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Re: [Amps] Tube gain vs frequency

To: jeff@wa1hco.net, kc0nxm@sbcglobal.net, amps@contesting.com
Subject: Re: [Amps] Tube gain vs frequency
From: TexasRF@aol.com
Date: Wed, 26 Aug 2009 23:56:33 EDT
List-post: <amps@contesting.com">mailto:amps@contesting.com>
 
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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