Cathode Bias Rules !

[Jammin'John]

I got in an arguement with a player last night. :BITCH
I was playin' my double deluxe and I told him that one of the reasons that I liked it was because it was cathode bias.
I told him that you can "feel" cathode bias and that it's kind of like compression as far as how it "clamps down" when you hit a note hard.
He said that 4x6v6's put out the same amount of power if they are fixed bias or cathode bias.
He said the type of bias doesn't matter.
I did not have a technical explanation to give him so I had to bite my tongue & smile. :jo
I really would like a technical explanation for this phenomenon
that I can lay on this dude.
He is your classic "know it all" and the only thing that will convince him is something technical.
Every amp I have tried with cathode bias was more compressed and sweeter for blues & rock.
Every amp that I have tried with fixed bias was tighter,louder, and more trebly,with very little compression.
Do you guys know "why" ? :confused:

JJ

[mad dog]

I'm no techie, but certainly know the difference. Had a fixed/cathode switch and bias points installed on my Sewell Wampus Cat, play with this all the time. My understanding is that more voltage hits the plates without the cathode bias circuitry in the loop. I could see the extra power on my amp tech's oscilloscope when we tried it out. You certainly can hear the difference. In stock, cathode form, this amp is 27 or 28 watts. Fixed, probably closer to 35. A little extra volume with fixed, not much, but noticeably different tone and headroom. Tighter, a little more solid in the bass. Whereas the cathode setting delivers the fat, juicier, wider tone. At home it's always set to cathode. On a loud stage, switch to fixed.

[dazco]

I don't know the technical reasons, but i agree 100%. My amp was fixed and after modding it to CB the amp sounded MUCH better. A whole other world thats spongier and more natural sounding. Sorta like going from a SS amp to a tube amp if i had to decsribe it in a few words. I'm totally sold on CB.

[Ronsonic]

It isn't about max power, that belongs to fixed bias.

Cathode bias is a very dynamic thing - as the power demand goes up, the current through the tube goes up which changes the bias point. This does two things, it keeps the output stage from completely clipping out and it does compress when pushed hard. The combination is just irresistable for some people.

Ron

[jlummaa]

I really cannot say if i like either of them more. I use them all: fixed bias, cathode bias and amps that are both fixed and cathode biased at the same time (at least Vox AC50 but also /13 RSA23 if i am not badly mistaken). However when i choose the amp that i want to use i must say the crucial factor is not the biasing method ;)

Sorry cannot really help with the original question but if i remember right Gerald Weber in his book 'Tube amp talk for the guitarist and tech' had a quite good descriptions of the methods. I just don't have the book here right now.

[John Phillips]

It's pretty simple...

In a cathode-biased Class AB amp (NB, true Class A amps don't do this), the bias voltage is dependent on the signal strength, because it's generated by the tube current passing through the cathode resistor. The more current flows, the more bias voltage is generated, which then tries to reduce the current flow. This is like a form of automatic gain control - the more signal, the less gain... ie, compression.

The reason it's unique to Class AB is that in a true Class A amp, the same total current flows at all times, because the tubes never go into cutoff at any point (this is the definition of Class A). Hence, no compression and one of the reasons Class A has very real advantages for hi-fi. Class AB runs the tubes cooler at idle by having them biased below the halfway point between cutoff and clipping, so you can get more power when they're driven hard - at the expense of the compression. For hi-fi, this would be a bad idea, because compression is a form of distortion, and needs to be avoided... but for guitar, it's an advantage.

So the real irony is that the desirable 'chime' and compression of a cathode-biased amp like an AC30 is quite specifically because it is Class AB :).

It's also responsible for the greater apparent loudness for the rated power - because the tubes are running more wide-open at idle, the initial note attack is louder than would otherwise happen if the tubes were running at the same bias as they are after the level rises. This makes them very 'present' in a mix, even though they don't always 'fill a space' as effectively as a genuinely more powerful amp.


In a fixed-bias amp, the bias voltage is 'fixed' (which is what it means, not necessarily 'non-adjustable'), and so no compression occurs with increasing signal strength - at least until the power supply starts to sag at very high power, which again it will because the current draw increases in a Class AB amp (and again, a true Class A fixed-bias amp won't do that either - these do exist, Class A and cathode-bias are not related).

Fixed-bias can achieve greater maximum power from a given set of tubes because it's possible to bias the tubes closer to Class B (which is more efficient) than is possible with cathode-biased - the closer you get to cut-off with cathode-bias, the less stable the bias becomes, since in theory, at cut-off, no bias voltage would be generated at all. So cathode-biased amps are usually run hotter at idle than fixed-bias ones, which reduces the maximum power available for audio output. It also does mean they are closer to Class A, but that's not the same thing as being Class A. Many higher-powered fixed-bias amps are run as close to Class B as possible without causing crossover distortion (which happens when one tube cuts off before the other has turned on).

So...

Cathode bias: lower maximum power but greater initial attack giving a perception of loudness, and a noticeably compressed feel.

Fixed bias: higher maximum power but little or no compression until high volume is reached, so a 'bigger' sound but not quite as apparently loud.

