explanation cathode vs fixed

[Phil_S]

70% is for fixed bias. Cathode bias generally operates at 100% and is not very efficient. You can bias at 100% because cathode bias is said to be self limiting. It is hard to know because you didn't offer any info like plate voltage and primary impedance on the OT. If you look this up, a pair of 6L6 at Va=360, Zout=9000, then Pout = 24. I'm going to guess you have a <50W amp. More likely is is south of 40W, which is still frikin' loud.

[rp]

They sound and respond very different, you're just gonna have to get hands on experience. Build any of the tweeds with 2 6l6s and fixed bias, or even the later fenders like 6G3 or Blonde or black Bassmans and then add a switched cathode bias set up. It's fun. I prefer fixed in my 5F6A when turned up but at medium and low volumes the cathode bias is very nice to have, and I hate multitaskers with lotsa switches as my idea of added functionality is building an other amp.

[pdf64]

2 6L6 at max dissapation would be 50 watts. Biased at 70% would be 35 watts a channel. So 70 watts stereo is a good guess

Static dissipation / max plate dissipation / power output don't correlate, eg power output doesn't increase as bias voltage reduces (given a starting point of basic functionality).
50 watts from a pair of 6L6s doesn't necessitate a static dissipation of 100%, rather it would be a bad idea as plate dissipation will then be over 100% for some signal levels.
Similarly, swapping in tubes of a higher max plate dissipation rating won't (significantly) increase power output (all else being equal), eg putting 35 watt 7581A into a 5F6A bassman (designed around 23 watt 5881) won't make it a 70 watt amp.
Pete

[teemuk]

http://ken-gilbert.com/images/pdf/fixbias.pdf

Amen.

[jelle]

http://www.radiomuseum.org/forum/df97_p ... urves.html

Not sure if this point was raised already... See what raising the supressor grid voltage does to operation of the tube. And many newer tubes need higher cathode resistors to function properly.

[jazbo8]

ok thats better. I'll leave it at that for now. i could ask constant questions. As because has never worked for me. So the next one would why wouldn't i be able to get power with a cathode bias, whats happening that it limits it. see where this could go. i'll let it sit. dont want to be a nuisance. This is how i learn best unfortunately. me and books are like oil and water. reference only. unless it a really good novel.

Where does the power go? This one is easy, even I can answer it! The cathode resistor consumes the power = heat. With fixed bias, there is no resistor, so less wasted power.

[tubeswell]

Cathode biasing also allows an additional method of frequency shaping via selective frequency boosting through bypassing the cathode resistor with whatever the desired capacitance. If unbypassed, you also get wide-band cathode current feedback for signal smoothing.

Whereas fixed bias doesn't offer these possibilities, but does give higher gain for a given HT voltage.

Does that help?

[tsutt]

Takes a long time for all this to sink in, got a hard head full of mush. Thanks for the input guys.

[Cygnus X1]

Cygnus

2 6L6 at max dissapation would be 50 watts. Biased at 70% would be 35 watts a channel. So 70 watts stereo is a good guess.

How does it sound as a stero amp for palyback rather than organ?
I'd love a nice tube stereo amp, maybe another build for the list...

See ya!

It's passable.
Nice and clear and powerful enough to drive my PA monitors.
12" + horn.
Perfect for the garage and for jamming along to backing tracks at good volume.