Even though this subject has been covered at some depth in the past, I find
myself at a loss for a solution to my latest predicament. A little
background information may help keep the "help" on track.
My rack of amps is modular in design. Each amp deck contains only basic RF
and filament supply parts. Everything else is switched between each deck as
required. This allows one HV supply to be used for all the amps. My "old"
supply is choke input and provides 3100 volts at about 650 ma. all day
every day.
When I brought up the YC156 amp, I discovered that it would quite happily
pull several amps of anode current when out of tune! The "old" supply
complained bitterly about that treatment, I got tired of resetting the
breakers, and built a bigger supply.
Against my better judgement, but since no suitable choke was available, I
used a capacitor filter. The design criteria was for 3100 volts at 2 amps
peak to handle the tune up problem. I used 1N5408's in the diode string
paralleled with metal film resistors. The diodes were all from the same
lot, but are from a "no-name' vendor.
The failure happened in the middle of a CW QSO when the total load current
including bleeder and anode was just under 700 ma. One leg of the bridge
simply shorted out. Now I've had many diode stack failures in the past. I
could probably fill a gallon paint can with bad diodes. Each and every
failure in the past was because of a turn-on transient or amp big-bang
disaster. I've never had one just quit while running at 1/3 the design current.
So here's the question: why did the highly acclaimed 1N5408's fail? Are 3
amp diodes in a bridge not enough for a capacitor filter supply with over 2
amps DC output capability? If that's not good enough, what is? Are "Silicon
Alley" diode stacks good enough for that level of current? I think not from
the published specs I've seen. Several vendors listed in Digikey offer 1000
volt/6 amp diodes. Would this be a better way to go?
This level of current in a capacitor filter supply is beyond my previous
experience and obviously design by book solution didn't work out well, so
how about some suggestions? Low cost suggestions preferably!
Thanks and 73,
Larry - W7IUV
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