On 12/3/2010 3:04 PM, Jim Brown wrote:
> On 12/3/2010 12:37 PM, CSM(r) Gary Huber wrote:
>> But I was trying to find out if anyone had used the WD7S HV-2 board
>> for the 3CX800A7s..... the HV-2 board has been used to provide higher
>> voltages for surplus 3CPX800A7s in the TITAN 425.
> One thing you would need to consider is what other high voltage
> capacitors are in the RF deck that might need to be replaced with parts
> rated for higher voltage.
> As I understand it, the main reason for using higher voltage with 3CPX
> tubes is that you can achieve legal power with less plate current. I
> don't know if there would be reduced drive requirement, or less grid
> current. Since the primary failure mechanism in these tubes is excessive
> grid current, that's what I would pay the most attention to.
Now there's a slippery slope. Raising the plate voltage and lowering the
current means a higher load impedance, and so to do it properly means
redesigning the PI-L network for that higher impedance. Else you use the
same tuning C and inductance and more loading C raising the loaded Q of
the output network increasing the circulating current in the output
networrk and so its heating.
> Also, I don't know what's in this board, but to get higher voltage, you
> would probably have to replace the power transformer as well.
Or significantly increase the value of the filter capacitor hoping to
improve the drop typical of the capacitor only filter (characteristics
are in a graph at
http://www.geraldj.networkiowa.com/papers/Capfilterps.jpg copied from an
ARRL Handbook power supply chapter. Often the limit on voltage drop
under load is the source impedance of the transformer. Still just
running a lower current will get the higher voltage too because the
regulation of the simple capacitor filter is so poor.
> The only condition under which I would look at this board as a
> replacement is if most of the power supply went up in smoke. :)
> 73, Jim K9YC
73, Jerry, K0CQ
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