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 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'
>
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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