>>The Eimac advice assumes that you are not using a regulated screen
>>supply,
>
>? Eimac 8660 specification sheet dated 6-1-1967, page 4, "The screen
>voltage must be maintained constant... ...".
>'
>
The advice that Leigh was quoting comes from Care & Feeding, Section
2.2, which doesn't recognise the need for shunt-regulated screen
supplies. Yet another major area for revision... but don't get me
started on that!
>>...only a "dumb" bleed resistor - but for good linearity and tube
>>protection you MUST regulate the screen voltage closely.
>>
>>When you use active regulation of the screen voltage, you no longer need
>>a 70mA bleed. 10mA is plenty, and its only function is to clamp the
>>screen voltage while the T/R relay contacts are actually changing over.
>>
>? Typical screen current is stated to be Negative 14mA.
That's under drive conditions, with screen voltage applied. I wasn't
talking about that situation - except to agree that the screen supply
must be designed to handle that reverse current without losing voltage
regulation.
I was talking about what happens when you use a changeover relay to
switch the tube to cutoff. (To make this clear, the common contact is
connected to the screen, the "TX" contact is connected to the regulated
screen supply, and the "RX" contact takes the screen down to cathode
potential.)
What happens during the few milliseconds while the relay contact is in
motion from TX to RX? The screen isn't connected to anything except the
bleeder resistor and the screen bypass capacitor, which is initially
fully charged. If the screen had been sourcing negative current due to
secondary emission, there is a risk that it might continue to do so - at
least initially. If the bleeder resistor wasn't there, the negative
current would charge the bypass capacitor and the screen voltage would
rise... maybe enough to blow the capacitor or cause some other kind of
damage.
The bleeder resistor must sink enough current to ensure that, during
those few moments, the screen voltage is always heading downward, never
upward. This ensures that the tube will start to shut itself down as the
screen voltage falls. A moment later, the relay contact will close, and
then the screen is nailed down to cathode potential.
I've looked at the screen voltage with a storage 'scope, during the
period while the relay is switching over. About 10mA of permanent bleed
current seems to handle this situation very safely.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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