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Old 7th April 2013, 10:16 PM   #1
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Default Cathode Bypass Capacitor

Hi folks,

I need some advice on choosing a make/model of cathode bypass cap. I need a 100uf 10-25v, preferably axial.

Is there much reason to use anything other than a low impedance, high temperature, general purpose? I've done some reading on what spec to look for, and on recommended caps, but a lot of what I've read has been contradictory.

Thank!
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Old 7th April 2013, 10:49 PM   #2
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Radial electrolytics will have slightly lower inductance, and higher voltage ones will have slightly higher ESR (to over-generalize) but neither is too significant for audio use. 105 C rating is sure nice to have inside a hot amplifier chassis. If you have some steady DC voltage across one, and it's very large electrically, so that no signal appears across it, an electrolytic really isn't terrible, despite their rep.

All good fortune,
Chris
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Old 7th April 2013, 10:59 PM   #3
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Having to replace some electrolytic caps 3 time in 42 years on my dynakit amp, I am partial to long life caps. Most of the ones that have an 8000 to 10000 hour spec have decent ESR also. Unfortunately they only come in radial lead, not tall cans or axial. I'm partial to buying from farnell.com as they have the hours life in the selection table. Other vendors you have to download the datasheet, and then read it carefully to see if the size you are buying has the best hours life. Sometimes when I need something from mouser that newark (farnell usa) doesn't stock, I go to the trouble of downloading the datasheets. I've ended up lately with various panasonic, nichicon, and rubicon radial lead caps, nothing under 3000 hours service life. Every cap I've installed has made the organ or amp sound better or produce more power, even the 1000 hour caps I bought in 2009 before I figured out how to install radial lead caps on solder terminal strips below the deck. This includes some tiny 22 uf 25 vdc caps I used to replace 25 uf 3 vdc cathode bypass caps the Hammond amp came with.
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Old 7th April 2013, 11:28 PM   #4
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Thanks for the info.

Let me clarify my question a little further, I'm new to vacuum tubes so bear with me. I guess what I'm more asking is how much this cap will contribute to the sounds/performance of the amp, and in what ways. Will a general purpose cap like a Nichicon VX, or some sort of Sprague, or Mallory yield any different results in this circuit than say an Elna Cerafine/Silmic or Nichi Muze of some flavour?

Using an axial will make things a little easier for me, but "better" caps are available as radials.
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Old 8th April 2013, 05:29 AM   #5
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If you´re restoring an old commersial amp a little bit of calculating of the value is a good thing.

Caps was expensive back then and the company wants to make money

Otherwise i agree with the previous posters that a good longlife cap is the way to go.
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Old 8th April 2013, 07:56 AM   #6
M Gregg is offline M Gregg  United Kingdom
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Well,

Different caps will give different results...it depends where in the circuit it is used. In the bypass of a cathode resistor in a preamp you will here it..and it depends what type of sound your looking for. There is always the LED trick (so people that say the caps don't make a difference and then say but you can hear an LED??)..but if your looking for the old tube sound a standard 105Deg cap will give a warmer sound warm bass. The silmic have two "Flavours" I like the Silmic II its a sort of cross between old warmer sound with detail..similar effect using Hex diodes in a bridge ...The Cerafine are sharper slightly more edgy..<<HIFI talk.

Elna Cerafine and Elna Silmic Capacitors homepage

I have no connection with the above..Muse are also good..

From a mix point of view think one type for the pre sections and another on the power section to balance the sound..you might like Silmic II in the OP section and Cerafine in the pre..again it depends on type of coupling cap etc...You wont get wax paper sound with polypropylene etc. High temp tend to give longer life as does using higher voltage values.. Again watch mounting position don't mount close to hot/warm components it reduces the life of the cap<<dry out.

Regards
M. Gregg
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Old 8th April 2013, 08:23 AM   #7
M Gregg is offline M Gregg  United Kingdom
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I should also add,

The type of cathode resistor will also make a difference..(Off topic)
In the pre section this will show..how much bass, how clean, how warm etc...Low wattage mills..carbon...tant.etc or LED..
I have also heard some marked changes with adding feeder resistors to LED's in the cathode of a preamp tube. (increasing current)
The effect is a warmer sound with the feeder..you won't know this until you try it..

Regards
M. Gregg
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Old 8th April 2013, 09:15 AM   #8
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The use of an electrolytic cathode bypass capacitor makes no discernible difference to the sound other than the value affects the frequency response. The choice of resistor also makes no change to the sound other than carbon types might add a little noise.

I agree with other posters that a good long life electrolytic is the way to go. Just a not about quoted life hours. Be aware these are usually quoted at the rated operating temperature so that if you operate it considerably below that temperature the life will be extended.

Cheers

Ian
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Old 8th April 2013, 09:44 AM   #9
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Quote:
Originally Posted by ruffrecords View Post
The use of an electrolytic cathode bypass capacitor makes no discernible difference to the sound other than the value affects the frequency response.
To you, it doesn't. To many people, it does! Please accept the fact that we are not all the same - some of us *can* hear the sound of a capacitor.

If you must use a bypass cap, the Elna Silmic II.would be a good choice. A CCS anode load+LED cathode bias would be a good way, too...
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Old 8th April 2013, 11:53 AM   #10
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Quote:
Originally Posted by Vincent77 View Post
To you, it doesn't. To many people, it does! Please accept the fact that we are not all the same - some of us *can* hear the sound of a capacitor.
We are not talking about the sound of a capacitor but the sound of a cathode bypass capacitor. It is well documented that in equalisation circuits and poorly designed coupling and bypass circuits that different capacitors do have different sounds. In properly designed bypass and coupling circuits there is no evidence of any discernible difference. If you have such evidence I would be pleased to see it.

Cheers

Ian
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