At 09:39 AM 12/9/97 -0500, you wrote:
>What is the best type of resistor to use in the HV lead
>to a power amp. A good wirewound should be able to
>act as a fuse as well, right? Any recommendations
>as to where to find a wire wound resistor that'll handle
>kV and have ample dissipation to handle legal output?
>
> 73 de Walt Kornienko - K2WK - zak@goes.com
>
First we must know how much voltage you are talking about, Walt.
This will determine our choices in selecting the proper resistor.
You do NOT want the resistor to act as a fuse! It should be robust
enough to withstand a direct short circuit in the B+ upstream from
it. For most table-top ham type amps with less than 3000 volts of
anode voltage, the Ohmite 200 series vitreous enamel wire wound
resistors are ideal. I have used the 10 or 12 ohm 12 watt units many
times with good results. The Allied Electronics part number is 296-1500
for the 10 ohm 12 watt units. If you want to construct a fuse for
the anode lead, a one inch length of #30 or smaller copper wire between
two insulators will work fine.
Never use the resistors that are square, or have a rough sandpaper-like
finish on them. If they fail they blow shrapnel into all the nooks and
crannies and make a real mess! The slick glass-like units will break
neatly in half when they fail, with no debris.
In the really large industrial type amps you will find 10 ohm 100 watt
(and up) resistors in the anode lead as it exits the power supply section.
Most tube manufacturers will state the value of R recommended for each
tube type, but not the type or wattage rating to use. I hope the above
real-life personal experiences of mine will help in your selection of
a resistor to do the job. Of all the mods one can think of for an amp,
IMO this is the most important addition of all. There is an unbelievable
amount of energy stored in those filter caps just waiting to wreak havoc
upon components in the RF deck. The worst case I have heard of was in
an Alpha 77SX with an outboard power supply. The flash-over in the RF
compartment was so great that it even welded the bearings in the blower!
(((73)))
Phil, K5PC
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