How much difference does switching between fixed and cathode bias make?

[Figaro]

Wondering how much switching between fixed and cathode bias makes in a Fender amp? Do you know of any examples on YouTube?

 

[00JETT]

I always thought this vid was good.

The results depend on the tubes used, but this vid shows how close they can be yet different in feel and compression.

Edit* Just realized this was Leon, hope he doesn't mind me posting it!

 

[Figaro]

Just what I was looking for. Thanks! Should have known Leon would have one.

 

[Timbre Wolf]

In my experience with BF-Fender based Carr Slant 6V, which can switch from one to the other (or both) - cathode-bias has more harmonic complexity, fixed-bias is a tad louder, with tighter lows, and more fundamental tone.

- Thom

p.s. Cool demo Leon!

 

[critter74]

In my experience with BF-Fender based Carr Slant 6V, which can switch from one to the other (or both) - cathode-bias has more harmonic complexity, fixed-bias is a tad louder, with tighter lows, and more fundamental tone.

- Thom

p.s. Cool demo Leon!

Kind of my (limited) experience as well with Tweed and a few BF type amps that I've had a fixed/biased switch on. Cathode bias can more easily "sag", especially with a tube rectifier, sooner than fixed with a rectifier.

 

[Leonc]

Hey 00JET - no problem.

I'm hardly an expert on this subject; I've only played through a couple amps that provide this feature. I think like most other things, just how big an affect it has is going to be a function of the rest of the circuit design and the way you play and/or the material you're playing.

Let's say you're playing real clean r&b all the time. IMO, the difference between fixed and cathode bias is going to be negligible in that scenario. Sure, you'll notice it...but just barely. But if you're really pushing the amp hard, you're probably going to notice it more.

I do enjoy having the choice and in this 5F6a bassman-type amp that I built (which is really bone stock other than this option, IIRC), I like both approaches...not sure I really prefer one over the other.