{Collins} Re: 30S-1 Screen voltage questions

Greetings to Tom and the list:

   Tom writes:

> Negative current or secondary emission actually counteracts
> primary current, so we cannot calculate screen dissipation
> by a simple P=I*E
> when secondary emission is involved. The normal process is
> to rapidly shut down the tube and measure emission from the
> screen (or other grid) to get an idea what the screen (or
> grid) temperature actually is. This is how dissipation
> ratings in power grid tubes are determined when initially
> establishing tube characteristics.

   I would agree with the above, except that I do not consider screen
emission normal, and in the case of positive screen current the formula
works.... otherwise, the tube manufacturer would not specify a dissipation
in watts.   This dissipation rating undoubtedly contains a margin for other
heating causes and therefore the calculation is quite usable providing, as I
said, that the current is positive.

> >   In the meanwhile, I personally do not like to see
> > negative screen
> > current.   It can be zero, or positive, but negative
> > screen current means
> > screen emission.   The screen cannot emit electrons unless
> > it is running
> > hot, and this is a condition best avoided, in my humble
> > opinion.
> Not so.
> Secondary emission can occur from a hot element, but it also
> occurs from a cold element when the element is bombarded by
> electrons. It can even occur from insulators! Kinetic energy
> of electrons can dislodge other electrons without the
> element being hot. The anode of the tube emits secondary
> electrons. The secondary emission electrons from the anode
> can be collected by the screen grid.

   I would not agree with the above, quite.   Certainly, kinetic energy of
electrons, if dissipated in the screen via elastic collisions with electrons
in atoms comprising the screen grid material can cause heating of the screen
grid.   They can also eject electrons from the screen grid.   Secondary
emission from the plate can also be collected by the screen; however, if
this occurs, the resulting screen current is positive.
   It would be necessary to analyze the energy budget for impacting and
ejected electrons to determine the resultant screen current.   If the
kinetic energy of an impacting electron is sufficient to eject more than one
electron which ultimately leaves the screen, then the resultant current will
be negative.   If the ejected electron multiplication factor is less than
unity or if the ejected electrons are preferentially reabsorbed by the
screen, then the result will be a positive screen current.   As Tom states,
this gets to be a bit complicated to calculate.
   Negative screen current can result from a number of factors, but it
usually indicates internal damage to the tube.   Modern power tetrodes are
usually constructed with vertical wire or bar elements in both the control
and screen grids.   These grids are so designed and constructed that each
wire or bar of the screen grid is on a radius of the tube directly outboard
of a control grid wire on the same radius.   This puts the screen in the
electron "shadow" of the control grid, greatly reducing the possibility of
direct electron impingement on the screen.   With this design, negative
screen current usually results from one of two causes.... excessive
temperature of the screen due to excessive dissipation within the tube, or
mechanical displacement of one or both grids such that the "shadow" effect
is no longer present.   This allows ejection of electrons from the screen
due to collision energies, with resultant negative screen current (current
from screen to plate).   In view of the fact that the 4CX1000A is known for
having a fragile control grid, loss of the "shadow" effect with resulting
negative screen current is most probably a result of damage to the G1
   In my experience as a broadcast engineer (before we got rid of beam power
tetrodes in favor of power MOSFETs), negative screen current meant that an
after-midnight tube changing session could be anticipated shortly.

   If the tube still works, by all means run it, and don't worry about the
negative screen current.   However, it would be prudent at this point to
start budgeting for a replacement tube.

> The entire process is very complicated and varies with the
> equipment design. This is a good reason why, unless we
> happen to be an experienced PA designer with suitable test
> equipment, we should just leave the voltage where they are
> and tune according to manufacturer's instructions.
> IMO it is a bad idea to alter the screen voltage in a 30S1,
> and I certainly have not seen any rational arguments for
> doing so.

   I would heartily agree with the above.

Jim T.

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