This has brought up an intersting question.
What is the dissipation of a screen grid if it is producing reverse current.
Secondary emission does not produce dissipation, the power dissipation is due
to the kinetic
engergy of the electrons that are striking it. Lets say the measured current is
-10ma. That means that the secondary current exceeds the
primary current ( dissipative current) by 10 ma. But what is the actual
That depends on the ratio of primary current and the current due to the
secondaries that have not
been recaptured by the screen grid.
An interesting problem. It is not only a matter of how many secondary
electrons are produced by
each electron that strikes the screen grid because the all produce secondaries
if they have more than
50 or so electron volts of kinetic energy. It is just that most are secondaries
are recaptured and
don't affect the net current flow.
By producing an "electron shadow" that the screen grids usually set in a
condition is set up where
it is possible to produce many secondaries that are not recaptured. In other
tubes many electrons
strike directly on the screen grid. I suspect under some condiition the
electron optics change such
that the "shadows" collapse some what and you get glancing or side collisions
that produce some
secondaries that have enough kinetic energy in the direction of the plate to be
captured by the
plate's electric field.
But, these secondaries themselves do not produce any dissipative power on the
Secondaries have very low kinetic energy compared to those coming from the
Please pardon the rambling, I am very tired tonight but it is an ineresting
From: firstname.lastname@example.org [email@example.com] On Behalf Of
Sent: Saturday, January 28, 2012 12:57 PM
Subject: Re: [Amps] 4CX1000A
How the bleeder resistor would help, if the reverse current exceeds 37 mA?
At 325 V screen voltage this makes 12 W - the maximum screen dissipation.
I have seen many brand new tubes with much higher maximum reverse current,
which they use to achieve at moderate power levels. They are good only to
work at full power in industrial devices.
73, Val LZ1VB
>A bleeder resistor to ground at the tube screen connection should help.
> Don W4DNR
>> I forgot to mention that most of these tubes run reverse screen current
>> with low drive and power levels. Mine (4CX1500B/FU728F) only goes to
>> positive screen current near the US legal limit output. At that point I
>> tune for max screen current.
>> Roger (K8RI)
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