rich Wrote:
>- Bargain-price/cheapie/fallout zeners diodes are basically units that
>fail to pass the manufacturer's final test. I was given a box of them
>which was marked "semiconductor scrap". Some were usuable. Some were
>not. None were first class.
My remark about "cheap zeners" came from an earlier statement by Rich:
>- A series of 5W. 20V - 30V zeners are a good choice for such a shunt
>regulator because they can be mounted on a perf-board, they are
>reasonably priced, and the screen V can be adjusted in 20 - 30 V steps.
It had nothing to do with the alleged quality of the semiconductors. Put
it down to the "common language" barrier.
If the well-known 1N5300 series of 5W zeners are chained together as
Rich describes, it looks at first sight like they can make a voltage
regulator with a low dynamic resistance - which is the change of screen
voltage, for a given change of screen current. The dynamic resistance of
the diode chain is equal to the dynamic resistance of one diode,
multiplied by the total number of diodes in the chain. If you use say 12
* 30V diodes for a 360V regulator, the dynamic resistance of an
individual diode has to be multiplied by 12.
The data sheet for the 1N5300 series shows that the dynamic resistance
is a strong function of diode current. It increases sharply when the
diode current goes down, which happens when the screen current is
greatest. So the regulation is worst on speech peaks, right at the very
moment you need it the most. That's my objection to a chain of "unaided"
zeners for a screen supply.
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.
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.
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.
>- The (your?) reported IMD difference between using a precision shunt
>screen regulator and a zener-string regulatior was too vast to be true
>with quality semiconductors.
>
The person who made the IMD measurements is more than able to respond to
that one.
--
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|