Tube amps - what 3 things…

The designer/builder of our custom amps told me that fixed bias provides more "snot" than cathode bias. He meant power by snot. I know you’ve stated in this thread that there is no power difference between these two biasing configurations. Can you please discuss the bias configuration elements related to power magnitude?

@xenolith 

Fixed bias does have that reputation (and FWIW, we use it in our amps as well).

The power output is the same for a given bias point, but for cathode bias you need a higher power supply voltage since a voltage is dropped across the cathode resistor and that might have to be 30 or 40 Volts in a lot of cases.

The cathode resistor has the additional issue, if its unbypassed, of creating degenerative feedback. This makes it harder to drive. The feedback means it will be lower distortion, but that might not mean much if the driver circuit makes more in order to drive it.

So there are a lot of variables and each has to be managed properly to pull it off. I would expect it to be pretty difficult to do a side by side audition if that were the only variable, on account of all the secondary variables involved.

I know that cathode bias is still cathode bias with any cathode capacitor value. But the cathode to ground impedance is realy very different. On 20 Hz with standard 100uF the impedance is Z=80 Ohm, 10000uf Z=0.8 Ohm, 100000uF Z=0.08Ohms.

@alexberger The impedance does not set the bias value- the DC resistance does. I'm sure the capacitor (and its quality) affected the sound quality! A tube amplifies due to the difference between the grid and cathode voltages- essentially as a differential amplifier. The cathode bypass allows the ground 'signal return' to be part of the gain structure of the circuit- essentially increasing its gain.

If you enjoy this sort of tinkering, here's a tip: figure out what the timing constant in the power supply is that feeds that tube. Then make sure that the timing constant of the cathode circuit is a little bit higher (by at least an octave) than the timing constant in the power supply. To this end, to prevent phase shift at 20Hz you'd want the cathode circuit to be good to 2Hz, so the power supply leg should go even lower. In this way the tube will be less able to modulate the power supply leg, which will reduce IM distortion.... Have fun!!

Cathode bias is not a good thing in input and driver stages, not just in the output stage.

I tried one solution in my DIY SET 300B amp. There is 6sn7 input tube and 6F6G in triode mode driver tube with coupling capacitors between stages. The solution is very big, around 100000uF cathode capacitance in the input and in the driver tube. It gave a huge improvement in everything: bass, soundstage, separation, high resolution.
It is a pity the same solution can't be used in the output tube cathode, because of high current start-up issues. The Maximum capacitor I can put in my 300B cathode is 5000uF. It improved sound but it can't exchange fixed bias in terms of SQ. 

@alexberger I've yet to see a voltage amplifier or driver circuit that isn't cathode biased. When a cathode bypass capacitor is used, its still cathode bias. Its not cathode bias when you have a manual bias adjustment to make, or the amplifier is employing some form of autobias (both are forms of 'fixed bias' since the bias adjustment is made via the grid).

I've read controversy around the sonic differences between cathode bias and fixed bias but IME its really about design and how well the circuit is executed. Both types work just fine.

ralph, curious to know what is your take on auto biasing... now increasingly employed in high level tube amps, audio research’s newer amps for instance

the feature certainly offers a major convenience factor for users, but what do you see as the tradeoffs or downsides sonically or operationally, if any? (of course we note the obvious add’l cost and complexity of the feature)...

also, does auto biasing get the power tube biasing as precisely correct as manually biasing via multi meter (as arc used to do it for years?)

I see automatic bias as a good thing. The less the user has to fiddle with the amp the better- makes it easier to live with.

If designed right there's no downside except the additional circuitry itself. How precise it is depends on the design; in some circuits the precision is more important than others. So its a case by case basis.

The downside of manual bias is the tube can drift while you're not looking, assuming you got it right in the first place. An auto bias system takes care of that. FWIW, cathode bias is a form of automatic bias. This can makes things confusing, because many autobias systems typically use some sort of monitor on the tube and adjust the bias via the grid rather than the cathode of the tube. For this reason, most modern autobias systems are considered 'fixed bias'.

