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In Reply to: RE: Cathode bypass caps with Aluminium Polymer caps posted by akltam on January 16, 2017 at 04:55:49
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
Follow Ups:
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.
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.
Edits: 01/17/17 01/17/17
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.
Edits: 01/17/17
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"
Thank you for correcting my ignorance. Much appreciated.
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
Edits: 01/17/17
Although I agree with you listening makes the ultimate choice of value of cap used.
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
Double ??
I often end up with SIX caps as a bypass cap bundle !!!
Jeff
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
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
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