...
>I have been following with great interest the thread about the
>B+ glitch resistors and guess what happened yesterday?
alas
>My glitch
>resistor opened up with a very loud bang spreading chunks of
>ceramic all over the bottom plate of my amplifier chassis. I have
>a home brew 4CX1000A amplifier with 3200 volts (that drops to about
>2700 volts under load) on the plate. I have only been using it for
>a few months now and the 4CX1000A that I put in it was a
>"previously owned" one. The glitch resistor was a 10 ohm 10 watt
>wire wound ceramic covered type ...
In ancient times, we used 10 ohm, 10w ceramic wirewound resistors, but
they had a habit of breaking down and throwing schrapnel about during a
glitch. We switched to glass-coated 10 ohm 10w glitch resistors. They
seem to do okay with up to about 3kV, depending somewhat on the size of
the filter C. For more voltage or more than average joules, we recommend
using two such resistors in series to reduce the turn to turn voltage
gradient during a glitch.
>...and I was adjusting the load capacitor
>on 20 meters when the resistor let loose. I did not notice any
>odd or unusual occorrunces when the resistor let loose. The plate
>current did not jump up or anything unusual like that....just a big
>pop and no more B+ or plate current.
>
It may be prudent to try lowering the VHF-Rp of the parasitic suppressor.
The price is roughly a 1% power output decrease at 29MHz.. OTOH, there
could be something wrong with the tube.
>... I guess the only way to see if the 4CX1000A is bad is to
>try loading it up again....but I don't want to have to keep replacing
>the glitch resistor just to test the tube out.
You could check it out prior to blastoff with a high-pot.
- The problem is that the glitch resistor arced between turns before it
let go, so the tube may have been unprotected during the glitch.
>
>BTW...I will probably end up using a "previously owned" 4CX1500B
>that I just acquired after testing my existing 4CX1000A out. I plan
>on burning the filaments for about 12 hours before loading up
>the 4CX1500B.
>
>Any suggestions that you can make to me would be highly appreciated!
>
Ok, here's one, Mike: Measure the anode/screen leakage current at
6000v with a high-pot. before you burn in the replacement tube. Then
measure the leakage after burn in. I'm betting that the leakage current
does not decrease more than 2% during burn in. If the leakage current is
above 10uA cold, I would toss the tube in the trash bin.
rich---
R. L. Measures, 805-386-3734, AG6K
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