bypass cap cathode bias 6L6

[sbirkenstock]

Hi again,

one more question.
I made my amp project switchable between cathode bias and fixed bias.
To try out different bypass caps I installed a switch.
I only had a 50mF 100V cap (guess a 25mf is used mostly).

If I put in or out a 25/25 at a preamp tube, a huge difference is audible.
If I take (switch in my case ) out the one at the power tubes, I can not hear any difference.
I checked all connections multiple times.
Shouldn´t I hear a fairly large difference?

Thank you guys for helping me out!

Stephan

[ChrisM]

Bypass caps on the output tubes cathodes are not like preamp ones. The cap is there not to increase gain but rather to stabilize the cathode and it's NFB affect.

[pdf64]

I guess that the amp in question is a push-pull using a shared cathode bias resistor?
With a shared resistor, at lower signal levels, the tubes effectively bypass the resistor, due to the inverted signals at either grid appearing on the cathode and cancelling out.
Only when the signal level goes high enough for one side to cut off (for some part of the wave) is that effect lost.

I suggest that you re-try the comparison with the amp putting out full power.

Bypass caps on the output tubes cathodes are not like preamp ones. The cap is there not to increase gain but rather to stabilize the cathode and it's NFB affect

That's an interesting viewpoint. Is there any further information on it?
My understanding is that cathode bypass works the same for power and pre-amp tubes.
A point to bear in mind, which often gets overlooked, is that the tube's cathode impedance is in parallel with the cathode resistor, and so should be taken into account when calculating a suitable bypass cap value.

[pdf64]

Merlin provides some interesting tidbits of info about cathode bypass cap value here http://www.valvewizard.co.uk/pp.html
eg 'A small capacitor (less than 100uF say) also increases non-linear distortion, which may be significant in hifi. Using a very large capacitor (greater than 470uF say), or using no capacitor at all, reduces this effect'

I'd be love for this section to be expanded, eg does it apply only to shared p-p cathodes, or more generally?
Are there any useful reference sources on this topic?