>
>Greetings,
>
>I recently completed prototyping my (all tube) screen
>supply/regulator and the question came to mind, "How
>much regulation is sufficient". In initial testing it
>would appear that the circuit is capable of < 0.5%
>regulation ( less that 2 volts @ 400V ) at load
>currents ranging from 0 to just over 200mA.
>
>Would this be considered adequate?
Yes, especially for SSB where the duty cycle is such that avg screen
current is c. 20% of that for AØ operation. However, for tubes with a
400v screen, reverse screen current is often considerable, so a 25% (of
screen-I) bleeder should be connected across the screen.
cheers, Randy
Not sure about the requirement for a bleed resistor id the design is
a shunt-screen stabilisor since it will have the ability to both sink and
source current. Does the design handle -ve current and remain in
regulation?
There was a good design for this type of regulator that used a 741/748
op-amp and EL84 tube in Oct 1977 RadCom - I built several of
these for use with '250B amps and they worked very well. The "trick"
with the design is that the cathode had two resistors in series which
were used to generate the supply for the op-amp.
0.5% regulation sounds good but is this under static load or with
a load that mimics the syabillic rate of speech? A failing of some
screen supply designs is that they can appear good under static
conditions but when presented with the more complex source/sink
current with speech modulation from the screen grid they wander
around, causing poorer IMDs etc. Check the time-constants (if any)
in the curcuit at AF.
Mike G8TIC
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