Filter cap resistors

[macula56]

on Fender amps there are 220K resistors across the first 2 filter caps. what purpose do they serve and why does Marshall not do this? thanks, JMac

[RicharD]

Fender used 100uF 350V caps stacked in series to economically create an effective 50uF 700V cap.  The 220k resistors are also stacked in series but parallel to the caps.  This creates a voltage divider that equally distributes the voltage across the 2 caps.  W/o them, 1 cap could see more voltage than the other depending on their internal resistance.  Marshall used 550V caps.

[Tiny_Daddy]

I have been using 270K, 2 Watt. Works fine with less heat.

[eleventeen]

Many call these "balancing" caps. As Buttery said, they work to balance the voltages across the two caps by forming a 50/50 voltage divider. They need not carry anything more than a mil or so of current. I have both used and not used them. They are useful in that one can use let's say 2 qty 350VDC rating caps in series across a 500 volt supply which are much cheaper than caps rated > 500 voilts. Because caps can vary quite a bit, even the same style/manufacturer, +/-20%, it's considered good form to include the balancing resistors so neither cap's voltage is exceeded.

[macula56]

do you think i should use them in a marshall build? i have a 50/50 cap can with both caps in parallel.

[eleventeen]

Caps in parallel or in series? It's when the filter caps are in series, allowing you to use lower-voltage-rating caps, and of course, dividing your ufds in half at the same time. Balancing resistors aren't needed when the caps are in parallel. Some folks like to have a bleeder resistor to discharge the filter caps at the output side of the p/s as a safety measure, but there, you want more current draw, more like 10-20 mils. Not everybody can or should spare those mils to a resistor that's after all going to be hooked up all the time and robbing your amp of precious mils. Like a leech!

[RicharD]

Igt was really a question of economics.  Back in the day, big uF caps at big DC were costly.  Kinda still are when looking at FP type caps.  If your 50/50 is rated for 550V, run with it in parallel for a 100uF, unless you have a weaker tube rectifier not rated for 100uF.  A 50/50 with a common neg obviously cannot be series stacked unless you have 2 of them.  As noted, balancing resistors don't have to be 220k.  This value was selected because nominal B+ was about 440ish.  2 - 220k stacked in series = 440k.  That gives a current draw of about 1mA.  Negligible in the big, easy to count on 1 finger.

[macula56]

these are in parallel per Doug's Plexi power schemo. if the resistors are not really necessary then i will not worry about them. thanks everyone.