My tube amps don't care if they're running 4 ohm or 8 ohm loads, have had both, and the 4 ohm loads do not cause any issues with greater loading, heating, etc, with the tube amps

I think you'll find that using a thermal imaging camera that on a 4 Ohm load, the output transformers are indeed running warmer.

I would be careful of auto biasing.  I think it tends to suck the headroom out of tube amps.

If you find this happening I would expect there's a problem in the circuit. Autobias shouldn't affect the power the amp makes.

For example: does any manufacturer use an inductor filtered power supply instead of the much cheaper (and lighter) capacitance filtered power supply? None that I know of.

@xenolith As far as I’m aware, chokes are always used in tandem with capacitors. If you know of a topology that uses only inductors, I’d love to learn about it (Google failed to find any such thing). We’ve been using chokes (inductors) in our power supplies for decades and we are by no means the only one- the use of chokes seems to be pretty common IME.

FWIW, if the amplifier is class A and also ultralinear, you won’t need any adjustment since the correct bias can be automatically obtained using a cathode resistor. Regardless of the transconductance of the tube, if the cathode resistor is correct, the tube will always operate in the A region.

Of course, if you want to manually bias the circuit and run a bias supply (and no cathode resistor) you can, but the tube will be set at the right/same current anyway. One thing that will happen though is that as the tube ages and the transconductance drops off, if you always set to the same current the tube will be over-biased in time.

The ability to switch between modes (triode, ultralinear, pentode) is nice.

I do not agree with this statement. The problem is that to do this properly, the output transformer (and the rest of the output circuit) has to be set up to handle the different operating points, and in practice this rarely happens.

If this is a concern, FWIW the original patent for ultralinear operation showed that the result had the same linearity of a triode with about 90% of the efficiency of a pentode output section. A lot of manufacturers realized the benefit, but to avoid paying the company that held the patent, moved the screen taps of the transformer to a slightly different spot, resulting in less linearity and simply called it good, thus bypassing the patent.

That patent has long since expired and the company that held it is unknown to most audiophiles and transformer manufacturers. As a result, its rare that the correct location of the screen tap on the winding is ever used. But if it were, ultralinear would be the only way to go if you are using an output transformer. Any competent transformer manufacturer could work this out though. Tradition of using the wrong position on the winding is literally the only thing stopping them!

If you are planning to use a tube amp, do yourself a favor and avoid low impedance speakers. Your amplifier dollar investment will thank you.

Tube amps using output transformers can lose as much as an octave of bass response going from an 8 Ohm load (using the 8 Ohm tap) to a 4 Ohm load (using the 4 Ohm tap) and the output transformer will run hotter (since more of your output section power will be heating it, not a good use of that power). The amp will make more distortion too and the speaker cables will play a far more important role (and will need to be heavier).

In the old days when tubes were the only game in town, speakers were much easier to drive- they were more efficient. Efficiency is helpful because you get less thermal compression in the voice coils- a more vivacious sound results (IOW this is both measurable and audible).

So a speaker of 8 Ohms (particularly in the bass, so be careful of speakers using dual woofers that might be rated '8 Ohm compatible') or better yet 16 Ohms, along with higher efficiency (+90dB) will help you get the most out of your tube amplifier investment (this is helpful for transistor amps too, since they also make less distortion into higher impedances...).

If you are planning an SET, its helpful to be aware of what you're getting into: If you want the most out of an SET, your speakers must be so efficient that the amp never makes more than about 20-25% of full power. At power levels above that, the higher ordered harmonics start to show up on the transients (where the power is), causing the ear to perceive the transients as 'louder'. This is why you read so much about how 'dynamic' SETs are; it really distortion masquerading as 'dynamics'. To avoid this you need a speaker efficient enough to prevent this phenomena occurring. Of course, SETs have troubles with bandwidth when built to make more power; usually the bass is curtailed in favor of higher frequencies in the output transformer design. This is why the lower powered SETs are known for better sound.

My advice is that the mark of a good system is that it doesn't sound 'loud' even when it is. The dynamics comes from the recording, not the electronics! To that end, the speakers must be easy to drive so that the amplifier is spending most of its time loafing.