>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
>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.
** Rich has never seen it in tubes that use under 10kVDC.
>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
** impressive indeed.
> 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.
** this discussion is about tubes that are commonly used in Ham
amplifiers -- and I don't mean JA contest stations that run 8349s and
the like that use 22kV, 10A anode supplies.
>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
>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. "
** you won the bet, John. However, Ham tubes don't generate X-rays
and the stuff you work with could do chest X-rays of an elephant.
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,