Tube amps - what 3 things…

Cathode bias is not a good thing in input and driver stages, not just in the output stage.

I tried one solution in my DIY SET 300B amp. There is 6sn7 input tube and 6F6G in triode mode driver tube with coupling capacitors between stages. The solution is very big, around 100000uF cathode capacitance in the input and in the driver tube. It gave a huge improvement in everything: bass, soundstage, separation, high resolution.
It is a pity the same solution can't be used in the output tube cathode, because of high current start-up issues. The Maximum capacitor I can put in my 300B cathode is 5000uF. It improved sound but it can't exchange fixed bias in terms of SQ. 

Hi @atmasphere ,

I know that cathode bias is still cathode bias with any cathode capacitor value. But the cathode to ground impedance is realy very different. On 20 Hz with standard 100uF the impedance is Z=80 Ohm, 10000uf Z=0.8 Ohm, 100000uF Z=0.08Ohms.
I did an expiriment by changed cathode capacitor value in the first stage of my SET amplifier. I compared sound with 100uF, 10000uF and 100000uF. It was a big difference in SQ between all 3 cases.

Hi @atmasphere ,

In my case cathode resistor was 820 Ohm, so:

with 100uF  - f = 2Hz,

10000uF - f = 0.02Hz

100000uF - f=0.002Hz

In theory, even 100uF is good enough. But in the real life the difference between 10000uF and 100000uF is very clear and significant.

Hi @ideal8592 ,

In my case 100uf/16v were Black Gate CE W (M).

10000 were the same Black Gate  + Nichicon Muze 10000uf/16v.

100000 were the same Black Gate, Nichicon Muze + Nichicon KA 9x10000uf/16v + Elna 3300uF/10v

It wasn't my "invention". At least dozen DIY people tried it many years before me. You can try yourself and it works!

Hi @ieales,

My friend does the same thing with 300B SET with Shishido schematics: ECC82 SRPP - direct coupling - 6F6 - interstage trans - 300B fix bias. The same excellent results.

 The ESR would be quite different with parallel caps...

Yes you right, but it is important haw to do it. When I parallel a bunch of big ~10000uF capacitors with one "bypass" 50-100uf capacitor it works. But when I tried to parallel 100uF BG with 50uF AN Kaisei Non Polar it causing sound smearing.