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Old 25th October 2012, 10:06 AM   #1
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Question Filter capacitors - motor run?

I am planning to build a WE91 clone based on 300b. I saw some designs that have used oil filled motor-run capacitors for filtering. Is this a good practice? In general what is the recommended capacitor type for filtering?

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Aner
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Old 25th October 2012, 11:45 AM   #2
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Motor run caps are polypropylene. Also often called "film caps". They can be oil filled or not. They come in various qualities, but in general ALL polypropylene caps sound better than all electrolytic caps. If you have room use all polypropylene caps in your PSU. They are bigger, and if you don't have room, the final capacitor in the chain, e.g. CLC, should be polypropylene. I believe this sums up good practice as usually expressed in these forums. No doubt others will advise in more detail.
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Old 25th October 2012, 06:23 PM   #3
kevinkr is offline kevinkr  United States
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The motor runs work great, my preference is for oil filled ASC and GE types. Do not confuse these with motor start capacitors which are generally much more compact and contain short lived electrolytic caps not intended for continuous duty.

Preferably purchase ones that have a DC voltage rating in addition to the AC operating voltage. Generally you are safe running them at up to DC values equivalent to the Vpk value represented by the stated rms value.. In many cases the safe DC voltage may be a lot higher than this, but unless disclosed this would be the safest way to go.

In tube amp supplies operated conservatively they will probably outlast you..
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Old 26th October 2012, 06:36 PM   #4
Cassiel is offline Cassiel  Libya
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Me testing a non electrolytic amp. Sounds softer (not happy about this) but it might be because it's tube rectified. The original is not. Too soon to say but starting to think not worth the additional space.
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File Type: jpg testing.jpg (403.2 KB, 531 views)
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Old 26th October 2012, 06:59 PM   #5
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Try the Clarity Cap TC series or Mundorf M Tube Caps in the power supply, huge upgrade over electrolytics, even bypassed...
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Old 26th October 2012, 10:02 PM   #6
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I used two Genteq 97f5211 50 uf 440 VAC film caps, to replace 47 uf 450 VDC rated electrolytic caps in my organ. I have two 5AR4 tube rectifiers. It can be run really loud, particularly the bass. The caps show a 1000 VDC rating stamped on them. Huge, I put them in a separated chassis connected to the original by twinax 18ga cable. Ground shielded one end, plus up and back in the same cable, minus up and back in the other cable. I have 40 ohm chokes (original) between the rectifiers and the caps, 47 ohm 5 watt resistors (not stock) between the transformer and the plates. The load is a pair of 7591 tubes on bass and two pairs of 6BQ5 tubes for the treble, and 17 other tubes for signal processing.
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Old 22nd December 2012, 07:16 PM   #7
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now i'm a bit confused, i need another 100uf cap, and i have to choose between JJ electrolytic and a big polypropylene motor-run. same price cca 11eur. what to do ? thanks
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Old 22nd December 2012, 08:26 PM   #8
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If you have room, use all motor runs. If you are tight for space make the last cap motor run.
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Old 22nd December 2012, 11:43 PM   #9
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A capacitor rated 440v AC should be OK for 1.4 x that figure in DC, in other words around 600v.
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Old 22nd December 2012, 11:48 PM   #10
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Quote:
Originally Posted by andyjevans View Post
A capacitor rated 440v AC should be OK for 1.4 x that figure in DC, in other words around 600v.
thank you. ( i deleted my post before i saw you reply, because i found out i was wrong in that calculation)

ok, so im going happily for those big white chunks. at least i'll feel more serious!
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