 
bulb.  But the temperature difference is not as dramatic.

The next chart on page 39 seems to indicate that 220C bulb temperature for a 
6AQ5 gives a lot longer service life then say a 260C bulb.  So every bit of 
temperature drop below about 220C increases service life but I am not sure that 
there is better circuit performance from electrostatic shielding.  As an 
example, I have an R-390A with no tube shields at all which seems to perform 
quite well.  I guess I could scrounge up some tube shields and do a before and 
after test but that is more work then I want to do.

http://www.radioremembered.org/edspeaker.htm has the information you asked for 
about hum-bucking coils as used in electrodynamic speakers.
Regards,
Jim





________________________________
From: "sgt88 at atlanticbb.net" <sgt88 at atlanticbb.net>
To: njarc at mailman.qth.net
Sent: Sun, January 13, 2013 5:40:59 AM
Subject: [NJARC] tube shields and bucking

Just remember
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_________________________________________________________
Obviously, the metal tube shields protect the tube from stray EMF.

Has anyone done a 'before & after' - ie with & without shields to see what 
actually occurs?

Saw a note under the Philco 116 speaker drawing...'when replacing any part of 
the speaker, the hum bucking coil connections should be connected for minimum 
hummmmmmmm...

Hmmmmm...I see no part number for this "bucking coil"...nor ID on the 
drawing..."what's up?"

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