[ARC5] Modifying 1625s

[Richard Knoppow]

----- Original Message ----- 
From: "Dennis DuVall" <scr522 at earthlink.net>
To: "ARC-5 List" <ARC5 at mailman.qth.net>
Sent: Tuesday, January 10, 2012 11:23 PM
Subject: Re: [ARC5] Modifying 1625s


> All,
>
> More accurately beam "confining" plates.  This subject 
> comes up about
> once a year.  For more than you ever wanted to know about 
> beam
> tetrodes see <http://www.r-type.org/static/btet.htm>
>
> Dennis D.  W7QHO
> Glendale, CA

     This is an interesting summary of how beam tetrodes 
work. Its interesting that a very similar explanation is 
given in the later editions of the _RCA Receiving Tube 
Manual_ for instance RC-24. In it the plates are referred to 
as "beam confining plates". However, in the 1938 edition of 
_RCA Transmitting Tube Handbook_ they are called "beam 
forming plates". The later book makes it clear that the 
suppression of secondary electrons comes from the formation 
of a virtual cathode by the design and position of the 
screen grid wires in relation to the control grid and that 
the beam plates are used to prevent electrons from straying 
outside the area controlled by this beam forming action. The 
entire design of the 807/6L6 type tube, of which the 1625 is 
an example, uses planer cathodes so that the tube is 
arranged as a two-sided structure. This is probably the 
reason for the plates. Later beam tetrodes use cylindrical 
cathodes and overall construction so such plates are 
unnecessary. The reason for the planar cathodes is probably 
that they have some desirable characteristic for smaller 
tubes. The article referenced above states that the reason 
for the plates being necessary was that the manufacturers 
wanted to continue to use old manufacturing technology and 
not have to re-tool. I suspect this is not true because the 
same arrangement persisted until the end of the vacuum tube 
era, being found in sweep tubes, etc. The cylindrical type 
heater is found in many older tubes but is also a practical 
form for larger tubes using thoriated tungsten 
heater/cathodes rather than the indirectly heated cathodes 
found in most smaller tubes.
     In any case, RCA called the plates both beam forming 
and beam confining plates but the latter term is the one 
that seems to have survived in their publications and is 
probably more accurately descriptive of what they do.
     Now to drop a rock: Penta Labs made beam pentodes. The 
claim was that they were significantly more linear than beam 
tetrodes. The type has not survived so I wonder if a 
modification of the four-grid design perhaps accomplished 
the same thing with a simpler structure. I have not tried to 
find the Penta Labs patents which might shed some light on 
this (or might not considering the way patents are written).


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com