[Amps] 2 ways of Gettering...
dave at uk-ar.co.uk
Wed Sep 14 06:54:01 PDT 2011
Gettering is a chemical process, the only "Thermal" thing about it, is
that the "Getter" is fired/activated by heating, to create the active
area that absorbs impurities etc, with glass envelope tubes at least..
In external Anode(Plate) tubes with a "Plate Getter", it is *NOT* the
same Plate(Anode) where all the electrons end up! Overheating those
tubes will lead to pain!
That running a tube that wasn't designed to have it's Anode(Plate) at a
cherry red temp' can damage it beyond redemption, if just by physical
dimension changes that may not fully recover when it cools. The amount
of radiated heat internal to the tube, can also damage grid structures
in the same way, even overhead the Cathode, leading to early failure.
Cathode material evaporating and condensing on one or more grid
elements, leading to all sorts of odd behaviour when the amp is driven
I've even seen the glass envelope soften enough on some internal
Plate(Anode) tubes to suck-in at one spot, then Bang!
As mentioned, hot things in a vacuum "Outgas" releasing trapped gas
molecules. In normal use it's not a real concern, as it's at such a low
level, the electron flow would ionise those molecules, where upon they
will then be attracted to and bond with one internal structure or
another where they will then stay. Until such time as that structure
itself is cooked re-releasing them into the vacuum again. Plus, with
most power gridded tubes, the volume of the vaccum is huge, in
comparison to the outgassing. Unlike some more specialised devices,
TWT's for example.
With most vacuum tubes, one of the most important factors, is *Not* to
overheat them. As if nothing else, it stresses the metal/glass an/or
metal/ceramic seals way beyond what they are designed for. Ultimately
resulting in fractures, and a "soft" tube, or sudden total failure.
Re-fireing a Getter (if even possible in some cases) is a specalised
process, and cooking the Plate(Anode) will more often than not, not have
the desired effect, but will have other undesired ones!
Best also to use "Tempilaq" or similar on the tube seals of a new amp,
or one being repaired or modified, if you have suspicions on the cooling
front, or are going to run it that hard. Different rated coatings melt
at different temperatures, so you can (with some effort) gauge how hot
your unobtanium tubes are getting in use.
No affiliation, other than a happy user in the past. Good stuff.
Don't forget the Filament connections and seal area too! I've seen
professional amps, where the plate cooling was well thought out, but the
tube base cooling was not. Resulting in early failure.
Remember, like a car engine. If you want more power out, just feeding
it more fuel and air, though that will get you what you want, will also
liberate more waste heat that must be removed, or Bang!.
I was once told by a tube manufacture, to keep the seals down to 200'C
or lower. If the resulting airflow caused the tube to leave the
socket! They had parts available to prevent that!
Also, not to use any stainless steel fixings that would be carrying RF,
or securing parts carrying the RF currents, in particular, in grounded
grid cavity amps at VHF.
Please please please, "Trim" replies to the group. One message
recently had over 100 lines of commented text, most of it totally
irrelevant. Remember, not everyone is using a PC on the website.
------- Original Message -------
Actually the getter is there all the time and what you are speaking
about is how to reactivate the gettering process.
By heating "some" metallic getters such as coated plates, they attract
more ions and molecules of gas in vacuum device. Running the plate with
color is risky in itself as it may liberate additional molecules from
the plate which then have to be absorbed. Some tubes may be worsened
after this process.
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