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Cathode bypass caps with Aluminium Polymer caps

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Posted on January 16, 2017 at 04:55:49
akltam
Audiophile

Posts: 1057
Location: Hong Kong
Joined: September 29, 2002
 Has anyone tried cathode bypassing their pre-amp tubes with Aluminium Polymer Caps instead of normal electrolytic caps ? Is it worth to try this ?

Alan

 

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RE: do you mean Panasonic Oscon, etc., posted on January 16, 2017 at 08:34:00
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 They are good in the cathode bypass position but don't expect much of a difference. I use them in el84 tubes because i only have some low voltage ones and they are ok. Most of the time i bypass the electrolytic and even the Oscon with a film cap.

 

RE: Cathode bypass caps with Aluminium Polymer caps, posted on January 16, 2017 at 12:07:32
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 You need very high quality FILM cap across the Rk, ONLY and always.

The bypassing will require a multitude of caps, as each cap plays in a different frequency range.

Jeff Medwin

 

RE: Cathode bypass caps with Aluminium Polymer caps, posted on January 16, 2017 at 14:20:59
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 Yes... Actually, I have never used electrolytics.

I have only tried two types, both from the same manufacturer, and they sound rather different in ways I did not expect. Although I don't share the Dr's views WRT multiple bypasses of various types, based on my experience I do believe cap differences are significantly audible in the cathode circuit.

I am starting to question whether poly Al metalised foil can present music in a way that seems natural to me. I have a couple of Clarity MRs set aside for my next build, but am curious to try modern PIO instead. I'll be trying a few things in my next build. I really have to get moving on it, BTW.

Cheers,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: do you mean Panasonic Oscon, etc., posted on January 16, 2017 at 23:00:21
akltam
Audiophile

Posts: 1057
Location: Hong Kong
Joined: September 29, 2002
 Yes, I mean similar caps like Nichicon FPCaps, United Chemicon ...etc

 

Where to find large value uf film caps ? , posted on January 16, 2017 at 23:17:28
akltam
Audiophile

Posts: 1057
Location: Hong Kong
Joined: September 29, 2002
 My Rk is 100 Ohm. To get a freq down to 20Hz, I need a film cap of 100uF.

Any recommendation ? 100uf film cap is huge in physical size and getting very expensive.

 

RE: Where to find large value uf film caps ? , posted on January 17, 2017 at 03:26:22
sony6060
Audiophile

Posts: 1465
Location: USA
Joined: August 8, 2014
 Are you sure you need 100uF? Most preamp tubes have lower than 20Hz response with 25uF. You could likely use 20uF and have 20Hz response. Many poly caps are rated at 10uF. Use two 10uF capacitors.

If you really need 100uF, use Obbligato oil film. 2x 47uF are $29 each. The size is large, so not sure they would fit. I believe these capacitors would outperform any poly type capacitor.

 

RE: Where to find large value uf film caps ? , posted on January 17, 2017 at 09:06:02
akltam
Audiophile

Posts: 1057
Location: Hong Kong
Joined: September 29, 2002
 My Rk is 100 Ohm.

If you use the formular Freq = 1 / ( 2 * pi * Resistance * capacitance),

with Rk = 100 Ohm, you need 100uf to go down to 15 hz.

And film cap with value 100uf are huge in physical size and expensive.

 

RE: Where to find large value uf film caps ? , posted on January 17, 2017 at 09:33:00
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005



Its nice that you know the Formulas. Now, SCREW the formulas and use yer ears !!!

The caps you wanna use are pictured in my December 2016 amp build - underside. Use WIMA brand DC LINK MKP-4 caps, at industrial prices, stocked by Mouser and DigiKey in the USA. Get the four terminal ones, sound better than the two terminal ones, IF you wire amp right. SHORT leads rule, I redid mine shorter,.... mucho better.

Order a 100 uF, 50 uF and a 25 uF and LISTEN to all three of them, gain experience from your ears, not only your eyes. In the above photo, I LAYED DOWN the caps at the bottom, main Rk caps for the driver on their sides, and used a blue 14 AWG Mil Spec wire, UNDER 1.5 inches long, to connect to 9 Pin driver tube socket. WIMA DC LINK caps, 25 uF.

Any 25 to 100 uF film cap on a Rk will require bypassing, to get the fullest musical expression, but that can be done later on, after the amp breaks in some...carefully done .

Have FUN, I am !!

Jeff

 

RE: Where to find large value uf film caps ? , posted on January 17, 2017 at 09:39:08
mcgjohn@yahoo.com
Industry Professional

Posts: 501
Location: Midwest
Joined: February 5, 2008
 would agree with drlowmu. Film bypasses on the cathode cap makes a huge difference. The quality of film cap also makes a difference, i.e. try to avoid metalized film and use a film and foil type.

If you want to go nuts you can even experiment with double bypasses on the cathode cap....

J

 

RE: Where to find large value uf film caps ? , posted on January 17, 2017 at 09:52:56
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 Double ??

I often end up with SIX caps as a bypass cap bundle !!!

Jeff

 

RE: Where to find large value uf film caps ? , posted on January 17, 2017 at 10:13:28
mcgjohn@yahoo.com
Industry Professional

Posts: 501
Location: Midwest
Joined: February 5, 2008
 you've outdone me then.....have tried 3, but have not gone beyond that.
as for film and foil larger value caps, Jupiter makes some as does SCR, but SCR larger value caps are all metalized.

Also problem with larger film and foil is their physical size. At some pt they become so large, you can't squeeze them in....

have fun.

J

 

You know the OP was not talking about film caps, posted on January 17, 2017 at 21:32:37
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 He was inquiring about the different type of electrolytic that is a polymer based instead of the chemical based electrolytics. These are Panasonic Oscon and others. I just thought you all should be aware of that since he wanted to know if using that type of cap was worthwhile.

 

That equation tells me the magnitude of the cap values I needed, posted on January 17, 2017 at 23:23:27
akltam
Audiophile

Posts: 1057
Location: Hong Kong
Joined: September 29, 2002
 Although I agree with you listening makes the ultimate choice of value of cap used.

 

RE: Where to find large value uf film caps ? , posted on January 19, 2017 at 22:40:29
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
Joined: February 9, 2002
 Two points.

1. The -3db point would be 15.9Hz. That means the phase would be shifted all the way up to 159Hz. A -3db point of 2 to 5Hz is normal practice.

2. What needs to be bypassed is not the value of the cathode resistor, it's the value of the cathode resistor in parallel with the impedance of the cathode.

The formula, Cap value in Uf = 1000000/(6.28 * r * the -3db frequency you want)

r is the value of the cathode resistor (RK) in parallel with the impedance of the cathode (rk).

The impedance of the cathode is, rk = (plate load + plate resistance)/ (mu+1)


Tre'


Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Where to find large value uf film caps ? , posted on January 21, 2017 at 00:03:10
91derlust
Audiophile

Posts: 1101
Joined: December 25, 2014
 That's because you use metalised foil caps - they need all the help they can get. The conductor is deposited on a substrate - why would anyone think this is great. Even at the bottom of the barrel there are slightly better and worse.

Cheers,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

 

RE: Where to find large value uf film caps ? , posted on January 21, 2017 at 10:18:53
akltam
Audiophile

Posts: 1057
Location: Hong Kong
Joined: September 29, 2002
 Thank you for correcting my ignorance. Much appreciated.

 

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