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Old 10th November 2010, 11:02 PM   #1
diaz028 is offline diaz028  Canada
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Default Electrolytic vs Polypropylene cathode

1. If using an Electrolytic or Polypropylene in a cathode bypass for a 12AX7, is there a difference in gain given everything is the same including the capacitor value? Example: If I use a 1u/100v Poly cap, vs a 1u/50V Electrolytic in the cathode bypass stage of a regular triode stage, will it have the same boost in gain?

2. If using an electrolytic capacitor for cathode bypass, what are characteristics to look for for best audio performance? Are Sprague premium caps any good for this application?

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Old 10th November 2010, 11:04 PM   #2
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1. Yes.

2. Low ESR.
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Old 10th November 2010, 11:27 PM   #3
tomchr is offline tomchr  United States
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If the capacitance of the cathode decoupling cap is the same, there will be no difference in the gain of the circuit.

Low equivalent series resistance (ESR) and low dielectric absorption (DA) are things to look for in inter-stage coupling caps and decoupling caps. I use polypropylene (polypro - PP - MKP) caps for this application. Teflon (PTFE) is another commonly used dielectric type for this.

Another option would be to add a reasonably sized polypro cap in parallel with your electrolytic cap. Say 1 uF polypro...

~Tom
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Old 11th November 2010, 02:23 AM   #4
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1. No. You're lucky if you can measure anything different, even for rather awful electrolytics.

2. Doesn't matter. Get the cheapest. Tubes are far more forgiving than switching supplies.

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Old 11th November 2010, 04:09 AM   #5
godfrey is offline godfrey  South Africa
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As a practical matter, electrolytics are less reliable and likely to die sooner. Their tolerance also tends to be lousy. I would consider plastic film to be a better bet - even cheap mylar / polyester. YMMV
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Old 11th November 2010, 05:31 AM   #6
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No big deal in tube circuits -- as long as you don't overheat them (now in a tube amp, that might be a good reason to get the 105C caps!), they'll last 20 years or more, which is better than can be said even for preamp tubes! Remember, they make mil-spec electrolytics, so there's nothing inherently worthless about the technology.

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Old 11th November 2010, 11:35 AM   #7
DF96 is offline DF96  England
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Tolerance is rarely an issue with cathode bypass capacitors, except for those cases when they form a significant part of establishing the LF roll-off. It is not a good idea to use an electrolytic for establishing the LF roll-off, so their wide tolerance should not be a problem in a well-designed crcuit.

Low ESR may help. The ESR ends up in series with the effective cathode impedance (roughly 1/gm), so you either want a linear ESR or one which is sufficiently small when compared with 1/gm. Note that this means that high transconductance valves, other things being equal, will be more fussy about bypass electrolytic quality. Other things are not always equal.
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