Cathode vs Fixed

[I360up]

What is the difference in tone from one to the other. I know that fixed makes more power, generally speaking.

I am thinking about building something tweedish ~20 to 30w 6L6/5881

[Enzo]

Why not make the amp switchable, then you can have both?

Seriously. If you build it fixed, all you need add is a DPDT switch and a cathode resistor with bypass to have both. One side of the switch connects the grid resistors to either the ground or the bias supply. The other side of the switch connects the cathode to either ground of the resistor and cap.

Just make sure the two sides of the switch are in agreement as to which way is which.

[I360up]

Cool idea, so what s the difference in tone between the 2??

[stokes]

To put it simply a cathode bias has a "spongier" attack,and to me the note seems to "blossom" more than with a fixed bias.I also find the cathode bias has a "softer" distortion.Fixed bias tends to be "tighter" to me.Of course there is a definate difference,but we will all describe it differently,the best way is to let your ears decide.The bias switch Enzo described is very good,I put one in a Boogie MKIIC+ years ago,its a great way to A/B the two.

[Jammin'John]

Feel

Great description.
I prefer cathode bias myself.
I like the way it feels.
There is a nicer breakup when you push it over the top.
I like it with 6V6's the best but 6L6's are also sweet.


JJ

[chocopower]

Hi!

In a 2 6L6 Twin Reverb clon, what resistor and cap values do you recomend?

[stokes]

250 to 300 ohm will get you in the ballpark.The cap should be about 25 to 50uf.

[drewl]

One of my recent builds started out as a Matchless clone with 2 6V6's Cathode biased....for the heck of it I changed the tonestack to a Fender style, so it's kind of a Tweed deluxe on steroids....but in a good way!

Cathode biased amps just seems to have a roaring output to them, and I guess I like them better than i would have thought since I have and have built quite a few....AC30, AC15, Marshall 18w.....

[Ptron]

If the cathode is bypassed with a large cap, isn't the primary difference between cathode and fixed bias simply lower headroom? Not saying that that's not exactly what we're going for, but if you eliminate the inherent negative feedback developed across the cathode resistor, isn't that pretty much what your left with?

[tbryanh]

the note seems to "blossom" more than with a fixed bias

I have heard the term "blossom" before, and I was thinking it meant the note increased in volume gradually during the sustain part. Is this the correct meaning?

[Chuck H]

tbryanh,

Ptron is right to some degree and your perception is also part of it. Cathode bias adjusts itself as the attack is happening. You do get lower headroom, but you also get a small volume swell. More like a volume cut on the attack, really. But the effect is still very close to the same for the listener. Not as much for the player, but pretty cool just the same. Good players can imagine an ideal attack/decay balance. And even if whats actually happening is a muted attack and the amp catching up to the note, good players can exploit this as if it were the ideal. The end result is a sort of compression without the sustain.

Chuck

[Jammin'John]

Cathode Bias

I like it better than using a compressor for my style and for blues especially !
The added cool thing for me is when I'm pushin' the amp a little I get a nice overdrive with the psuedo compression without a pedal.


JJ

[tbryanh]

Doesn't Carlos Santana's sound have alot of blossom (volume swell) in it?

[Jammin'John]

Santana

Thats very different.
He uses a cascading gain preamp setup.
Most of his sustain is in his hands and the preamp.
He doesn't use cathode bias amps.

JJ

[tbryanh]

Thats very different.
He uses a cascading gain preamp setup.

Do you mean cascoded instead of cascaded as in the preamp stage of the FireFly? http://annex.ax84.com/media/ax84_m276.gif

Here the first preamp stage has 2 cascoded tubes. The plate of one tube is connected to the cathode of the other tube.

Do you know which anps Santana uses that have the type of preamp you mentioned?