>Rich Measures wrote:
>
>>>GW4FRX's measurements quoted in my article (October QEX) show that a
>>>zener string emphatically does NOT "work best" from the viewpoint of
>>>IMD!
>>
>>Volts are volts. It matters not what type of regulator they come from,
>>unless the regulator is performing poorly.
>
>You surely don't mean that, Rich.
I plead guilty, your honor. Simply stating that shunt regulators result
in IMD does not suffice.
>Under conditions of varying current
>demand, some types of voltage regulators obviously have a lower output
>impedance than others.
>If you look at the datasheets, you will see that a string of high-
>voltage zeners cannot have a output impedance less than a few hundred
>ohms - a load pull will confirm this. A few hundred ohms is big enough
>to have a very noticeable effect on the IMD.
>
>For the benefit of people who haven't seen the QEX article, the diagram
>(which is also on my web pages) shows three sets of IMD plots. The two-
>tone measurements were made by GW4FRX using a pair of 4CX250Rs at 500W
>PEP output. The best IMD was obtained using an electronically regulated
>screen supply with a very low output impedance. Second-best IMD was with
>the same screen supply, but with its output impedance deliberately
>raised by a 150-ohm series resistor. You can see a clear difference in
>IMD levels.
>
>Worst was a string of zeners, and that is exactly what you'd expect from
>the known output impedance. VR tubes would be very similar.
>
>>>
>>>> For larger tetrodes, a 723-based positive
>>>>floating series pass regulator has advantages. You can find both types at:
>>>>http://www.vcnet.com/measures
>>>
>>>But note also that a series regulator cannot handle reverse screen
>>>current, which many tetrodes can generate. You MUST use a shunt resistor
>>>from screen to cathode, that permanently passes the maximum negative
>>>screen current that the tetrode can generate. ...snip...
>>
>>True indeed, Mr. White. But note that essentially the same information
>>can be found in *Amplifiers* at the above URL.
>
>(Yes, but in a different file.
Shunt vs. series regulated screen supplies are discussed in Part 4:
----------
Quoting:
"Advantages of Shunt Zener Screen Regulation:
Limits the maximum current that can be drawn by the screen.
Protects against reverse screen current.
If the high voltage disappears, so does the screen voltage.
However, for larger tubes with higher screen current and screen voltage
requirements, a Zener shunt
regulator is somewhat impractical. A continuously-adjustable
series-regulator screen supply is a better solution. To protect against
reverse screen current, a shunt resistor/bleeder must be connected across
the
screen supply. A bleeder current flow of roughly 20% of the normal screen
current seems to be adequate. 25% might be safer. To protect against
excessive forward screen current, a fast acting fuse or
magnetic-type circuit breaker is incorporated in the primary of the
screen supply power transformer. An adjustable series regulator circuit
is provided."
--------
>You might add the bleeder resistor to the
>schematic as well.)
The screen bleeder resistor, as well as the glitch protection diodes, Ds,
are on the simplified diagram for the Class AB1 amplifier, Figure 5. .
http://www.vcnet.com/measures/D.a.05.GIF
Rich---
R. L. Measures, 805-386-3734, AG6K
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