I wanted to add to Manfred's comment. I have noticed that if you
severely overload a tube, it may flash over from the evolution of gas
also. The gas is, in this case, evolving from overheated elements
which are taken to a higher temperature than they have been
accustomed to or were processed at in the factory. This gas may get
consumed into the getter inside the tube after a short while, and the
tube will continue to work fine for a long life. Having a gassy tube
doesn't always mean failure.
I had a 3-500Z in the switch tube socket of a Continental 314R1 PWM
transmitter do this. It was overloaded because the damper diode was
shorted so that when the switch tube began pulsing at 70 KHz, with
very low modulation, it got very hot and the anode glowed bright
red->almost yellow hot. There was a deep glow in the tube and the
circuit breaker kicked the HV off along with a plate overload trip.
Once the defective diode was replaced, the tube has continued to play
fine. Later I switched its location with one of the RF tubes, as the
plate voltage would be lower there.
At work where we have large tubes (superpower triodes and tetrodes
from Burle) we monitor the internal vacuum continuously with a ion
pump. When we are conditioning a new tube or an old tube that has
remained in storage cold, we slowing increase the duty factor of the
RF until we are at least at 100% or more of the future operating
point. We hold this for a number of hours. During this conditioning,
the tube electrodes and envelope continue to outgas and cause flash
overs which trip off the power supply and fire a crowbar in shunt
with the B+ to discharge capactors without depositing a lot of energy
in the arc.
If we didn't do this, the tube would have a short life indeed. The
gas goes away, it is taken by both the getter and by the ion pump in
this case.
73
John
K5PRO
>Message: 3
>Date: Mon, 23 Apr 2007 02:41:16 +0000
>From: Manfred Mornhinweg <mmornhin@gmx.net>
>Subject: Re: [Amps] Rehabbing that dusty old
>To: amps@contesting.com
>
>> Thought I saw a faint blue glow after an extended keydown
>> into the dummy load. The first CW QSO, however, into a resonant
>> antenna brought a brighter blue flash in one tube, popped breakers on
>> the primary, and the that familiar cooking something electronic smell
> > ('The smell of progress!').
>
>Sounds like you have a gassy tube.
>
>> Any way other than destructive testing to figure out if the tube's
>> going to arc again?
>
>Applying some high voltage through some sort of current limiting device.
>But it's not simple. You need to apply at least twice the normal high
>voltage of the amp, no RF drive, current limited through a fast high
>voltage fuse, and then adjust bias to control the current... not very
>practical.
>
>> Any other suggestions ( I have a couple of NEW 3-500zg's, but really
>> want to see if these Eimacs can be put back into service )?
>
>In my experience, when a 3-500Z starts flashing over, it's ripe for
>mounting on a nice wooden base and exhibiting in your living room,
>rather than in the amp. They make great decoration! But they are no
>longer good for practical use at high voltage. They will flash over
>again and again.
>
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