The first question requires the longest answer:
The foremost frequency wise limiting factor is the transit time: the
time it takes for the electrons to travel from the cathode to the anode ( a
few nanosecond). The period of the RF cycle should be about 10 times that in
order for the tube to operate according to theory. This is the reason high
frequency tubes have such close spacing between the various elements.
The second factor is the length of the wires connecting the various
elements to the outside world. Their inherent inductance is a very serious
impediment to the tube's operation as you go higher in frequency. You can
alleviate that in 2 ways: make the wires shorter and/or wider. It was done
in the tubes intended for VHF/UHF operation. In extremis, the wires ended up
as the rings you see on power tubes today.
The issue of how you cope with the output capacitance pertains to
circuit design. I'll just say that the technique includes "absorbing" part
of it in the tank design and tuning it out with the help of phasing lines,
disgusting!!
Everything else is voodoo.
As for lower frequency tubes: tubes don't have an intrinsic mechanism that
limits gain as a function of frequency. So a tube able to deliver power is
doing what it wants versus frequency. The task of band limiting it rests
with the circuit designer. It stinks, but it has to be done!!
Alex 4Z5KS
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Stirling Schmidt
Sent: Thursday, August 27, 2009 5:21 AM
itanceSubject: [Amps] Tube gain vs frequency
Hi all:
I'll try to keep this short, but have several questions. First, what
is/are the factor(s) that determine a tube's upper frequency response? I
tried thinking (uh-oh!) but only deduced that ceramic tubes, with their much
larger plate structure, should only have more stray capacitance to a nearby
ground plane than a glass tube, and therefore much less usefullness into the
vhf-uhf region - obviously not the case, so what gives? Second, wouldn't it
be beneficial to employ a tube for an HF amp that naturally begins to lose
efficiency above HF (seems like the 833 would be a prime candidate)? Third,
if RF flows mainly on the surface of conductors, why don't ceramic tubes
have an insulator at the top (it seems as though all the RF flowing around
the bottom edge of the plate would concentrate heat exactly where it's least
welcome - at the seal - a top insulator would at least divide the current
flow in half)? Bear in mind these are beginners'
questions - Thanks in advance for your consideration.
73 de kc0nxm
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