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mj452312

Cathode Bypass

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Looking for opinions here.  I currently use a single ended amp (el84) that is cathode biased via cap/resistor.  The bypass cap values are large (100uf input, 220uf output) so electrolytics are currently being used.  I'm wondering if anyone has experience bypassing electrolytics with higher quality film caps?  Also, it looks like there's a few polypropylene caps in the ~100uf range (albeit likely the size of a soda can).  

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Guest wdecho

Electrolytic's are perfectly fine for cathode bypass. You can bypass them with a small value film cap .01uf if you choose but doubtful it will make any difference being they are not in the signal path. 

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Isn't the cathode bypass cap in the circuit to give the amplifier more gain at AC frequencies then the cathode resistor would allow? I guess what I am saying is it kind of is in the signal path.

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Guest wdecho
11 hours ago, babadono said:

Isn't the cathode bypass cap in the circuit to give the amplifier more gain at AC frequencies then the cathode resistor would allow? I guess what I am saying is it kind of is in the signal path.

The coupling cap in a tube amplifier is considered in the signal path. Technically any and all components in a audio circuit are in the signal path but generally unless a component is directly in the signal path it is not considered in the signal path. 

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Guest wdecho
14 minutes ago, Don Richard said:

It certainly shouldn't hurt to bypass the electrolytic with a film cap and wouldn't cost a lot, so...

A small film cap should not effect the circuit that much. If you increase the capacitance too much you will get away from the designers intent. One can certainly use film caps to replace an electrolytic, if it is the same value, anywhere in an amplifier but the gain vs cost is debatable. In the power supply of tube amplifiers many use film caps instead of electrolytics claiming a better sound but the value is much less than if an electrolytic is used. If so one must include some big expensive chokes instead of resistors to smooth out the ac ripple. Again cost vs gain comes into play. If one wants to play replacing an electrolytic cap for a film cap I say go for it but the designer did not feel it was necessary. Not something I would do. 

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Guest wdecho

Electrolytics get a bad rap on this forum being that this is a speaker forum and electrolytics are not used in X-over networks. At least not in the signal path. The reason being that electrolytics have a slight amount of leakage which makes them unfit for X-over or coupling use in a tube amplifier. In circuits, such as notch use in x-over networks, where a slight amount of leakage does not matter, they are generally fine to use. There are audio grade electrolytics by Nichicon and Elna which are excellent caps. Another alternative if one wants to play. 

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I have the suspicion that as electrolytics degrade with age, a paralleled film cap comes further into play for maintaining linear response in the upper frequency range.   Much less noticeable, if at all, when both are new.   Try it with a 20-50 year old electrolytic that otherwise measures and functions fine.  

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The cathode bypass is very large because it is shorting the full spectrum AC signal (20Hz - 20KHz) on the cathode and eliminating the degenerative feedback and raising the gain. If the cap is now 220uF there would be no predicted effect by adding say, .01uF of any type of cap, because ALL the HF are already being grounded by the original cap.  

 

Bypassing electrolytics in a DC power supply is sometimes useful because it shunts HF periodic noise to ground. But there is no such noise on the cathode. It won't hurt, but it also won't help anything. It would be better to replace them with high quality new electrolytics if there is any chance they are old and possibly not perfect.

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2 hours ago, wdecho said:

The coupling cap in a tube amplifier is considered in the signal path. Technically any and all components in a audio circuit are in the signal path but generally unless a component is directly in the signal path it is not considered in the signal path. 

 

The cathode resistor bypass capacitor is directly in the signal path and should be considered as so.

 

One of the inherent weakness of cathode resistor/bypass capacitor bias designs is the frequency dependent impedance variation of the output stage of the amplifier versus a fixed bias design. While the impedance variation is small when considering the total impedance of the typical output stage it does affect the performance/sound of the amplifier.

 

miketn

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Appreciate the insight here.  As with most things in audio, it seems as though I may not get a definitive answer on electrolytic vs film bypassed electrolytic vs huge film (in this application) until I try it in the circuit.  The designer mentioned bypassing with a small film cap may help at passing higher frequencies, but wasn't sure the result would be audible.  It seems like most agree that film caps alone will outperform electrolytics sonically, but I don't think I'm convinced yet to go out and get some huge film caps to squeeze in :).  Has anybody tried that with positive results?

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There is a difference in dielectric absorption, which contributes memory effects like a battery storing voltage, but whether you hear it or not, or care, is a different question.  

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Bump this thread.   I have some reasonable experience with Rk bypassing. 

 

Mikebse2a3  in this thread ...............also got the right answer.   

 

                        The Rk bypass cap in such a  SE amp is IN the circuit, ......totally audible

 

FIRST  THINGS FIRST , ( before the Rk bypass part !! ) 

 

I see from your Profile, you use a system Attenuator.  So do I. 

 

IF your amp has any user " volume control - potentiometer  " before the Input stage, the FIRST modification priority in such a SE amp is....... to replace the Pot with a high quality 1/2 Watt single fixed-value resistor.   

 

If there is a high mu tube,  ( like there is - often a 12AX7 as the driver tube, in a two stage amp  eg:  for an Almorro 205 or a DEC EL-84 amp ) then whatever is done to improve the FIRST stage has dramatic impact on the amp"s overall sound.  That mu of 100 Input tube,  ( followed by a Finals tube with a mu of 10 ), is easily is the MAIN signature of the amp, in all of my direct A-Bing experience. 

 

The input tube HAS to be optimized for best overall amplifier performance.  EZ to do, just select better Rs and better Cs, and simplify / improve the wiring, RCA jack to control grid of the input tube. 

 

The thoughtful IMPLEMENTATION of the input wiring to the Input tube's control grid, is Job #1.  After all, you GOT to get the ENTIRE music signal to the tube's control grid, UN altered !! 

 

The Rk bypass optimization is Job #2. 

 

On Rk bypassing, the first thing we need to know is :

 

(1)   What are the two Rk resistor values, associated with each uF bypass cap value???

 

(  VERY OPTIONAL :  If possible, post a schematic - not totally needed, but often helpful.  Also a below-deck photo or two.  )

 

 

People would not STAND for an electrolytic coupling cap, to listen to music through, would they ??  Well then, WHY have a nasty electrolytic as the Rk bypass cap,  when its IN the circuit on a SET amp such as you use ??????  

 

With both  Rk Ohmic values,  I can show you all HOW to compute ( or find on line, my choice ) the -3dB down point,  between any  Rk and a Rk bypass cap combination.  

 

90 percent of the time, the EE's selected  "C" value is needlessly too high !!  LOL. 

 

This is EZ stuff to to, to improve overall amp ( and your audio system's ) overall music playback performance. 

 

'Will try to assist .

 

Jeff 

 

Rk FILM bypassing bundle.jpg

Edited by Jeffrey D. Medwin
PHOTO - FILM Rk bypass bundle, 2018 SE DC amp

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