>
>Bob Johnson wrote:
>>
>>So my last mod before shooting the juice to the finals is to add the
>>"Glitch Resistor" !!!
>>
>>It has been recommended on this reflector that RFC-2 be replaced with a 15
>>to 20 Ohm real enamel wirewound resistor.
>>
>>My questions are:
>>Is 15 to 20 ohms "Cast in Stone" ?
>>Would 25 ohms be OK ?
>
>Not cast in stone.
>
>Eimac recommend a resistor that would limit the peak short-circuit
>current out of the power supply to 40A (for tubes up to 1500W
>dissipation). So for example, if the B+ is 2.5kV, at first sight the
>resistor would need to be 2500/40 = about 60 ohms.
>
>However, you can also "take credit" for the internal resistance of the
>smoothing capacitors, which allows you to reduce the calculated value of
>the glitch resistor. In practice 25 ohms will take most of the sting out
>of a current surge, so that's a typical value that people aim for. Some
>would go as low as 10 ohms, others a bit higher.
>
>>What wattage should be used ?
>>
>Enough to handle the full-time normal anode current with something in
>reserve, so I^2R would say a minimum of about 25W... but that isn't the
>whole story.
>
>>I have checked the Mouser catalog and they stock the OHMITE Series 270
>>Power Resistors in 15 and 25 ohms for the 25 Watt version (L25J).
>>
>I don't have that catalog, but the rule is to use a long-bodied resistor
>to avoid a direct arc from end to end. Having decided on the resistance
>and wattage, choose the longest resistor you can find.
>
Sound advice.
>>If this resistor is suitable, would it be better to go with the 25 ohm
>>value on the basis that the peak discharge current would be lower or would
>>the higher resistance cause other unforeseen problems ?
>>
>Higher values give less peak current and more protection. Lower values
>waste less DC power and less anode voltage. It's a judgement call.
>
>Something often overlooked is that even if you fit a glitch resistor to
>handle current surges, there also needs to be some way of interrupting
>the power input from the mains. I've heard of situations where someone
>dutifully added a glitch resistor, but when the B+ was shorted the power
>supply kept right on going because the fuse/breaker ratings were set too
>high.
Not sound advice. The Heath HV supply is not capable of generating 40A
@3000v.
end
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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