[Amps] "Tubes 201" - How Vacuum Tubes Really Work

[R L Measures]

On Jul 23, 2006, at 7:18 PM, Chris Howard wrote:

>
>
> I don't have a dog in this fight.  I've enjoyed and appreciated
> you guys educating me.
>
>> From where things ended up, it looks like some body of
> people say that the grid fuse/resistor idea really works
> and they are convinced their experience proves it.

Since the original article on anode circuit parasitic oscillations  
appeared in the October 1988 issue of *QST*, 6000 or so HF amplifier  
owners have installed one of our low VHF-Q parasitic suppressor  
retrofit kits.  These kits have two purposes:  Lower VHF gain by over  
50% and protect uncheap amplifier components such as tubes and meter  
movements from possible damage. Tube protection is afforded by a  
frangible grid fusing R and a sturdy current-limiting R in the HV+ PS  
lead. So far, no user has reported the occurrence of a catastrophic  
event taking place when a grid fusing R blew.  IOW, the grid floating  
catastrophe theory doesn't float.
>
> There is another body of people that use reasoning/logic/etc
> to say that it could never work.

There were many who said Igor Sikorsky would never be able to get his  
helicopter to fly.
>
> I myself have a bit of a hard time with the universal
> negative: "that will never work" type of argument.
> But I appreciate that folks on both sides have a lot
> more experience that I do.
>
> The real lesson for me was that there is such a fundamental
> disagreement on what is going on inside that little glass
> box.  It definitely doesn't look like settled doctrine.
> And that's OK with me, I can live with a little bit
> of ambiguity.
>
> The floating grid issue seems kind of intriguing.
> Maybe I'll get out some old octals or something and play
> around with that and see what happens. The "Tubes 201"
> guy seems to go both ways, one sentence says never float
> the grid, and the next sentence seems to allow for it.

Some pesky problems with the floating grid catastrophe theorem are:
1. The two most common grid materials, Au and Mo, are poor emitters  
of electrons.
2. How can the grid get a low-emissivity element hot enough to begin  
emitting a catastrophic quantity of electrons if the fusing element  
opens and current though the grid stops before it gets hot?
3.  Arguing that a floating grid is a bad (after a glitch blows the  
grid fusing element) because numerous books advise against floating  
the grid during normal operation sounds a whole lot like straw-grasping.

cheers,Chris

>
> 73
>
> Chris
> w0ep
>
>
>
>
> On Sun, 2006-07-23 at 13:04 +0200, Peter Chadwick wrote:
>> Tom said:
>>> Not quite a peer reviewed engineering text is it Will?<
>> He does tell you not to leave grids open, though.
>> When you have grid current, you have grid dissipation. Which is  
>> why tubes designed for Class C had carbon coatings or whatever to  
>> reduce emission. See the RCA TT4 Tube Manual. It's fair to say  
>> that flash over dumping a lot of current into the grid could well  
>> heat it enough to get emission.
>> Whether 'arc' is the right term is arguable, in that 'arc' implies  
>> a plasma effect, which you don't get in a vacuum. However,  
>> 'flashovers' in high power tubes with a good vacuum have been  
>> written up many times over the years, with a number of  
>> explanations. In tube manufacturing circles, they were said to be  
>> 'barnacles', and could be removed by controlled energy flashovers  
>> - there was an article in about 1934 in the IEE journal. As I  
>> recall, flashovers are more prevalent with new tubes.
>> Sorry, I just don't buy grid fuses ( and even more so, resistors  
>> as fuses) as a good idea. I believe I'm not the only one, though.
>> 73
>> Peter G3RZP
>> _______________________________________________
>> Amps mailing list
>> Amps at contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>

R L MEASURES, AG6K. 805-386-3734
r at somis.org