[Johnson] 866A catastrophic failures?

[Roy Morgan]

At 09:41 AM 1/8/03 -0800, kwylow zinjanthropus wrote:
>Can anyone cite any catastrophic failures that occur with equipment using 
>866A's (merc vap recs) that's a direct result of the same?

Not catastrophic, thanks to the engineers, and with 872A's:  A TMC GPT-750 
runs 3000 volts in the plate supply at about 6/10 amp.  A pair of them not 
fully re-vaporized caused the plate over current relay to trip multiple 
times till the darned things got fully warmed up.  The typical "failure" 
mode is "flashover" where the tube(s) conduct pretty much equally in both 
directions.  In this transmitter, the plate transformer is quite overrated 
for the job and no harm was done.  The plate overload relay did its job and 
kept any damage from happening.

What protection does the Courier have in it's plate supply 
circuit?  Fuses?  Over current relay? Both?  How much overload can the 
plate transformer take with no harm?

>Just wanted to know as I'm looking to retrofit a Courier with a pair of 
>these jewels.

866A's are nice. I like the glow,and run them in my Valiant.  The Valiant 
does not come anywhere near the max ratings of the 866.  I suspect that the 
Courier is the same  I cannot say from either experience or hearsay what 
the life of a pair of 866A's is.  You will be wise to run them with just 
filaments for an hour or two before applying plate voltage.  A pair if 
3B28's is much safer. Much.  Solly state rectifiers, if robust, are safer 
yet, and the increase in plate voltage is non-existent or insignificant.

Roy


- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
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