Tube amps - what 3 things…

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.

@alexberger The impedance does not set the bias value- the DC resistance does. I'm sure the capacitor (and its quality) affected the sound quality! A tube amplifies due to the difference between the grid and cathode voltages- essentially as a differential amplifier. The cathode bypass allows the ground 'signal return' to be part of the gain structure of the circuit- essentially increasing its gain.

If you enjoy this sort of tinkering, here's a tip: figure out what the timing constant in the power supply is that feeds that tube. Then make sure that the timing constant of the cathode circuit is a little bit higher (by at least an octave) than the timing constant in the power supply. To this end, to prevent phase shift at 20Hz you'd want the cathode circuit to be good to 2Hz, so the power supply leg should go even lower. In this way the tube will be less able to modulate the power supply leg, which will reduce IM distortion.... Have fun!!

I compared sound with 100uF, 10000uF and 100000uF. It was a big difference in SQ between all 3 cases.

Were they:

• the same technology
• the same size
• formed and capacitance verified

Any of the above could change the SQ... and what reference other than different?

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 @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.

Thanks very much for your input Ralph. The designer/builder of our custom amps told me that fixed bias provides more "snot" than cathode bias. He meant power by snot. I know you’ve stated in this thread that there is no power difference between these two biasing configurations. Can you please discuss the bias configuration elements related to power magnitude?

I’m very confused as I view both he and you as extremely well on informed on this subject so am quite puzzled at how such diametrically opposed assertions could come from you and he.

Thank you.