>
>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.
>
Chortle
>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.
To do the job at hand, a zener must exhibit a high dynamic resistance
with only a small change in V. Otherwise it would not be able to
maintain a constant V as the screen current varies.
>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 has to be or it wouldn't maintain constant V
>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.
>
You electronic logic on zener regulation appears to be somewhat
illogical, Ian.
>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.
Which is why good engineering practice is to choose a value of series R
to the HV+ that will limit screen dissipation to a safe amount during a
glitch.
>This may or may not damage the tube,
>but it will damage signal quality and it needs to be detected.
Unregulation of the screen-V can be easily detected with a DMM.
>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.
A voltmeter has no such problem.
>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.
>
Valid point.
>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.
>
I use active regulators for tetrodes with handles and zener shunt
regulators for tinier tetrodes.
>>- 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.
>
Did he measure screen voltage regulation?
cheers, Ian
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
end
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