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Old September 3rd, 2012, 07:05 PM   #1 (permalink)
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CATHODE vs. FIXED BIAS?

Would someone explain the difference between cathode bias and fixed bias. I know this is TUBE AMPS 101, but I am confused. Do they both need biasing when changing tubes? Thanks in advance.

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Old September 3rd, 2012, 07:56 PM   #2 (permalink)
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Cathode bias has the current through the tube run through a resistor before it gets to the ground and returns to the power supply. When you have current running through a resistor you get a voltage drop across it. This voltage drop is used to bias the input of the tube (I won't go into how). If one tube draws more current than another in the same circuit you end up with a greater voltage drop (more current more of a voltage drop across a resistor) the tube gets biased more negative and this has the effect of reducing the current through the tube (which then reduces the voltage drop). In the end things balance out and unless the tube is way out of whack it finds its happy place.

A fixed bias amp does not have this resistor, the bottom of the tube is connected to ground, so one tube may pass X amount of current while another may pass Y given the same bias voltage on the tube grid. The bias circuit does not change with a change in current (it is oblivious to what is going on in the amp unlike the resistor in the cathode bias circuit) so if it is set for -20V on one tube giving X amount of current you may have to change it to -18 for the next tube to get X amount of current.

Cathode biased amps generally do not need to be looked at but fixed bias amps probably should. A replacement set might be bang on but then again it might not.
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Old September 4th, 2012, 06:57 PM   #3 (permalink)
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It's my understanding that cathode biased amps do not need to be biased if you are using the same type tubes and each tube is reasonably within spec of each other.

Typically you will find cathode bias amps in lower power EL84 configurations although there are exceptions ie: Carr Rambler etc,

There are many far more amp savvy folks on this board though I'm sure you can get even more detailed information
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Old September 4th, 2012, 07:50 PM   #4 (permalink)
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It is pretty common practice to purposefully mismatch output tubes in an 18 watt Marshall, which is cathode biased, with 2 EL84's. Makes the amp "grindier".
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Old September 4th, 2012, 08:10 PM   #5 (permalink)
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Cathode bias does not need to be adjusted. Fixed bias must be adjust correctly to perform its best.

Fixed bias can get more power from a given pair of tubes than cathode bias.
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Old September 5th, 2012, 02:18 AM   #6 (permalink)
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just to clarify, or confuse further...

cathode-bias amps do not usually NEED to be adjusted when replacing tubes. However, it is possible to adjust the bias by changing the value of resistance between the cathode and ground.
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Old September 5th, 2012, 05:58 AM   #7 (permalink)
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Tonally, a cathode biased amp will have more compression. When signal is applied to the output tube and current flows, the voltage drop across the cathode resistor increases, which changes the bias such that the gain reduces. This effect increases at higher signal levels, and the net effect is that the sound is more compressed than in a fixed bias amp, which tracks the preamp signal much more exactly in dynamic level.
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Old September 5th, 2012, 11:28 AM   #8 (permalink)
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Quote:
Originally Posted by jhundt View Post

cathode-bias amps do not usually NEED to be adjusted when replacing tubes. However, it is possible to adjust the bias by changing the value of resistance between the cathode and ground.

+1. You are right.

Many amp techs either don't understand this, or refuse to admit it.
If a power tube is very efficient, or very inefficient, the value of the
cathode resistor needs to be changed accordingly.
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Old September 5th, 2012, 11:37 AM   #9 (permalink)
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Thanks for the info; I have a Pro Jr. (Cathode bias I believe?) and I've often wondered what the deal is.
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Old September 5th, 2012, 05:15 PM   #10 (permalink)
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Pro junior is fixed biased.
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Old September 5th, 2012, 06:58 PM   #11 (permalink)
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Pro junior is fixed biased.
I see...
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Old September 7th, 2012, 05:45 PM   #12 (permalink)
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Tonally, a cathode biased amp will have more compression.
Yes and no... that's true if you convert an amp from one to another, but it would be wrong to say that all cathode biased amps have more compression... Compression can come from a lot of different sources rather than bias method. The stiffness of the power supply can really affect that, and you can build a cathode bias amp that is stiffer than a fixed bias amp. Since we tend to cathode bias amps really hot, many actually have LESS sag/compression than some of the classic bluesy fixed biased amps such as a deluxe reverb, fender bassman etc...

More than anything, cathode bias vs. fixed bias is a biasing method. As such, they don't necessarily have a "sound" to them...
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Old September 7th, 2012, 09:02 PM   #13 (permalink)
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More than anything, cathode bias vs. fixed bias is a biasing method. As such, they don't necessarily have a "sound" to them...
Using cathode bias on an output stage will certainly sound different than an essentially identical circuit with a fixed bias.

