>
>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.
Did you see the precaution against using so-called equalizer resistors in
the post 1994 Handbooks? What value of R ans watt rating?.
thanks
> 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.
Did you have a glitch resistor in series with the positive lead that
limits peak fault I to <200A? Do you have a step-start circuit?
> 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?
You did not state the manufacturer.
>Are 3
>amp diodes in a bridge not enough for a capacitor filter supply with over 2
>amps DC output capability?
They work fine at up to 6A in FWB if they have adequate cooling and no
parallel circuit that unequalizes reverse currents. (equalizer resistors)
>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?
>
The longest life diodes for high temp. applications are glass passivated.
For ham use, I doubt that they are necessary.
later, Larry
- Rich..., 805.386.3734, www.vcnet.com/measures.
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