> >
>>> 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
I mentioned that simply to show the progression over the decades Ian, from
basics to modern circuits that werent possible in the 50's or took up
considerable real estate and high cost.
Carl
KM1H
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