>
>Hi:
>I'm getting ready to put a NEW old SB-220 on the air that has been sitting
>around unused for 15 years.
>
>The unit has been slowly brought up on a variac and run for a week with the
>plate connections to the 3-500Z's disconnected.
>
>The capacitors have been checked and the voltage drops across bleeder
>resistors all fall within 5% so I plan to replace them with 75 ohm 5W MOX
>resistors.
>
>The old red grid bypass capacitors have been replaced with 120 pf Silver
>Mica caps.
>
The old capacitors were 200pF. In order to use a grid fuse, the total
grid bypass needs be c. 1500pF.
>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.
>
Whatever it takes to limit the fault current to under 200a, plus the
ability to carry 800mA during tune up.
>My questions are:
>Is 15 to 20 ohms "Cast in Stone" ?
10-ohms, 10w seems to work ok.
>Would 25 ohms be OK ?
sure, if the resistor can dissipate the power and not break down during a
short.
>What wattage should be used ?
>
Are you not familiar with Ohm's Law?.
>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).
>
>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 ?
>
P = I x E.
>Please NO PARASITIC discussions on this message !!!
>
Glitch resistors are primarily needed the prevent stored energy in the
filter caps from causing fil. / grid shorts during such an event. (see
photo of grid/fil. shorted 3-500Z on p.15 of "Parasitics Revisited",
September 1990, QST Magazine)
cheers, Bob
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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