[SignalOne] 8072

[EDWARD SCHUMACHER]

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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