>
>Rich Measures wrote:
>
>[snip - agreed, or near enough]
>
>>>5. The tube is sourcing EVEN MORE negative screen current. The series
>>>regulator is already supplying ZERO current so the voltage across the
>>>bleeder resistor will start to rise. There is NO voltage regulation.
>>>
>>The solution is to fire the "designer" and increase the screen bleeder
>>current to the tube mfg. recommendation.
>
>Yes, again we agree.
>
>>>
>>>OK, I accept what Rich and Steve say about the possibility of using a
>>>transistor shunt stablizer from a supply that's higher than its
>>>breakdown voltage... but I still don't feel comfortable about it. To be
>>>reliable, it certainly needs more careful design than most amateurs
>>>would be prepared to put into a screen supply.
>>>
>>So, operate a series string of name-brand 20v - 30v 5w zeners at no more
>>than 2w, biased from the anode supply through a suitable R, and one has a
>>foolproof screen regulator for smaller tetrodes.
>
>But also unfortunately an inadequate regulator, from the viewpoint of
>obtaining the best possible IMD and protection for the tube (as
>demonstrated by GW4FRX for 4CX250s, and shown on my web pages).
>
However, one need not use the same obviously-flaky zeners that GW4FRX
used . I use a zener-string (400v total) shunt regulator for the screens
of the pair of 4cx250Rs in my 2m 1kW pep out SSB amplifier. The IMD
appears to be quite acceptable. The screen potential appears to be
stable. - note - The zeners receive cooling air.
>>For tetrodes with
>>handles, the positive floating 723 pass circuit works well.
>
>Indeed it does. Either circuit can be made to work well, if designed
>well.
Agreed..
- cheers
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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