Cathode bias vs. standard Bias

[Demanic]

I know something about tube amps, but I was wondering if anyone would care to further enlighten me on the differences between cathode biased amps and those amps whose bias is set with some sort of resistor? Curious as to the tonal and technical differences between the two.

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[JeffB]

Re: Cathode bias vs. standard Bias

Cathode is tough on tubes and you can simply plug in new tubes withoit having to manually bias.

As for tone...Id ask someone who builds the things....certainly I can talk class a vs a/b based on my experience ...but unsure how fixed vs.cathode vs, adjustable bias would differentiate tonally in the exact same amp...or if it could even be done and not require enough circuit changes to allow one to pinpoint type of bias tonal changes and make any reliable conclusions...? I.e. would the circuit changes to accomodate the different types of bias be more influential on the tone..vs. the bias method itself?

 

[Demanic]

Re: Cathode bias vs. standard Bias

Well, I suppose all those items mentioned bear upon my interests.

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[Demanic]

Re: Cathode bias vs. standard Bias

Let's start with something simple: why is cathode biasing tough on tubes vs. standard biasing?

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[misterwhizzy]

Re: Cathode bias vs. standard Bias

The weirdest thing for me is the terminology. Fixed bias is the one that you need to adjust.

 

[Demanic]

Re: Cathode bias vs. standard Bias

Right, and even then, some fixed bias amps don't have a variable bias resistor.

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[beaubrummels]

Re: Cathode bias vs. standard Bias

I have a hand-wired JTM45 with a switch the flips it from cathode to 'fixed' adjustable bias. I haven't heard of cathode bias being hard on tubes before, but I do know the cathode bias doesn't run the tubes in their sweet spot for tone. Runs a bit colder in temperature and darker sounding in cathode bias - it's not bad at all, still a joy to play, but just not the record-busting awesome that it could be when set to the correct value via the variable resistor.

 

[Demanic]

Re: Cathode bias vs. standard Bias

Now that is interesting. More info as to the how's and why's from anyone who knows please.

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[Scott_F]

Re: Cathode bias vs. standard Bias

There is a cathode bias resistor that is important. In beau's case, perhaps his cathode resistor needs a slight value change.

Some fixed bias amps, especially the older ones, simply required the tech to change the resistor when tubes were installed. Then it became so much easier to drop a 10K or 50K pot in the path to ground in order to "adjust" it.

I don't pretend to be an amp theory genius (plenty enough of those guys out there). I'm simply an amp mechanic that does my thing and I'm all about quality control and repeatability. I can report back to you on amps where I've installed a cathode/fixed bias switch, I can hear the difference easily. The cathode bias gives up a bit spongier/looser sound whereas the fixed seems a bit more defined. I'm actually more partial to cathode biased amps than fixed. They seem to have more give to them. Putting tone into words is difficult.

Randal Aiken is one of my go-to guys when I'm digging into this type stuff. His white papers are excellent. Check this one out.

http://www.aikenamps.com/the-last-word-on-biasing

 

[Demanic]

Re: Cathode bias vs. standard Bias

Cool, thanks!

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[crusty philtrum]

Re: Cathode bias vs. standard Bias

It should be mentioned that cathode bias does not necessarily mean 'Class A', and fixed bias does not necessarily mean 'Class AB'. Either can be used for either.

 

[jeremy]

Re: Cathode bias vs. standard Bias

scott build me a 5e7 inspired amp years ago with a cathode/fixed bias switch. cathode biased sound spongier and looser as scott said but also seems to have more mid range and a softer bottom. fixed bias sound bolder, louder, tighter with a bigger bottom and sharper clearer top end. both sound great but are pretty different. i usually prefer the juicier cathode bias tone for my blues/rock wanking but its nice to have the fixed option if i need to cut through a mix thats more dense. this is just my experience with one amp and im sure changing the value of the cathode resistor would change things just as how i adjust the fixed bias does.

 

[DonP]

Re: Cathode bias vs. standard Bias

scott build me a 5e7 inspired amp years ago with a cathode/fixed bias switch. cathode biased sound spongier and looser as scott said but also seems to have more mid range and a softer bottom. fixed bias sound bolder, louder, tighter with a bigger bottom and sharper clearer top end. both sound great but are pretty different. i usually prefer the juicier cathode bias tone for my blues/rock wanking but its nice to have the fixed option if i need to cut through a mix thats more dense. this is just my experience with one amp and im sure changing the value of the cathode resistor would change things just as how i adjust the fixed bias does.

This is a very good description of the tone difference between the two.

Cathode bias is normally used for lower power amps. Some of the potential power of the amp is lost across the cathode resistor. A cathode cap can recover some of this.

Fixed bias is more efficient, and lets larger amps deliver their full potential.

Cathode bias is usually colder simply to play it safe since it is normally not adjustable.

 

[Lazarus1140]

Re: Cathode bias vs. standard Bias

Fixed bias is more efficient, and lets larger amps deliver their full potential.

Is it generally true then that a fixed bias amp will be rated for higher output in Watts vs an otherwise identical circuit with Cathode bias?

Is there any point at which the cathode resistor could make a Tweed amp tonally and response-wise more like a Black-Face amp?