> Using a string of zeners to bias a shunt transistor is not necessarily
> a good way to improve voltage regulation. Typically the diode current
> is no more than required to supply base current to the transistor (a
> few milliamps) but at low currents the dynamic resistance of the
> zeners increases dramatically. The improvement in regulation from
> adding the transistor may be largely lost by having reduced the diode
> current.
Why would that be true, if you run nearly constant current through
the diodes???
> Also, a voltage regulator such as a string of zeners ("amplified" or
> not) makes it very difficult to protect the amplifier against screen
> current going outside the normal range, due to overdrive, incorrect
> loading or a range of other causes. This may or may not damage the
> tube, but it will damage signal quality and it needs to be detected.
> But any kind of current sensor that you put between the zener chain
> and the screen grid will add to the dynamic resistance of the screen
> supply, and spoil the voltage regulation. The main reason why I'm so
> keen on active regulators is that you can put the current sensor
> *inside* the feedback loop, so it has no effect on voltage regulation.
If the source voltage is at least twice the screen operating voltage,
the maximum possible screen dissipation is equal to the standing
dissipation in the shunt regulator.
What could be more simple?
> I rate voltage regulation and current-sensing protection as equally
> important. The amplifier needs both. Having tried just about every
> kind of screen supply from VR tubes on upwards, an active regulator is
> the only way I've found that can deliver both.
See my comment above.
73, Tom W8JI
W8JI@contesting.com
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|