Alex and Gerald pretty much gave you replies that explained this. Here is more
'free' information. Usually the output circuit is the most difficult to design
as you push tubes in Power x Freq. Output being taken between the following
terminals, screen grid and anode (plate) for tetrodes and between control grid
and anode for triodes (no cathode followers or inverted amplifiers in this
discussion). The transit time is influenced by the electric field inside the
tube between these elements, which is of course determined by the spacing of
the electrodes and the HV applied (and more complicated, by the plate high freq
AC voltage component superimposed on the DC). Higher the voltage gives faster
electron movement, but this cannot be continued as breakdown between elements
occurs in vacuum and on the insulators outside the tube (especially there). So
you can reduce the spacing. This is contrary to raising the HV, so there is a
trade off of parameters there. As the operating freque
ncy and
voltage causes traditional current flowing back and forth (electrons actually),
eventually will get to a point where the physical transit time is too long and
the gain drops off. I know this is simplified, but there are plenty of texts
out there that explain in great detail. Both ARRL and other professional texts
like Terman explained this.
The output capacitance is fixed, from anode to the other elements, so that it
is accomidated in the circuit design, as others pointed out. At lower
frequencies, it is absorbed into the tuning and coupling network parameters by
design. At some point, in a transmission line UHF circuit, the first voltage
standing wave minimum will not be far enough away from the peak standing
voltage inside the interelement spacing, so designers will use higher
fractional-to-full wavelengths in the lines, such as a 3/4 lambda circuit,
which moves a tuning short further away from the tube connections. Otherwise
tuning contacts would be right against the tube surfaces! You'd be surprised
but the stray capacitance of a cylinder or ring contact above another ring
contact can be minimized to a degree, but as you pointed out, short fat tubes
will have some capacitance to deal with. In older glass tubes, the capacitance
was mostly from the elements inside, and less dominated by the connecting
cylinder
s. But
this can be dealt with, as discussed above.
The lead inductance in tubes is a considerable difference in old tube designs
and modern ceramic metal tubes. In older designs tubes were long with leads
coming up from the socket to the elements. These leads become part of the
circuit at higher freq. This is directly related to the ongoing discussion here
on GG and GK amplifier topologies. In reality, as the frequency goes up,
neither is correct terminology, as both the grid and cathode are above ground
by this series inductance, so the actual voltage across the interelectrode
spacing in the tube is not what is measured or calculated at the socket. At VHF
and UHF, this can be a real problem, leading to amplifiers that have poor gain
& efficiency. Fat wide straps and cylinders, of course, will improve this. Or
resonating with these inductances to reflect a ground inside the tube, via a
standing wave in the circuit. All sorts of tricks can be played but at some
point, lead inductance may be the limiting factor in a particular
tube. Of
course, as others and I have alluded to on this forum, they can also cause
parasitic VHF resonances in the circuit/tube combination that should be
accounted for by design.
A practical limitation of high power gridded tubes is the power dissipated in
the screen grid or control grid if it is part of the output circuit. The
circulating currents are high, and due to skin effect, concentrated in the
lead-in to the tube elements. At some point, this will cause problems, limiting
how high in Freq and Power they can go. Water cooling, forced air, and other
techniques apply here.
Finally, when tubes get bigger, to handle high power, there are internal
waveguide modes that can happen in the large spaces of the output elements
(remember, anode to screen or grid) that will restrict performance unless
accounted for in the design of the circuit.
73
John
K5PRO
> Date: Wed, 26 Aug 2009 19:20:37 -0700 (PDT)
> From: Stirling Schmidt <kc0nxm@sbcglobal.net>
> Subject: [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 fo
r your
consideration.
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