>
>Tom Rauch wrote:
>>
>>> A different aspect to consider - what happens when there is
>>> anode-screen flashover. I inherited a PSU (with 10x33V/5W zener screen
>>> supply) from a friend. A dud tube caused a flashover and lots of
>>> damage. After some investigation, I came to the conclusion that 375V
>>> VDR clamps on the screen didn't protect anything, as their clamping
>>> voltage at 10s Amps is around 600V+. The arc current went through the
>>> screen meter and shunt and down the zeners, taking some of them and
>>> permanently damaging the meter and shunt. Yes, there was a suitable 50
>>> ohm resistor in the plate supply.
>>>
>>> One advantage of an 'electronic' screen supply is that you can build
>>> in a current sink limit and make the VDRs work for a living.
>>
>[...]
>>On the other hand, you could also protect a zener supply just as
>>easily. It is not an advantage at all to use a more complex circuit.
>
Amen, Tom.
>You can't generalize either way on the basis of complexity alone.
>Complex circuits can offer design opportunities that don't exist in
>simple circuits; but they also have more to go wrong. In the end, it
>always comes down to the *specific* design details.
>
>>Think about the problem a little bit.
>>
>>The screen has to rise to probably a thousand volts or more to give
>>any transient protection time and voltage to act.
>>
codswallop
>Fortunately that's not true, because VDRs act on a timescale of
>nanoseconds. Their problem is - as identified by Steve - that they don't
>clamp hard at the limiting voltage. That simply means the supply circuit
>has to be designed to survive that situation (which Steve's inherited
>supply evidently couldn't).
>
>Gas-arc surge protectors are slower to strike, but after the arc strikes
>it clamps at a lower voltage than the striking voltage. It's very
>difficult to tell which is better for screen protection, VDRs or gas
>tubes. I tend to use multiple VDRs - at least one large one connected
>right at each screen terminal - but wouldn't argue strongly against gas
>tubes. Either can work well.
>
>One circuit which has some of the advantages of both is a thyristor
>crowbar (once again credited to GW4FRX and associates). This is very
>much like the overvoltage crowbar circuit you see in 13.8V DC power
>supplies. If it is set to trigger a little way above the normal screen
>voltage, it will nail the screen to ground if the voltage starts to
>rise. The parts cost is minimal, but once again it needs to be connected
>close to the screen that it's supposed to be protecting. The thyristor
>can also be triggered externally, eg by a circuit that monitors for a
>rising edge in anode current.
>
>Returning to strings of zeners, one useful feature is a large
>electrolytic capacitor (say 50uF) in parallel with the zeners. This will
>help to absorb the rising edge of any voltage surge. In the last amp I
>had that used zeners, I had several arcs from B+ but never lost any more
>zeners after fitting the capacitor.
>
>>You better be very careful what you are doing, and not be foolish
>>enough to assume current limiting will protect anything.
>>
>There's nothing inherently wrong with current limiting, or any other
>design features. They can all be made to work.
With a zener-string screen regulator, operated through a high resistance
from the anode supply current limiting is automatic.
> The foolishness is in
>*assuming* anything, when we should be *designing* everything!
>Everything needs to be designed twice over, once for normal operation
>and again for a range of fault conditions.
>
>Good flashover protection for the screen requires at least four separate
>design features:
>
>1. B+ surge resistor to limit the maximum possible current. This gives
>you a basis to design the rest of the protection features.
>
>2. Something to protect the screen itself (and the bypass capacitor)
Why protect the screen in a bad tube?
>against overvoltages when the flashover connects the screen to the
>anode.
>
>3. Protection for the screen supply against (a) the overvoltage coming
>back from the screen, and (b) any actions that screen protection circuit
>takes (eg applying a short to ground).
>
>4. Shutdown circuits (fuses, breakers etc) to impose a cutoff on the
>whole scenario.
>
>Note: this is AND logic! To prevent a microsecond arc from turning into
>a soldering iron and checkbook job, the amplifier needs ALL of the
>above.
>
>So maybe it isn't so simple, after all...
Why would an arc stop after one microsecond? If the tube is gassy,the
party is over.
>
cheers, Ian
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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