>
>
>>>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.
? yes.
> I wasn't
>talking about that situation -
? Does screen current exist without drive?
>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.)
This is hardly a logical method to cut the tube off, Ian. Tis easier to
switch grid bias. Ordinary relays are more suitable for switching 90vdc
than they are for switching 400vdc. Better yet, sense RF relay coil
current with an optoisolator and use it to control a grid bias switch
transistor.
>
cheers
- Rich..., 805.386.3734, www.vcnet.com/measures.
--
FAQ on WWW: http://www.contesting.com/ampsfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|