[SignalOne] 8072

Sat Dec 30 15:02:23 EST 2017

Don,

Whether filament only degassing will work depends upon how the getter is activated.  I build a homebrew amp about 15 years ago using three Russian 4CX800 tetrodes and the filament does activate the getter and I ran them for 24 hours (as instructed by the factory) before putting them into service and they have been fine.  But a lot of tubes, like the popular Eimac 3- series triodes designed for zero bias grounded grid operation, use the anode as a getter and it doesn’t do anything until it is showing some color.  Older tantalum plate tubes are the same in using the plate to absorb gas; some tubes use the anode material itself as the getter while others use a substance applied to the anode.  Tantalum is an excellent getter except it has to operate at very high temperature for it to be effective which is why you don’t want to operate those old tubes too far below their ratings on a continuous basis because the anode won’t get hot enough to do its job.  The material used for more modern tubes like the 3-500Z activate at a dull red barely noticeable color unlike the yellow glow seen and needed with the pre-war popular tantalum plate high power tubes.

With these anode activated tubes the safest way to bring old ones back into operation is to either build a test stand or temporarily convert the transmitter/amplifier so that the tube can be encouraged to dissipate a lot of power at low voltage.  The goal is to generate heat without having to use a high enough potential to result in flashover.

For example with the SB-220 temporarily wire the plate transformer so that it is operating with its primaries in series as they would be for 240 volt operation but feed them only 120 volts however the filament transformer and fan MUST be fed the correct voltage so that filament voltage and fan speed is correct.  Run the amplifier in the CW/tune position which should provide a bit over 1,000 volts potential with the rewired primary fed with 120.  Temporarily short across the zener diode use to provide operating bias.  Put the amplifier into transmit and note the plate current, it should be sufficient to cause the plates to “blush” after they cook for awhile.  If the current draw isn’t high enough you may have to go further and try some positive bias on the grid.

A test stand for burning in older tubes would be nice but if you only rarely need to do it a simple temporary and completely reversible modification to the equipment may allow de-gassing of those tubes that use anode activated gettering.  Note that you should always supply correct filament/filamentary cathode voltage and proper cooling air during this process.

Note that low power transmitting tubes and receiving tubes with the “silvered” getter deposited on the envelope do not require heat activation.  If the getter material is clearly reached its maximum or there is a broken seal then that tube is worthless.

I have heard reports of people applying a high potential between the cathode and anode of an otherwise unpowered transmitting tube to ionize and remove the gas but if this actually works I do not understand how it works.  The getter works by bonding with the contaminants but I don’t think you can otherwise make gas disappear via ionizing it.

Note that blue spots appearing during operation upon the glass of many beam power tubes is normal and is simply the electron stream causing something in the glass to fluoresce and this is commonly seen on 807 tubes and does not indicate a problem.  One of the weirder cases I saw of this was after completing restoration of a 1930s era Hallicrafters SX-17 and the blue spots on the glass of one of the two push/pull output tubes was winking on and off at about a 1 hertz rate.  The receiver worked with no audible artifacts but I replaced the tube with another which had steady instead of blinking spots.  Apparently some odd combination of that tube and output circuit were creating a sub-sonic oscillation.

Rodger WQ9E

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________________________________
From: signalone-bounces at mailman.qth.net <signalone-bounces at mailman.qth.net> on behalf of Don Merz via SignalOne <signalone at mailman.qth.net>
Sent: Saturday, December 30, 2017 1:03:15 PM
To: SignalOne at mailman.qth.net; EDWARD SCHUMACHER; EDWARD SCHUMACHER via SignalOne
Subject: Re: [SignalOne] 8072

This list is for discussions regarding collecting, repair, use and sale of Signal/One radios and accessories.

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.
 This all sounds like what I have read The gassy condition occurring when transmitting tubes sit unused for long periods is, I think, pretty much universal across all transmitting tube types. The conventional wisdom seems to say that all transmitting tubes that have been sitting unused for long periods should be run with filament voltage only to try to burn off the gas. I have seen recommendations to run the tubes both above and below the rated filament voltage. Does any of it work? Can anyone say for sure?
This is getting to be an issue like whether or not CDs will contain any readable data in 50 years...
73, Don Merz, N3RHT

    On Saturday, December 30, 2017, 10:58:33 AM EST, EDWARD SCHUMACHER via SignalOne <signalone at mailman.qth.net> wrote:

This list is for discussions regarding collecting, repair, use and sale of Signal/One radios and accessories.

Documentation on Signal/One radios can be found at https://nam04.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.hamanuals.com&data=02%7C01%7C%7C55653787cc7a4aad7a2208d54fb89b36%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636502576648728961&sdata=a8i4KnD2jLaSG8Y%2Bx8KaKmvCs4PJzMD4K0QLV7WFw00%3D&reserved=0 and several other sites on the Web.

.
I agree that it is very hard with the usual equipment to satisfactorily test any of the higher power transmitting tubes though my old tester will at least emission test up to the 811.That being said, it is somewhat risky in this case in my experience to test by use in the circuit.

The 8072's were notorious for becoming gassy sitting on the shelf and the problem was so well known that inventories would be cooked before being put into service. That is, plugged in to a bench setup where filament was applied for several hours in the hopes of the getter restoring the tube. A gassy tube can flash letting high voltages get into other circuits with disastrous results as happened with my CX7 with a NOS tube.

There are bench test setups for verifying the gassy condition but I have not yet set one up here because I don't have a circuit yet for the tetrodes. I would at the least go through the cooking procedure as a preventative measure before putting any NOS 8072 in service now. Took me quite a while to locate and repair the damage to my rig but at least it was repairable.
Later inspection of my failed tube showed what seems to be a flashover path externally on one of the ceramic base rings. This was probably the source  of faint popping sounds before the rig quit.
Another weak point of the 8072 was the torquing of the grids which also resulted in shorting hence the need to have the added fuses for screen protection which became a factory add also. However, my rig is still working with what I believe is the original final and that is what I keep ending up with after killing two other 8072's, a rebuilt that had been run too hot and the NOS faulty tube. Hope this helps.
73 ... Ed, WA9GQK

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