Tube amps - what 3 things…

I think one of the best things about tube amps is the fact that they're fussy. I love the sound of my Pass XA-25 but it just sits there. Looking at glowing tubes, swapping 'em out for others, adjusting bias (I used to do that with my Jolida) before my current tube amps no longer required it. Tubes, although often hot, are cool. I still really enjoy my little Dennis Had amp from time to time, and at least always have a quartet of old GEs glowing in my preamp. My guitar amps have tubes but you can't see them (the tubes, not the amps).

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.

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?