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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Better Cap = Better Sound?

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Have been playing some different cathode bypass cap with my headphone amp laterly.

The 6sn7-6bx7 wcf amp sounds smooth with a Panasonic TSHA for the wcf cathode bypass. When I added a WIMA 1.0uf in parallel, surprisingly it became harsh and edgy. Then I removed the WIMA, and it again sounds what I like better.

When I built my ef86-el84 amp, I used two pulled no-name 220uf elect cap to bypass the ef86 cathode because that was what I had at hand. It sounds so sweet, it is my current favorite. When the Muse cap I ordered from Canada arrived, I replaced the no-name cap with Muse || Wima. Again the result is very disappointing. So I now switch back the no-name cap and I am happy again.

I guess the better cap could make the amp response curve look better (my scope is not good enough to tell), but my ears like to be pleased by distortion, especially in the mid range.

Such is HIFI!
apologies for not being familiar with terminology. Is this a cathode follower? Then the bypass would be the series cap passing signal on to next stage (headphones?)

Sounds like there's some frequency dependent impedance effects here. I'd want to know the output impedance of the cathode follower, the impedance curve of the headphones and the Effective Series Resistance (Impedance) of the cap's you're using before making any judgment here. A schematic perhaps?

If I'm way out in left field just ignore me 8-*
Your experience is not unusual. When you improve one section of an amp it then has the capacity to show up all the "warts" in the other sections. Cathode bypass caps are a "classic". Improve those and all the faults/limitations in your power supply and front end are exposed.
Two choices:
Put the "lesser" quality cathode bypass caps back.
Start more mods by improving final power supply caps, coupling caps etc.

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And the best cap of all is no cap, it's relatively simple to re-arrange the white cathode follower for fixed bias on the lower tube of the pair eliminating the need for a cathode bypass cap. Led bias is another more transparent option if the an available led Vf lines up with the cathode bias voltage you need.

I designed a headphone amplifier using a D3A to drive a fixed bias 71A, and I had a lot of problems with cathode bypass capacitors in a previous design using this tube, so this time around I used a red led for bias - works quite well. Note though that this is actually a form of fixed bias because the bias voltage is largely independent of the current flowing through the tube. This technique is particularly useful with a CCS load because it does allow you to choose a specific operating point and know that you are on it.

FWIW: Large value electrolytics tend to have a lot of issues, (well known) and I have found the Nichicon Muse specifically in values larger than 220uF not to sound very good in cathode bypass use with very small dc voltages across them - the famed Black Gate (any grade) is better and in some cases you can actually observe a difference with an FFT or using burst wave forms and a scope where the Muse generated an awful lot of (visible) distortion. Why I don't know...
After trying a few other mods in my C3G headphone amp the biggest improvement in sound came from using oscon as cathode bypass cap. I even reduced the value of the cap from 1000uf to 440uF/10V (since those oscons are pricey...) with no issues in bass extension.


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I agree OSCONS really sound wonderful


be VERY carefull about the voltage rating and turn on surges etc.
OSCONS have zero overvoltage tolerance and they fail short circuit.

I found this out the hard way in a Morgan Jones Bevois Valley Amp.

The 2 x 68uF 20V OSCON cathode bypasses on the EL84 which attached to the top side of the concertina splitter would pop at HV switch on. Because it fails short circuit that meant no bias and a cooked tube.

I eventually went back to 50V Blackgate although simply deleting the Standby Switch in my amp would have probably stopped this happening. (when you turn on the HT you get a +ve pulse on the output tube grid attached to the top of the concertina, if the tubes are already warmed up you then get a big current pulse, therefore a big pulse at the cathode and that was taking out the OSCON bypass caps)

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