Regarding current flow from the anode to the top cap connection: RF will
flow on the surface of conductors. All of the rf then has to pass through the
ceramic to metal seal. The amount of rf flowing directly through the metal
that makes up the anode cooler is minimal. Remember how a thin sheet of
metal makes a great shield? Even .032" thick material offers many dB of
isolation when used to make a shielded plate enclosure.
DC on the other hand will flow through the metal parts. Because of
circulating currents the rf current will be many times the dc current flowing
at
the anode cooler.
So, it seems the rf path and dc path through the anode cooler are probably
separate. The dc has a path from anode to the cooler cap without ever
passing through the ceramic to metal seal. All of the rf current must pass
through this area.
Because the path for rf currents follow the surface, the distance involved
is greater and substantial inductance is created. At VHF and UHF this
inductance becomes a large part of the anode plate resonating structure. This
is
why tubes designed for UHF operation have the various connecting rings in
a coaxial manner. The rings provide a way to connect to the tube elements
with minimum added inductance.
At 432MHz and 1296MHz, even the finger stock used to make connections to
the coaxial rings can effect the resonance of input and output circuits. I
have one homebrew amplifier for 1296MHz that runs a coaxial base tetrode tube
at 1500w output. It seems a miracle to me that we can build an amplifier
of that power level at such a high frequency. But, we can and do. The main
effect of the high frequency is the loss of gain. It takes over 200w drive
for 1500w output. The loaded Q of that amplifier is around 250 so all losses
must be minimized as much as possible.
73,
Gerald K5GW
In a message dated 8/26/2009 9:45:04 P.M. Central Daylight Time,
jeff@wa1hco.net writes:
Stirling Schmidt wrote:
> 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?
Plate output capacitance doesn't matter too much because it gets
absorbed into the plate output network. The biggest consequence becomes
the high Q and high circulating currents that naturally occurs with
large output C at the operating frequency.
VHF/UHF plate circuits use a cavity structure with transmission lines,
often silver plated to handle the high circulating currents
efficiently. As an aside, all that plate circulating current return
though the grid ring, which explains the large low impedance grid and
screen grounding schemes.
> 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)?
The problem is that the electrons inside the tube work just as well at
VHF/UHF frequencies, but the large old tubes have highly inductive
internal wiring from element to socket. This means that external
amplifier design has relatively little control over the tubes at VHF/UHF
frequencies. Which leads to parasitic suppressors and all these
discussions that have a bit of the black magic about them.
> Third, if RF flows mainly on the surface of conductors, why don't c
eramic 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'
>
There's relatively little heat generated due to Ohmic losses in the
surface currents of the tube. The majority of the heat gets deposited
by the electrons landing on the plate. Even so, your question about
current flow is quite interesting. I've never seen a model of that. So
a question for the group... Imagine an 8877 Amplifier for HF with a
plate output connected to the top cap. How much of the current flows
through the anode metal and how much flows "on the surface" at the anode
ceramic to metal seal?
> questions - Thanks in advance for your consideration.
>
> 73 de kc0nx
jeff, wa1hco
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