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[AMPS] New 3-500Z

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Subject: [AMPS] New 3-500Z
From: (
Date: Thu, 27 Feb 1997 19:51:29 -0500 (EST)
n a message dated 97-02-27 08:32:04 EST, you write:

 I have not seen any published IMD data on the graphite anode versions and
 will leave that up to someone else.>>
Tests here indictated equal performance.

The Amperex version has slightly
 higher gain and could be a problem in an amp that is on the threshold of
 instability. There are no specs on the Chinese versions that I am aware
 of. >>

If anyone has a PA that close to the edge of stability it is going to be a
problem anyway. Gain goes up and down with anode voltage, drive level (MORE
drive, less gain), tuning and so on. 
Besides all that, VHF instability has nothing or little to do with HF gain.
HF gain is set by the input and anode operating impedances in grounded grid,
while VHF gain is a function of layout and construction of the tube's

>>I have however been told that the graphite tubes require careful control
 of the air supply and should not be used without chimneys. The reasoning
 is that when a graphite plate shows color it is releasing gas and slowly
 poisioning the tube.<<

There is a little truth to that. The graphine anode store heat longer, and
the seals can be damaged if the anode is real hot and air is removed
abruptly. That can eventually weaken the seals, but that is an extreme
condition of full dissipation and removing airflow abruptly. 

The anode is isolated by the vacuum in the envelope. Very little of the
cooling is direct conduction. All the airflow does is keep the glass from
melting, the seals from being damaged, and the components around the tube
cool. Blowing 500 cfm or 25 CFM across the tube, anode color will remain
essentially the same. The airflow prevents the glass and seals from failing,
it doesn't directly cool the anode.

Keeping the seals cooler in operation does give some margin for the thermal
lag problem at shut down.

Amp Supply's are particularly prone to this. The Heath SB220 has adequate
 cooling for the metal plate tubes but requires TLC with
 a few minutes of air cool-down after use before powering off. 

The 220 has adequate cooling for 600 watts of dissipation for TWO tubes. It
was designed to run 1 kW dc input (~600 watts output) and dissipate 400 watts
with a 50% safety margin. Keep that in mind.

73 Tom

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