[Amps] SB-220 update & gettering

[Rick Stealey]

You could try just keying up the amp with no drive and note the total plate power dissipated.  Something close to rated plate dissipation is best if the cooling is adequate.  Otherwise you may need to build up a little bias supply so you can vary the plate current.  If you did this, I suggest removing plate voltage from one tube so your power supply and cooling will only be working with one tube at a time.  (Your SB220 will not be happy pumping out 1000 watts of dc continuously).  Also in the event of an arc you will know which tube did it.  Do not use a variac on the whole amp as this would reduce the filament voltage.

These suggestions are for tubes like yours that are not known to be gassy,  If you had questionable tubes you would want to run them at more like 1000 volts, adjustable bias, and be prepared for an arc that would blow the line fuse.  You don't really want that to be occurring with a good, working amp at full plate voltage.

I've attempted gettering some 3-500s using a variac on the B+ supply.  Bad tubes can begin drawing a LOT of current in the range of 1200 volts.  Really gassy ones will like up like a mercury vapor rectifier in a Viking Valiant.

Rick  K2XT


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