For those of us who are interested in the restoration of vacuum tube equipment, many original parts are hard to come by, especially multi-section, twist lock, electrolytic capacitors whose voltage ratings are in the 450 to 600 volt range. Today's capacitors, are significantly smaller in size, use higher quality materials, however, most are made as single units and in order to replace a failed, multi-section capacitor, it may require that individual capacitors be wired in series or parallel to obtain the desired capacity and/or voltage ratings. Electrolytics are not created equal even if they are stamped with identical capacity and voltage ratings. There can be variations of 10 to 20 percent depending on the manufacturer. This was recently verified through some experimentation. When wiring electrolytics in series, it is ESSENTIAL to add swamping resistors across each of the electrolytics in order to balance the voltage across each one.
The next question is, of course - What value should be used? The best answer is "as low as practical". The lower the resistor value, the better it will swamp the leakage resistance of the capacitor. It does this, however, at the expense of current draw and power dissipation. One hundred ohms per volt would be a reasonable value but it also means that an additional 10 milliamps of current will flow through the resistor. If the supply can handle this extra 10 milliamps that's no problem. If not, you may have to use a higher value. Just remember to calculate the power dissipation of this extra bleeder and size the resistors accordingly. As an example, assume you need 50 uF at 600 WVDC and you have two 100 uF capacitors rated at 350 WVDC. Using the 100 ohm/volt rule-of-thumb, each capacitor will need a resistance of 30K ohms across it. This resistance will draw 10 milliamps and dissipate 3 watts.
Last updated July 3/97
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