What 1296 tube is that Gerald?
Carl
KM1H
----- Original Message -----
From: <TexasRF@aol.com>
To: <jeff@wa1hco.net>; <kc0nxm@sbcglobal.net>; <amps@contesting.com>
Sent: Wednesday, August 26, 2009 11:56 PM
Subject: Re: [Amps] Tube gain vs frequency
>
> 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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