I recently acquired a few Alpha amplifiers that needed repair.? Along the way I decided to test all 10 tubes I had on hand.?
I put each 3cx400A7 in one at a time, let the filament run for a while to burn in. I Applied 1500 volts for 10 minutes or so.. then Keyed up While monitoring Grid and Plate current for 5 minutes.
I noted idle current and then applied 15w RTTY drive on 20 meters and noted output power.?
Out of the 10, Two where bad, one had open filament and the other a grid short.? The other eight where all good.
Output power for those eight tubes:
175 w
175 w
175 w
150 w
140 w
120 W
100 w
75 w
I let them run each tube keyed for 5 minutes or so and power was stable, and no tuning would produce more.? All tubes ended up at pretty much exactly the same tune and load.
ONE of the new tubes made 50W but after a cycle of key OFF and Key ON power rose up. 75, then 100, then 120.? It ended up at 175 over about a 10 to 15 minute period.? Interesting to see it rise up like that.? Something was coming back to life.
ALL other tubes made power instantly.
What is interesting is that even the tubes that have lower output power and gain at 15w, make FULL power in the amplifier.? ?I can take the two weak tubes, 75 and 100, and put them in the amp and make 1500w out with a woodpecker.??
I can put the best three in the other map and make 1800w.? ?I can remove them and put the weakest three in the amp and make 1800w.
Is there a better way to test power tubes?? Or do you really need to just test them to full output?? I had reservations about applying 2700 volts to them right away, that's why I used 1500v and low drive. Some of the tubes where new and marked 1980 so some run time is probably advisable at low voltage.
I guess the drive and or added B plus is overcoming the loss of gain?
C