Cathode bias will introduce dynamic and harmonic distortion, and you will hear that; introduce an phase shift that you likely won't hear, and reduce the efficiency (output) of the circuit.

This distortion is a very different nature to the waveform clipping that we associate with overdrive.

Many people describe the sound of a cathode biased amp as "rounder" -- whatever that means -- I've heard cathode biased amps that sound pretty good -- it's really a matter of what kind of sound you like best.
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Old September 7th, 2012, 09:19 PM   #14 (permalink)
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To the extent that generalizations are not bs, I find cathode sweeter and fixed to be more pure and articulate. For distorted play, cathode will help you sound good while fixed demands better playing technique and does not collapse into mush as easily.
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Old September 7th, 2012, 09:41 PM   #15 (permalink)
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From guru billm in another thread:

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Your terminology is correct, but it confuses many people. Cathode bias is self-adjusting, so it's not fixed. It varies with the characteristics of the tube and the load. The cathode resistor that creates the bias voltage is typically not adjustable.

Grid bias is a fixed voltage, but the bias supply is often adjustable, so you can change the voltage at which the bias is fixed and optimize it for the tubes, essentially setting the "idle speed."

Clear as mud, right?
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Old September 7th, 2012, 09:41 PM   #16 (permalink)
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Using cathode bias on an output stage will certainly sound different than an essentially identical circuit with a fixed bias.

Cathode bias will introduce dynamic and harmonic distortion, and you will hear that; introduce an phase shift that you likely won't hear, and reduce the efficiency (output) of the circuit.

This distortion is a very different nature to the waveform clipping that we associate with overdrive.

Many people describe the sound of a cathode biased amp as "rounder" -- whatever that means -- I've heard cathode biased amps that sound pretty good -- it's really a matter of what kind of sound you like best.
What is dynamic distortion?

Why would cathode bias introduce any more harmonic distortion than fixed bias?

How does cathode bias introduce phase shift? Relative to what?

Detailed technical responses, please.
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Old September 7th, 2012, 10:00 PM   #17 (permalink)
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Why would cathode bias introduce any more harmonic distortion than fixed bias?
It should not under undistorted operation. What can happen though is related to the thing that goes with the cathode resistor, namely the capacitor. The capacitor bypasses the AC around the resistor so the resistor only effects the DC current through the tube. At least in theory. In overdrive when the grids are driven positive there is a voltage shift on the capacitor that changes the bias voltage. If the bias is set so that the voltage shift puts the amp in Class B rather than Class AB you will get crossover distortion adding harmonic content. If you have the tubes biased hotter so the amp does not go into Class B then you will not get the added distortion.

Now biasing the tubes hotter may give a different harmonic content but if you biased them hotter in fixed bias you should get the same effect. For stability purposes we bias fixed bias amps colder than cathode bypass amps.

The only two things I can think of off the top of my head.

One more thing. The identical amp with a power supply at say 300V that is cathode biased might really have 275V on the tube with the other 25V across the cathode resistor. The same amp but switched to fixed bias would have the full 300V on the tubes making it sound cleaner. The cathode biased amp would be a 'browner' sounding amp. due to the lower voltage.
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Old September 7th, 2012, 11:16 PM   #18 (permalink)
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What is dynamic distortion?

Why would cathode bias introduce any more harmonic distortion than fixed bias?

How does cathode bias introduce phase shift? Relative to what?

Detailed technical responses, please.
If the sound output waveform is shaped differently from the input waveform, this is harmonic distortion. Cathode biased power amps amplify different parts of the wave differently, as the bias changes dynamically with the current. This alters the shape of the waveform.

As others have noted, there is a compression-like effect that limits gain in a non linear fashion. This is dynamic distortion.

If I wanted to build an old school phase inverter, I would use a cathode biased tube, rather than a 50 cent IC.
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Old September 8th, 2012, 02:39 AM   #19 (permalink)
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If the sound output waveform is shaped differently from the input waveform, this is harmonic distortion. Cathode biased power amps amplify different parts of the wave differently, as the bias changes dynamically with the current. This alters the shape of the waveform.

As others have noted, there is a compression-like effect that limits gain in a non linear fashion. This is dynamic distortion.

If I wanted to build an old school phase inverter, I would use a cathode biased tube, rather than a 50 cent IC.
Technical references?
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Old September 8th, 2012, 08:33 AM   #20 (permalink)
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Technical references?
All you have to do is Google 'cathode bias' and you will find many references, some of them correct. The Wikipedia article is OK, and fairly easy to understand.
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