Bill and Ian,
Thank you for the enlightening of the question.
Than do we need the screen current protection to trip by the reverse current
and at what level?
For 4CX1000A the circuit trips now at +/- 35mA. Is it safe to increase only
the negative threshold to 50 mA?
73, Val LZ1VB
> 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
>dissipative current?
>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 screen grid.
>Secondaries have very low kinetic energy compared to those coming from the
>cathode.
>73
>Bill wa4lav
>Please pardon the rambling, I am very tired tonight but it is an
>ineresting discussion.
>
>Don,
>
>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.
>>
>> 73
>>
>> Roger (K8RI)
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