Hi Ian...
I have no doubt that we saw many "Rocky Point effect" flashovers in the
3CV1500A7 (vapor cooled 3CX1000A7) tubes we used in our first amps - the
ALPHA SEVENTY c. 1970. The BANG occurred much more often in standby (tube
biased beyond cutoff) than when drawing plate current, and it was LOUD.
Reason? The arc was in a vacuum. But until we got smart and installed
surge-current suppression resistors ("glitch resistors") in the HV leads,
the 25 uf filter cap charged to ~4 kV discharging through the tube arc
often blew the plate choke into 200+ neat half-turns of magnet wire!
Eimac engineers referred to the cause of the flashovers as "barnacles," but
your description is much more descriptive. These flashovers usually
occurred very early in tube life - more often than not during factory
checkout of the amplifiers - and were self-clearing after two or three
flashes. We also experienced frequent new-tubes-in standby flashovers in
early 8877s and 8874s through the late 70's, but in the last 15 years or so
they seem much less common.
But the BANG from those RF chokes disintegrating during a Rocky Point arc
during checkout of early '70V made several of us pretty jittery about
pushing the power-on button the first time! Proper fault current limiting
resistors tame the phenomenon to the point that many times the plate
overcurrent relay or breaker doesn't even trip.
73, Dick W0ID
-----Original Message-----
From: Ian White, G3SEK [SMTP:G3SEK@ifwtech.demon.co.uk]
Sent: Thursday, March 18, 1999 5:36 PM
To: amps@contesting.com
Subject: Re: [AMPS] More on the SB220
Carl wrote:
>
>
>
>On Thu, 18 Mar 1999 15:48:44 -0000 Peter Chadwick
><Peter_Chadwick@mitel.com> writes:
>>
>>Could it be that you have a 'flash arc' or 'Rocky Point effect'?
>>Although
>>the voltage may be considered low for a glass tube, the effects you
>>describe
>>fit the description.
>An arc in a vacuum would not go BANG.
>Neither would the opening of a 2W resistor.
>
>Sounds like a HV breakdown to me that is external to the tube.
There could have been a silent arc inside the tube, with the bang coming
from the resistor. IMO a 2W resistor standing in the way of a near-short
across the HV supply is very likely to exit with a bang!
Very small "bubbles" of gas that is loosely combined within the
structural metal of the tube can take months or even years to diffuse to
the surface and be released, which would explain why these events happen
at very infrequent intervals. (With good manufacturing processing and a
bit of luck, they may never happen at all within the working life of the
tube.) Heating accelerates diffusion, so the event is more likely to
happen when the tube is working; but it could also happen when the tube
is cut off, because the majority current carriers don't actually come
from the cathode.
It's quite possible to have a sudden local release of gas that raises
the pressure for the few microseconds needed to trigger an arc, and then
the gas could be re-absorbed over a timescale of seconds to minutes so
that it doesn't show by the time you get a high-pot tester on the tube.
Bent filament helix? The authors of the paper that Peter cited noted
that arcs to/from the anode of a triode will easily divide themselves
between the grid and filament, and also that the currents in parallel
conductors can be large enough to force the conductors together by
magnetic attraction.
As regards the other damage such as an arced bandswitch, it's possibly
due to shock excitation by the extremely fast falling edge of anode
voltage - I'm guessing that this is probably much faster than the
circuit ever sees in normal operation. It really needs a SPICE
simulation to put some numbers on this effect, to see if we're in the
right ballpark.
A lot of this is speculation, but I'm trying to find an explanation that
fits *all* the reliably observed facts, not just a convenient sub-set.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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