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[Amps] overloading screen TSPA

To: amps@contesting.com
Subject: [Amps] overloading screen TSPA
From: "John T. M. Lyles" <jtml@lanl.gov>
Date: Tue, 12 Jul 2005 11:13:53 -0600
List-post: <mailto:amps@contesting.com>
Ian White's discussion on the sources of vacuum arcs and anode to 
screen breakdowns in tetrodes is what I have witnessed in my years of 
working with high power tetrodes. It is standard practice 
(recommended by Burle, CPI/Eimac, and Thales Electron Tubes) to place 
a spark gap across the screen bypass cap, typically rated 20% above 
the highest DC voltage that will be used. The sockets available from 
Thales sometimes come with a built-in gap, for instance the TH18555 
socket for the TH555A tetrode has a small air gap with adjustable 
threads, typically set for 2200 VDC breakdown at the altitude above 
sea level where it will be used.

I have witness autopsied tetrodes which have blown screen wires, from 
an arc. We rarely (!) remove load on an operating amplifier to 
prevent sudden injuries to the engineer. However, the phenomina of 
ion bombardment (gas) is a real problem. Tubes are conditioned to 
allow the getter to absorb as much free gas before high power 
operation commences. Another source of gas which Ian may have omitted 
is when an element (typically screen, plate or grid) is 
overdissipating, and outgasses. This usually causes a fault, where 
the tube breaks down internally. There are also chemical getters 
which are cold, and do not need the heater or anode heat to make them 
function. RCA/Burle tubes made a lot of them inside their tubes. They 
are typically inside a glass ampule which gets broken open once the 
tube is being sealed up, where they attract residual gas molecules.

Now before someone says that these are only things that are 
experienced in larger tubes with handles or eye hooks, the principles 
of ceramic metal tubes are scalable, from small sizes on up. We have 
Seimens or Claire gas tubes across some other tetrode screens, with a 
small series RFC from the power supply. We use the RFC and a few ohms 
wirewound as a fuse, when the tube arcs anode to screen, and the gas 
tube fires. This prevents the anode B+ from backfeeding and 
destroying the screen bypasses, the power supply, the wiring, etc. 
Its a lot easier to replace the RFC than a tube or power supply.

Hams would be wise to use similar techniques if they use expensive 
or rare tubes for high power amplifiers. The cost of protection is 
small compared to the cost of the RF device.

As an aside, last summer I seriously overdissipated a 3-500Z in the 
PDM switch tube socket of a Continental 314-R1 (Collins 828C1) 
broadcast transmitter in my shack. It had a shorted damper diode so 
the ST was running into a dead short and was being driven into 
conduction a low DF by grid pulses that would represent carrier level 
only. The 500Z anode glowed nearly yellow, and a deep blue glow was 
seen around the edges. It had roughly 8500 volts across it! Luckily 
the HV breaker tripped along with the plate overload, but this 
happened fraction second later. The tube recovered OK, but it took a 
beating.

73
john
K5PRO
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