Cap bypassed cathode bias vs fixed bias?

[Hardtailed]

Just wondering, since I'm planning on building a modified version of a cathode bias amplifier... what if I simply bypassed the cathode resistor with a cap, instead of switching to fixed bias
(I prefer the sound of fixed bias)

Also, this way you can easily put the cap on a switch to alternate between the two sounds.

I notice cathode biased amps are quite often of the EL84/6V6 variety. For a 50W EL34 amp, the guy from London Power mentions you should go with a 10-20W resistor, but according to my calculations, it doesn't even dissipate a whole watt (ok, let's say 2w since there are two tubes in push-pull). Am I wrong?

[stokes]

You are measuring the dissipation at idle.When the amp gets kicking at full bore,things change.10-20watts is recomended as a big safety margin,and to increase the life of the resistor,they take a lot of heat and over time heat will take its toll.

[imaradiostar]

You could use some sort of power mosfet or other biggish transistor with a zener diode to create a fixed voltage drop that won't sag like a cathode biased circuit but will prevent you from having to have two separate grid bias circuits. You could even have it adjusted so that that cathode arrangement is nice and hot and voxy (closer to class A) and the fixed arrangement is a little colder for more of a fender or marshall type ab output feel. The lazy man's version would be a zener to create the bias and a tiny cathode resistor (pot?) bypassed with a large cap so you'd have a little bit of adjustment but not nearly as much sag when you lay into the amp and start pushing the output section.

I think there is a circuit floating around out there on how to do this. You could even use a DPDT center off switch for standby and have one half switch the B+ to the preamp and the other half switch the cathodes of the output tubes between the two arrangements for instant tonal changes. Hell, you could put it on a relay and a footswitch if you really wanted to. Why not switch the NFB while you're at it?

Uh-oh, sounds like it's gonna get messy now!

jamie

[MWJB]

"Just wondering, since I'm planning on building a modified version of a cathode bias amplifier... what if I simply bypassed the cathode resistor with a cap, instead of switching to fixed bias (I prefer the sound of fixed bias)" It still won't sound like fixed bias, just cathode biased with a bypass cap...many cathode biased amps were designed with a bypass cap and they don't sound fixed bias. Still, there's no reason not to have the cap switchable.

50W from a cathode biased, 2xEL34 amp might be optimistic? Whatever the wattage it will still likely struggle compared to a FB amp, due to FB's harder dynamics?

"For a 50W EL34 amp, the guy from London Power mentions you should go with a 10-20W resistor, but according to my calculations, it doesn't even dissipate a whole watt (ok, let's say 2w since there are two tubes in push-pull). Am I wrong?" Let's say that your 50W cathode biased amp devolps 40v accross the cathode resistor - to determine dissipation accross the resistor, square the voltage, then divide by resistor value - so we get 1600v divided by 300ohms (rough example) - 5.3W. Double it to be safe = 11W minimum.

To add a fixed/cathode bias switch you just need some wire, a DPDT, an electrolytic cap, a diode, a trim pot & 1 or 2 resistors depending on whether negative voltage is derived from a PT tap or B+ secondary winding - see londonpower.com, Kevin's FAQ and look for his cathode/fixed switch.

[Chuck H]

FWIW I'd like to add That you should go with a cathode bypass cap anyhow in cathode bias with EL34s. If you don't the resulting local NFB will make the amp cold and sterile. You'll still get quite a bit of cathode bias mojo and sag because the AC bypass cap won't eliminate the DC bias shift that happens with cathode bias. I just think you would find an unbypassed circuit very unsatisfying. Too Hi Fi. That's why you don't see many guitar amps (even true class A) that don't use a bypass cap.

Oh, and bias your tubes hotter in cathode bias than they will be in fixed. You'll like it better.

Chuck