To: <amps@contesting.com>
> Date: Wed, 20 May 1998 18:16:48 +0100
> From: Peter Chadwick <Peter_Chadwick@mitel.com>
> I'd suggest that Rich has presented more than enough evidence to suggest
> that the answer to 3 is YES.
Anything that causes excess grid current can damage the grid. That
includes oscillations as well as fundamental frequency drive...or
even dc.
Once the failure happens and the tube is pulled, you have no idea
WHAT frequency caused the heat. Just that it happened.
> To 2, I am still totally unconvinced that you can get enough current from
> the space charge and the filament in 3-500Z to do this. So I'm a flash arc
> believer.
Me also, since the saturated current isn't much more than ten
amperes.
> To 1, I'd say 'It all depends'.
That's safe.
> My idea of the parasitic problem is that the lead inductance (including that
> of the tune capacitor) resonates with the stray capacitance to give an anode
> tank circuit at VHF, which resonance is at some frequency where the feedback
> is enough to cause oscillation. Increasing the amount of stray inductance
> has been known to stop the parasitic by moving the resonant point of the
> 'stray' tank circuit to the right point in relationship to the self
> neutralising frequency of the tube. I suspect that's why some amplifiers get
> away without a parasitic suppressor - until the tube is changed.
Do you mean the type of tube?
I suspect the reason the amp acts up when a tube is changed is the
very same thing we run into during manufacturing. One particular tube
has about 60% that arc when either RF or enough dc is applied.
It certainly does not surprise me the user, when sticking a "new"
tube in his PA, might think the thing oscillated IF he thinks an arc
is always caused by a parasitic.
> After all this, I don't disagree that a good suppressor can be made with
> nichrome tape. I don't say that it's the only way, though. The measurements
> of Q, Rp etc don't tell us too much, because the tube sees the total VHF
> tank circuit, not just the suppressor.
Correct as I see it.
> By the way, has anybody experienced holes in the envelopes of 3-500Z caused
> by parasitics?
No, but the Chinese tubes used some very poor glass that experienced
a lot of melt-throughs. I ran temperature tests on the glass in
Chinese 3-500Z's (they were also sold as Russian tubes through what
was called mislabeling), and the glass failed at 185 degrees C.
Vectronics tried to use these tubes in their 3-500 amp, and
experienced virtually 100% failure over a few weeks of operation. In
desperation they installed nichrome suppressors, but the problem
continued. They cured the problem by switching to Eimac.
> I've seen it in other tubes, where a component close to the
> tube has enough capacity to allow a VHF current at the parasitic frequency
> to flow from the anode to the component through a localised area of the
> envelope: the dielectric losses are enough to heat the glass, and a pinhole
> appears. Seen it in 6146s and 572s.
I've seen a melt through in may inadequately cooled tubes. How do you
know it was a parasitic? Did you ever calculate current required to
heat the glass with RF heating?
73, Tom W8JI
w8ji.tom@MCIONE.com
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