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Old 11th January 2008, 11:50 AM   #1
liberty is offline liberty  Italy
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Default polypropylene caps

Since I have some large polypropylene caps (for motor runs), I need your opinions about a SE amp "electrolytic free".

I decided to use a pure single ended configuration, like this(for example):

http://www.penguinlovers.net/audio/2a3_sch.pdf

my questions are:

1)Do you experenced sound improvement using polypropylene caps in the cathodes instead of electrolytics(I have 50uF 400Vac one's, so I can use 2 of them for each cathode bypass), or it's a waste in that position?

2)Do you experenced sound improvement using polypropylene caps in a tube rectified supply instead of electrolytics, or it's a waste?

Thanks for you answers!
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Old 11th January 2008, 01:12 PM   #2
Klimon is offline Klimon  Belgium
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Haven't tried them as cathode bypass (yet) but love them in power supplies. On the Vaughn audio website (a production amp with PS oil-caps) the subject is addressed IIRC.

Simon
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Old 11th January 2008, 03:57 PM   #3
kevinkr is offline kevinkr  United States
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Yes!!!!! The answer applies to both questions in the initial post.

PPIO or PP make great psu filter caps and imo the difference is audible in unregulated supply designs - and unlike electrolytics if they are run within their ratings they don't deteriorate over time, and will probably never have to be replaced during the amplifier's lifetime. Surplus motor runs in particular may not cost any more than a comparable high voltage moderately exotic audiophile approved electrolytic.

For cathode bypass duties you may want to bypass large motor runs with a smaller film cap or better still use the hum canceling configuration:

Motor run cathode to ground, film cap for 2A3 between 1/4 - 1/5 the value of the cathode bypass cap from cathode to B+ feeding the output transformer. C_hum_cancel = C_bypass/mu+1 Use a high quality cap here as the motor run already has significant inductance which otherwise tends to soften the highs making them a bit euphonic. I used a 100uF ASC PPIO motor run for the cathode bypass and a 22uF Solen for the hum canceler. I'd recommend a Clarity SA here as they are more transparent in my experience than the Solen. You can get 20dB or so of hum canceling in some cases.

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Old 11th January 2008, 04:06 PM   #4
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I am in the process of building a grounded grid preamp, and have replaced the specified electrolytics in the last stage of each CRCRC power supply with polyproylene caps (2x 37.5uF per rail).

The preamp isn't finished but I would expect this to make a big difference...
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Old 11th January 2008, 06:01 PM   #5
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As said, makes a big difference for PSU and especially cathode bypass.

The only place I can't here so much of a difference is the first cap in the PSU, but follow that with polyprops.
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Old 11th January 2008, 07:55 PM   #6
rdf is offline rdf  Canada
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My only concern would be as a cathode bypass for the 6C45. A 100uF motor run is large, has high self-inductance and parasitics to case. It might be tricky taming oscillations with that tube.
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Old 11th January 2008, 08:14 PM   #7
jnb is offline jnb  Australia
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100uF for the first stage is minimally enough capacitance. Don't forget to try it without the cap at all. Alternately, with 1.85V needed you may find an LED that will fit so you get your gain back and don't need the cap.
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Old 11th January 2008, 08:31 PM   #8
kevinkr is offline kevinkr  United States
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I wouldn't advocate a 100uF motor run for the 6C45 - my comments were directed at the 2A3. I'd use a black gate or led bias there.
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Old 11th January 2008, 08:33 PM   #9
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I agree with the LED on the first stage. Those motor runs will be very welcome as the cathode bypass cap on the output tubes, however. I have gone from Panasonic to Solen in a previous 2A3 amp, and the improvement was great! Currently, I went from stock Panasonic lytics on my SDS boards to Solens (47uf-L-47uf-R-10uf-R-4.7uf).
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Old 11th January 2008, 08:49 PM   #10
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How about a simple current source under the cathode?
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