Choosing Ck for cathode biased 6L6's

[Chuck H]

What your experiencing is why there is no exact correct bias setting for guitar amps. Whatever sounds good and doesn't abuse the tubes (too badly) is correct. Certainly there is a correct setting for reference amplifiers because the goal is to minimize distortion and maximize bandwidth. These aren't the same criteria. Since a guitar amp is an instrument of it's own it should be set up to give the user whatever tones they find most inspiring. The ideal changes with each player and each amp. As a general rule a cooler bias (grid more negative WRT the cathode) will have more headroom, tighter bass, more dynamics and often more power. Also easier on the tubes. A hotter bias (grid less negative WRT the cathode) will be more compressed, softer attack, more pronounced in the mids and break up a little sooner. Harder on the tubes.

[wagdog]

What your experiencing is why there is no exact correct bias setting for guitar amps. Whatever sounds good and doesn't abuse the tubes (too badly) is correct.

Thanks for that info Chuck.

I do think I've found a nice balance in this amp. I'm going to wrap it up for this week anyway.

[1ampman]

Put a fan on it.....

[Enzo]

It iws fqast and easy to experiment with cap values. But figure to some extent what you are playing with freq-wise. If your cap switch selects a frequency of say 30Hz versus 20Hz, I am not so sure that will sound any different when used for guitar.

[jimboyogi]

On the topic of the effect of Ck's on Class AB cathode bias outputs, here are a couple of scope pics that I took.
Both are at max. output before clipping, identical quiescent specs, only difference is one has no Ck the other has a 50uF Ck.
Vp = 409V, Vs = 396V, Rk = 220R (shared), Vk = 28.2V, Plate dissipation is at 92%. Valves are a pair of JJKT77's.

First without Ck, notice the compression and the lower max. output voltage


Then with the 50uF Ck. Much less compression, and a significantly higher max. output voltage. But see the crossover distortion beginning!

[wagdog]

Those scope shots are very helpful - thanks for posting them!

[pdf64]

Well done in posting the scope shots; they are exactly what I've found as well.
Regards the perceived increase in crossover distortion for the bypassed version, try checking the Vk in both arrangements, at the onset of clipping. It will be higher for the bypassed version due to the increased output / current, thereby resulting in the more (visually) noticeable crossover distrotion.
So it's not that the bypass cap is causing crossover distortion, rather that it is facilitating higher output and it's the higher output creating squish, thereby causing the crossover.
Most importantly, how do you think the unbypassed arrangements sounds?
I'm not keen on it, seems dead / lifeless to me.
Pete.

[kleuck]

I thought that bypassing RK was also lowering the anode resistance, and therefore the output impedance of the amp, obviously giving a higher damping factor, i think this is probably providing a hearable difference in an amp without negative feedback.
I did a few experiment in two amps, a PP and a SE one, without feedaback loop, and the main differences i can hear are slightly differents with these two.
Using bigger cap with the PP (500µF vs 50 stock, actually 220µf for each cathode resistor as i use one for each tube, originally one 50µF for a shared resistor), the sound is "stiffer" and recalls a fixed bias amp, with less compression from the power stage on single notes or chords played loud for a short or long time.
With the SE (100µf vs 22 stock) the amp sounds more powerful and there’s less (no) compression with chords and on long times, no difference on short times.

[pdf64]

bypassing RK was also lowering the anode resistance, and therefore the output impedance of the amp, obviously giving a higher damping factor, i think this is probably providing a hearable difference in an amp without negative feedback

The difference may not be significant, as the output impedance of a regular pentode / beam tetrode amp is about 10x higher than the speaker's nominal impedance. So pushing it even higher (by the shared cathode resistor being unbypassed) may not make a significant difference. Also, with the PP shared unbypassed cathode resistor, the output impedance will change dynamically throughout the cycle, depending on its magnitude and whether a tube gets pushed into cut off at any point.
But it's worth looking into.

However, with a PP unbypassed cathode resistor, that rounding off of the wave occurs even when overdriven - the waveform can't get squared off in the normal way; to my mind that's a much more significant effect / difference to how the amp sounds.

Using bigger cap with the PP (500µF vs 50 stock, actually 220µf for each cathode resistor as i use one for each tube, originally one 50µF for a shared resistor), the sound is "stiffer" and recalls a fixed bias amp, with less compression from the power stage on single notes or chords played loud for a short or long time

I've not noticed that; I've listened hard for that type of 'compression' difference between large and small/regular bypass caps and just can't hear it; just a fraction more bottom end at higher power with larger caps but not anything that can be measured with my (very basic) test equipment.
Pete.

[kleuck]

I have one and only test equipment : my ears (and my friend's ears of course) and it's absolutely obvious with the PP, not so with the SE, but i did not try a lot of caps with this one.
I tried it without any cap though, and found the sound awfull (dull) and lacking bass a lot (damping factor ?) just tried once (6V6 in the se, my PP uses EL84)

[Bruce / Mission Amps]

The best piece of test equipment is and always will be the one between your two ears.