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Author Topic: NOS SVETLANA 572B TUBES  (Read 2922 times)
N0BUM
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« on: September 20, 2008, 04:06:30 PM »

HELLO ALL
I HAVE A CLIPPERTON L LINEAR AMPLIFIER THAT I JUST GOT A NEW QUAD SET OF NOS SVETLANA 572B AND I WOULD LIKE TO FIND OUT WHAT IS THE BURN TIME FOR THESE TUBE IF ANY BEFORE I PUT IT ON THE AIR.
THANK YOU
BOBBY
n0bum

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n0bum@comcast.net
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W8JI
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« Reply #1 on: September 20, 2008, 04:44:13 PM »

It won't matter. Those tubes getter by the anodes getting hot, and they won't get hot if you only run the filaments.

By the way, when Ameritron released the AL572 it had Svetlana 572's and more than half the tubes were bad out of the box. I hope the ones you have are better.
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G3RZP
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« Reply #2 on: September 21, 2008, 01:05:04 AM »

Tom,

What was the predominant failure mode? gas? Shorts? opens?
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W8JI
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« Reply #3 on: September 21, 2008, 03:47:45 PM »

I assume the predominant failure mode was gas. It could have been sharp edges or close element spacing inside the tube, but they acted like they were not pumped down enough. Whatever the cause, they had low breakdown voltage.

The 572B is supposed to run at 2200 volts or so full load.

This means the PEAK anode voltage under ideal conditions would approach 4400 volts. If someone underloads the amp, voltage can be higher. It can reach the peak RF voltage across the tank plus the supply voltage! Since the air variable breaks down at 3500-4000 volts, the peak anode voltage under conditions of load fault can reach 6500 volts.

In a good Cetron tube or a GOOD Chinese tube, we could normally high-pot 572's to 8 kV or more. They never arced.

With the Svetlanas, the vast majority broke down at around 5000 volts, far too low for a tube rated at 2250 volts operating voltage. Cooking them with hot anodes at low voltage did seem to help, but not enough.

73 Tom
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