[Festus]

Well, this player you mention is wrong. Check out the Carr amps website. They have some amps with different biasing options, and different power amp stats as a result of cathode vs. fixed biasing.

I have very limited knowledge when it comes to serious amps specs and such, but a fixed biased amp has a negative feedback loop going back to the power tubes in order to maintain consistency in tone and performance. It something to do with the power tubes drawing the same amount of current at all operating ranges - low or high volume.

Cathode bias essentially has no negative feedback in the power section, so current demands and such vary a bit with output volume. That's also why you can swap in any matched set of power tubes in a cathode biased amp and not have to "set the bias". They are considered self-biasing.

At least that's how I understand it. And I hear a difference between cathode biased and fixed biased amps, usually.

I got in an arguement with a player last night. :BITCH
I was playin' my double deluxe and I told him that one of the reasons that I liked it was because it was cathode bias.
I told him that you can "feel" cathode bias and that it's kind of like compression as far as how it "clamps down" when you hit a note hard.
He said that 4x6v6's put out the same amount of power if they are fixed bias or cathode bias.
He said the type of bias doesn't matter.

JJ

[unklmickey]

...I have very limited knowledge when it comes to serious amps specs and such, but a fixed biased amp has a negative feedback loop going back to the power tubes in order to maintain consistency in tone and performance. It something to do with the power tubes drawing the same amount of current at all operating ranges - low or high volume.

Cathode bias essentially has no negative feedback in the power section, so current demands and such vary a bit with output volume. That's also why you can swap in any matched set of power tubes in a cathode biased amp and not have to "set the bias". They are considered self-biasing.

At least that's how I understand it. And I hear a difference between cathode biased and fixed biased amps, usually.


hi Garbanzoman,

some of the facts you stated are true. but the hows and whys are mostly misses.

a fixed bias amp doesn't necessarily have a Negative FeedBack loop.

and a cathode biased amp can also have a NFB loop.

the presence of the cathode resistor itself, and how the voltage at the cathode (with respect to the grid) increases with current through the tube, is what stablizes the circuit, and makes it largely "self biasing". not the lack of NFB.

[Jammin'John]

Many Thanks to you Mr. Phillips !

I printed your response and will bring it the next time that I see this dude.

Also,Garbanzoman,I printed the Slant 6v details about different power levels.

JJ

[Old Tele man]

Cathode bias: lower maximum power but greater initial attack giving a perception of loudness, and a noticeably compressed feel....sorta like the "splash" and ripple of diving into a pool: initially the water goes way up (attack), then subsides down lower than original (compression), but evenually returns to normal.

[Born2Blues67]

My Allen Brown Sugar is cathode-biased,and sounds very
warm and lively.

[minesaguinness]

Many cathode biased amps have a capacitor across the cathode resistor as an ac shunt. This is to prevent the ac current flow causing too much change in the current through the resistor which would cause the bias point to change.
The dc quiescent operating point is controlled by the cathode resistor (as more current flows through the cathode resistor causes increase in voltage across it which in turn makes the grid voltage become more negative wiith respect to the cathode and so close the valve and reduce the current - hence the British term "valve")....

..... but the ac signal is shunted to ground via the capacitor - so the ac (or signal) equivalent circuit, to a first approximation, does not include the cathode resistor - in such an amp.... e.g. Vox AC30 has a largish decoupling capacitor (~250uFarad) across the cathode resistor (~47ohm) which all four EL84's share - in contrast the Vox AC50 has no decoupling capacitors (2xEL34) - or at least not in the one I had many moons ago.

Cathode resistor biased Fenders also tended to use decoupling capacitors (including single power valve amps).

Some do some don't.

Agree with comments above re Class A &cathode bias - Class A can be either cathode resistor biased or grid biased. Similarly cathode resistor biased amps can operate in either class A or class A/B depending on how they're biased.

Also agree with the comment re negative feedback - can be applied to both grid - and cathode resistor - biased amps.

Cheers
Brian

[Topcat]

I for one am so glad that the "Racket Scientist" rests his hat occasionally on this forum. Always the best and thorough replies.

Thanks John. Stick with us--you are appreciated.

Terry

[Flyin' Brian]

I for one am so glad that the "Racket Scientist" rests his hat occasionally on this forum. Always the best and thorough replies.

Thanks John. Stick with us--you are appreciated.

Terry

Another expression of thanks here as well John. That's the best explanation I've heard and answered a lot of questions for me.
Very much appreciated, and "it's pretty simple" because you made it that way.

Brian

[riverastoasters]

It's pretty simple...

Next stop: what if you have an ultralinear power section? (which can be fixed bias, cathode bias,...)

[Jim Salman]

I'm not familiar with the schematic of the Victoria Double Deluxe, but I'm guessing it is very much like a late-50's tweed Deluxe that has 4 instead of 2 6V6s. A lot of folks don't realize that the cathode biased output section of a tweed Deluxe is not the only contributor to its compressed spongy feel and singing sustain. Those amps don't have a choke. Instead, they use a rather large value (5K ohm) power resistor. This resistor creates a large degree of current limiting to the 6V6 screens and this results in quite a large voltage drop to the screens during transients (temporarily reducing gain in the output section). Thus, plenty of compression and sag.