[600MRG] How tough are your tubes?

[Brian, WA1ZMS]

No doubt..... "Vacuum Power FETs with Internal Pilot Lights" are tough! 

I design SSPAs at work. And built a water cooled 144MHz legal limit
2m amp with a pair of the latest Freescale devices.  They claim 65:1 SWR as
the upper limit, but the little secret is that is it's for pulse mode and
not for
CW. Same for NXP's latest BLF578XR.

But as an old FAA engineer once told me 30 yrs ago: "No matter which one
came first,
there would be good applications for both types when viewed from
the full world of RF." 

I think he had a good point.  Not sure how I'd fit a 12AX7 into my iPad!
hi...hi....

Now THERE'S a potential snake-oil market!

-Brian, WA1ZMS


-----Original Message-----
From: 600MRG [mailto:600mrg-bounces at w7ekb.com] On Behalf Of Ralph W5JGV
Sent: Friday, January 25, 2013 11:23 PM
To: 600MRG at w7ekb.com
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 




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