On Jul 12, 2005, at 2:14 AM, Ian White GM3SEK wrote:
> 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.
When load loss happens, the first thing to arc is the Tune-C. The
resultant metal-vapour arc reduces the anode potential to a few tens of
volts.
> With certain amps, you can make this happen anytime you wish:
> remove load, apply full drive - BANG.
The bang is not from an arc in a vacuum.
>
> If the RF voltage is high enough, the anode flashes over to the next
> lower-potential object within sight.
In a typical amplifier, the Tune-C breaks down at less than 1/3 of the
anode-grid breakdown potential -- so how can the anode potential keep
rising when there is a metal-vapour arc at the Tune-C which limits the
potential to 20 or so volts?.
> For a tetrode, this is usually the
> screen, so the screen supply experiences a very large negative current
> input from the tube.
Flashover from the positive anode produces negative screen current?
Now there's a new one.
>
> 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.
As I see it, the top priority is to dissipate most the energy stored in
the filter-C outside the tube -- i. e., in the glitch R. With 15-ohms
and 25uF, this happens rather expeditiously.
> 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,
No window-screen over the air inlet to the blower can do that.
> 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.
RIP
>
> 3. Internal arcs due to gas release. I know that Rich doesn't like to
> accept this;
The reason is that I have autopsied a fair number of kaput electron
tubes and I have yet to see any such arc craters or barnacles -- even
in tubes that are gassy. The idea that a big-bang being heard is
certain evidence of an arc in the vacuum of the tube's envelope is
laughable.
Rich Measures, 805.386.3734, AG6K, www.somis.org
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