[600MRG] How tough are your tubes?

Fri Jan 25 22:43:48 CST 2013

Love this post!!!

Those 4- series tubes would generally hold up pretty well if not for too 
long a time. Good thing it happened while you were close by the big 
switch!!!

Doc.

----- Original Message ----- 
From: "Ralph W5JGV" <w5jgv at w5jgv.com>
To: <600MRG at w7ekb.com>
Sent: Friday, January 25, 2013 22:22
Subject: [600MRG] How tough are your tubes?

I had an "interesting" anomaly occur this afternoon.

I was sitting in the hamshack, working at my business computer, when I
happened to notice a yellow reflection on the screen.

I turned around and saw a bright yellow light shining on the wall behind
me.  "What was it?" was my first thought.

Glancing to the left, I suddenly realized that the source of the
beautiful yellow light was the incandescent anode of the 4-400A residing
in my 600 meter amplifier!!

"OK S###T!!" was my next thought.

Knocking my chair backwards, I leapt up and dove for the amplifier plate
switch. As I reached for it, I noted that the plate voltage meter was
steady at the 3,200 volt mark, but the plate ammeter was PEGGED past the
1/2 ampere top of the scale! Cripes!  How long had THAT been going on?

I flipped the plate and filament switches to the off position, and
watched the glowing anode slowly sink into darkness. Maybe
permanently...

After a few minutes to let the blower bring the tube down to a more
normal temperature, I fired up the filament - that looked normal anyway
- and then cautiously flipped the plate switch on.

Peggo!  1/2 amp plus plate current.

I quickly determined that the negative grid bias had vanished, allowing
all those eager electrons to make a mad dash through the now wide open
grid to the plate.  I spun the grid bias adjustment knob back and forth
a couple of times, and things came back to normal.  I adjusted the bias
so the pot is now in a slightly different position than it was before,
but I will replace the bias adjust pot to eliminate that as a possible
cause of the problem.

...let's see now;  3200 Volts x 0.5 Amperes = 1600 WATTS PLATE
DISSIPATION!!   That's only 4 times the rated value for the tube.

Try THAT with your sand-state devices!

<G>

73,

Ralph W5JGV - WD2XSH/7

_______________________________________________
600MRG mailing list
600MRG at w7ekb.com
http://w7ekb.com/mailman/listinfo/600mrg_w7ekb.com