For slight growl/overdrive/distortion cathode bias is my preference. For clean they both sound great. For louder clean, the fixed bias would be preferable. They both sound great!
Cathode bias is also fixed bias, implemented differently but accomplishing the same thing, and set-able just like grid bias. Each method can be designed to overload asymmetrically if desired. These amps sound great, by the way.
No, cathode bias is different in that the voltage at the cathode swings with the signal going through the power tube, meaning it varies by a lot under load. Fixed bias is called fixed because the bias voltage at the control grid is stable and independent from any signal through the tube. Of cource it is overlapped with the signal at the grid but here we are talking up to 40 volts DC and negligible AC as the signal. Sound-wise i am not too sure myself, even from own expierence. But with a power amp clipping, there is an audible difference between its bias types. Contrary to popular belief though, power amp clipping is rare or at least rarely the (single) source of overdrive in a non-master-volume tube amp.
@@hanswurst9120 You are correct. I completely misstated. I was referring to the load following the cathode. Brain fart! Sorry about that. Power stage nonlinearity has a big sonic effect and is usually the result of the power supply being swamped by tube dissipation. It is a different sound to anything the preamp tubes cause.
BOTH sounded good, I thought the fixed bias amp had better attack, the cathode biased was a bit softer in the attack! (Assuming you put the SAME Lorantz speakers in both! ) \m/
Now try a chain of LEDs for cathode bias, instead of the usual resistor/caps method. Be interesting to see if the claims of it being quieter/more like fixed bias due to LED’s having low resistance is actually audible or not.
I thought the fix bias was a bit cleaner for longer, had slightly less sag, and a bit more overall punch, possibly because of slightly more power output from the same set of tubes. Do they both have the same negative feedback?
Cathode bias pulse jumping punch with faster low end decay, only all tube cathode bias can be this magic, im sorrow covert my tiny terror to estabilized cathode bias like as fixed ( fixed tension cathode)
Actualy i think both have a good tone but the cathode bias topology has a different gain structure, so my next question would be are both amps biased at the same dissipation for the wattage expected from both amps, if not, the lower volume of the cathode biased would be expected.
Yes true! The fixed bias is 20 watts and cathode bias is 15 watts output power (not plate dissipation) which is about as much as you can get from a pair of cathode-biased 6V6.
@@beareamps you stuck on the 70% bias thing, I'm not, but nothing you said was wrong just not too many people have built, tested and rebuilt too many amps, sometimes you just have to sacrifice a few tubes to get where you want to go instead of worrying about replacing tubes.
@@noel3422 the cathode-biased amp is biased to 100% plate dissipation and the fixed-bias to around 70%. I haven't found biasing hotter than 70% (fixed-bias) to yield sonic benefit whereas reliability and longevity are of utmost importance to me.
The terminology used in biasing schemes is super confusing. Cathode-bias is also known as “self-biasing” and involves the bias being created by a voltage drop across a cathode resistor. Fixed-bias (which is the one where you actually need to adjust it!) has the cathode grounded and a bias voltage is applied to the grid. The name “fixed-bias” comes from the bias voltage being constant regardless how much current is flowing through the tube. In cathode-biasing the bias voltage varies depending on the amount of current flowing through the cathode resistor.
Strait out of the gate the cathode bias sounds hotter then the fixed. Does this mean the benefit of fixed bias is you can just put New tubes without the need to bias your amp each time? And can you make a cathode bias fixed?
