[Boatanchors] Linear Amp arcing

[Bob W5UQ]

Have a liner that uses 811's or 572B's?  Has it arced and blown grid 
resistors, fuses, etc.?
  Here is something that I found out from experience.

I have had the Ameritron AL811 in the past and it blew a couple of 
times.  Now I have an AL811H.
Both had problems with the final tubes.  I thought it was me or 
parasitics.  Blew a hole in the plates and
put a dimple in the glass envelop.

Sure glad I had a spare pair of pants available when they blew.

Years ago I replaced the 811's in my AL811H with four brand new 572B 
tubes. They are more rugged.
Haven't blown holes in their plates, yet.

Recently there have been three occasions where one of the four grid 
resistors blew.  I heard arcing and loud pops.
Of course I turned off the amp immediately.
The problem of arcing occurred when the amp was just sitting there, not 
being keyed, but it was on, had been on for several minutes.... and was 
ready for keying. (I was on CW)
I had marked the tube that the resistor blew on the other occasions.  I 
had also moved the tube to a different socket each time.

The other day it blew again so I decided to do some testing.  First I 
checked each tube in my tube tester.  Three of them checked out at a 
gain of 70 to 75 and one maverick was at 85 to 90.  (Relative readings)
The maverick was the one that had the grid resistor blow.   Hmmmmm.  
Interesting.  So I concentrated on it.

No leakage.  No shorts showing up.... so I turned up the filament a 
notch.  (Heathkit IT17 Tester)
I left it to sit and COOK, thinking that maybe it was just a bit gassy.  
Thought I'd turn it up another notch later.

After a couple of hours, I went over to check on it and found it was 
showing a SOLID short.
Well.  That's a good indication of a problem.

I had just put the three 572B's back in the linear amp and have been 
using it.
Actually I can't notice any substantial loss of power.  Definitely not 
enough to matter.
We will see how that works as time goes by.  I'll let you know.

Good reading on linear amp arcing problems is on W8JI's website.  He 
does a good job of describing the problems.

Thought I'd pass this information along as I'm sure there are others 
experiencing these problems.
Bob W5UQ

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