It is unfortunate that bandwidth is wasted going over and over this
subject; some (one?) here don't seem to pay attention to the evidence
of effects in vacuum tubes, instead insisting on theories supporting
their own 'science'. In this example, however, it doesn't hold water.
Rocky Pt effect, arcs in vacuum tubes between grid and plate, cathode
and grids, cathode and plate, are commonly seen, heard and found
through autopsy.
I certainly agree with parasitics causing havoc in amplifiers. But
they are not the sole cause for internal tube arcs, as excess gas,
unconditioned materials, barnacles, outgassing parts as they are
heated, all contribute to such.
> >Selectively ignoring evidence will not make it go away.
>>
>The problem with Rocky Point Effect is that there is apparently rarely
>evidence to support it during kaput tube/valve autopsies. I have yet to
>autopsy a tube with an anode arc-mark and that includes tubes that are
>gassy due to a leaky seal. Another problem is that it seems unlikely
>that (since sound travels in air) an amplifier could produce a stentorian
>bang from an arc inside the vacuum. I have not yet found an amplifier
>that produced a loud bang that did not have a damaged VHF parasitic
>suppressor resistor. This tells me that there is a quite likely a VHF
>connection with loud bangs.
The key word is 'rarely'. In other words, it happens, but the writer
hasn't seen it often.
I have sent Rich photos of large power triodes with 0.25 inch square
holes burned through the grid, from a massive plate to grid and
cathode arc. I have seen the inside of tiny planar triodes with
similar chunks missing, although on the scale of a mm or less. One
sits in my neighbor's office at work.
I have heard internal arcs, or more likely the ping of the metal as
it is hammered by the discharge. Lucky they were all on current
limited supplies, so it was merely a ping. When our 4CW250,000B
tetrode series tube modulators arc over (happens in very old tubes
with high hours, somewhat more gassy, and having warped structures to
cause more intense e-beam on spot of anode) a champion JT-8 sparkplug
across the screen to chassis sounds like a quarter stick ((M80) of
dynamite. We grind off the tip of the plug to make a nice protector
from them, for 3-5 kV breakdown, by the way.
If I get around to it, which is unlikely in the coming 2 weeks, I
would like to photo a tube i just cut open, with a plethora of anode
arc marks to the grid structure. Its convincing enough to our team of
RF engineers and techs. Also to the manufacturer, Burle Industries.
When we have tubes rebuilt at Econco, they refer to the debarn'ing
step as part of their conditioning process. They successively raise
the voltage across a tube, while hot, and let discharges debarnacle
the whiskers and other trash which form as a new tube is operated. As
part of conditioning of all tubes, including smaller ones with
handles, it is routine to allow them to getter the gas molecules ,
while kicking up the voltage. And when they do arc, they tend to
improve threshold of breakover in successive bangs. Odd that all this
evidence doesn't seem to mean much to one ham, and even odder when we
all try and convince them. I suppose the thing to do is to let it
rest, and leave him to his theory, as odd as it sits against much
evidence. But when advice is given to newcomers and curious hams
which sits contrary to natural processes, its time to explain it ONCE
AGAIN. Here it is. Arcs do occur within tubes, due to many causes.
Parasitics, and all of the reasons explained above, contribute to
this.
I would bet that the response to this might be....."But those are
really big tubes with high plate voltages, not like what hams are
talking about. " Rocky point and vacuum arcs do occur in small spaced
elements in small tubes with smaller voltages, not as often as the
big boys.
73
K5PRO
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