wrote:
>
>On Jul 11, 2005, at 11:36 PM, Ian White GM3SEK wrote:
>
>> wrote:
>>>>>
>>>>> ... by stopping the flow of current into the screen PS.
>>>>>
>>>> If the "glitch" is an arc from anode to screen, the mains fuse won't
>>>> protect the screen at all.
>>>
>>> A tube that arcs from the anode to the screen is already a goner, Ian,
>>> so there is nothing of value to protect.
>>
>> That simply isn't true. Well designed power supplies can protect the
>> tube, the anode supply and the screen supply against a wide range of
>> faults, including arcs. Many of these faults are completely
>>recoverable.
>
>What condition would make the anode arc to the screen?
As many folks will remember from previous repetitions of this topic,
reasons for anode-screen arcs include:
1. Loss of load at a phase angle that results in extremely high anode
voltages. With certain amps, you can make this happen anytime you wish:
remove load, apply full drive - BANG.
If the RF voltage is high enough, the anode flashes over to the next
lower-potential object within sight. For a tetrode, this is usually the
screen, so the screen supply experiences a very large negative current
input from the tube.
For a GG triode, the RF arc will hit either the chassis if the arc is
external, or the control grid if the arc is internal. An external arc to
chassis probably won't harm the tube, but an interna arc to the grid
certainly can.
The so-called "glitch resistor" in the B+ supply is a vital part of
every tube amplifier, because it limits the peak current: it's simply
Ipk=V/R. This resistor is not intended as any kind of fuse - it simply
limits the maximum possible current for a few vital milliseconds, until
something *else* has time to switch off. Eimac Bulletin 17 covers this
very adequately.
If the power supply provides adequate over-current protection for the
screen and anode supplies, and adequate over-voltage protection for the
screen bypass capacitor, then arc faults due to loss of load can be
completely recoverable. Re-connect the load, press the RESET button and
the amp will come back online.
2. Bugs in the tube chimney. Don't ask me how they got in there, but in
FD situations I have sometimes found an earwig or a moth bridging the
anode and the screen. Again there's an arc and the amp trips out, but
this time it won't come back online until the charred corpse has been
removed.
3. Internal arcs due to gas release. I know that Rich doesn't like to
accept this; but I go along with the vast majority of tube manufacturers
and professional RF engineers who have seen enough evidence to know that
it happens - so we protect against it. Again, these faults are usually
self-clearing, and the amp will come back online when the RESET button
is pressed.
4. Any other reason the amp may choose! The most important practical
point is to accept that amplifiers occasionally DO arc, so they need
some protection.
--
73 from Ian G/GM3SEK
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