[Amps] Initial RF testing of 4CX1000A amplifier

[Ian White]

>
>> My meter reads -10 - 0 - 40. Actually it is a normal meter with a new
>> scale. There is a 10 mA bleed which brings the pointer to zero when
there
>> is no screen current. Very clever idea by GM3SEK.
>>
>> I'll be testing with full drive soon.
>>

>
>That screen bleed goes back to the 50's or so when tetrodes were more
>popular than triodes.
>
>Carl
>KM1H
>

Back in the 1950s, the screen bleed resistor was a rather poor effort to
provide some voltage stabilization. It never really worked, which is why
modern screen supplies are electronically stabilized.

The Tetrode Boards use an active shunt stabilizer. At first sight this
shouldn't need a bleeder resistor at all, and the bleed current (about
10mA) contributes nothing to the voltage stability... so why is it there
at all? 

The bleed resistor is not part of the power supply. It is installed
close to the tube itself, connected directly between the screen grid and
ground (cathode). Its purpose is to prevent the screen grid from
floating unconnected during the few milliseconds while the screen relay
contacts are in motion between the RX and TX positions. This is
particularly important for "problem" tubes like the 4XC1000A and the
4CX250/350 family whose screen grids are very prone to secondary
emission (negative screen current). Without that resistor acting as a
pull-down, the screen voltage can easily float upward causing a runaway
spike of anode current. 

The bleed current of about 10mA will move the meter needle up-scale, but
probably not enough to display the full range of negative current for a
4CX1000A. The best solution is to mechanically adjust the meter needle
further up-scale toward the center. As well as the front-panel
screwdriver adjustment, many meter movements have a coarse adjustment on
the rear pivot.

(That was the short version. Many more details are provided on my
website, both in the User Manual for the boards and in a QEX article
which explains what was wrong with the large majority of older screen
supply circuits.)

73 from Ian GM3SEK