[Amps] Tetrodes

[R@contesting.com]

On Jul 12, 2005, at 7:37 AM, Ian White GM3SEK wrote:

>   wrote:
>>>
>>> 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.
>
> Not in every amp.

A healthy 3-500Z typically has a grid-anode withstanding ability of 
 >20kV.  For a gratis pint of Guiness, name one 3-500Z amplifier that 
uses a 25kV Tune-C
>
>> The resultant metal-vapour arc reduces the anode potential to a few
>> tens of volts.
>>
> Correct.
>
>>> 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.
>
> I never said it was.
>
>>>
>>> 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.
>>
>> Flashover from the positive anode produces negative screen current? 
>> Now
>> there's a new one.
>
> Yes. Sit down and work it out.

Okay.  An 8171.  The anode potential is +8kV, the screen potential is 
+1500V.  Screen current is +120mA.  However, as luck would have it, an 
anode-screen arc takes place, temporarily raising the screen potential 
to c. =8000V. and now we have negative screen current?  Hello!
>
>>>
>>> 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.
>
> See the photos referenced in my other posting - which you have been
> pointed to several times before.
>
I discussed these photos in my post but it seems to have disappeared.