SET Cathode Biased vs. Fixed Bias?

[mbeards]

Hi everyone,

I have wondered about this time and time again..

How come most SET amplifiers, even new designs, use cathode bias rather than fixed? Cathode bias limits flexibility and decreases available output power but are there pros to this design choice or is it mostly keeping in tune to historical convention?

Thanks!

 

[kevinkr]

It's a good question, all of my SE designs use fixed bias in order to allow some ability to fine tune operating point, and to eliminate a large, problematic cathode bypass capacitor.

I think the fact that you can just pop a tube in without concerns about adjustment, and a certain amount of tradition explains the ongoing popularity of cathode biased SE output stages.

 

[loudnoise]

It's a good question, all of my SE designs use fixed bias in order to allow some ability to fine tune operating point, and to eliminate a large, problematic cathode bypass capacitor.

I have the same idea with the above, Fixex bias work well esspecially with high-power output SE amps. I have tried it in my 4D-32 SE amp and find fixed bias also sound more dynamic 🙂

 

[M Gregg]

I think it sounds different,

The same is true on mixed bias. Years ago people used to say that cathode bias was better sounding.
Not that I agree or disagree, however I prefer mixed bias on OTL to Fixed. That's just a preference.
I also have a preference to cap type, again each to his own.
Of course what travels through the cathode and what travels through the OP tx would be the same buts that is just an opinion. 😀

Regards
M. Gregg

 

[Hilmar53]

I use fixed bias since many years for output and driver stages. For me it sounds clearer and more "natural" then Cathode-bias. In Line stages or phono stages its different. Here C-bias sound better and is more "stable".
Merry Christmas,
Hilmar

 

[DF96]

Cathode resistor bias is self-adjusting. In most circuits it has only a small effect on available output power. The one snag with cathode bias is that the bias shifts with long loud passages, so the quiescent current has to be set a little high. Fixed bias is theoretically best, but practically inconvenient for all except audio DIYers.

 

[50AE]

DF96 said it all. If going commercial, I would build cathode biased amp for realiability and convenience, but fixed bias for DIY.

 

[lhgin]

It has different sound. Why you just build it and try it yourself to see which you prefer? 🙂

I've tried both and cathode bias appeals to me more in terms of sound characteristics.