The "huge cathode cap method" on 18watters... does it work?

[Silvio55]

With 2 * EL84s (shared cathode string) I have been using a 7.5V, 5W zener to ground, and using a resistor between the zener and the cathode.
The resistor value is adjusted by measurement to get your required plate dissipation. Eg. I have ~ 330V plate voltage, and a 47R resistor gives me ~ 80% of max. dissipation. If your B+ is lower then your could try a lower value resistor, higher value for higher B+.

I keep the resistor unbypassed, this has 2 effects here.
1 - this adds a little bit of compression to the signal peaks as they approach max. output, which is an effect that i like the sound of,
2 - the unbypassed resistor dramatically reduces crossover distortion.

With the above values, the bias supply voltage is ~ 70% "fixed" by the zener, and ~ 30% cathode bias. You may prefer a different proportion of fixed/cathode bias, so it's worth getting a range of zeners of between 5V and 10V, and a range of resistors from say 10R to 150R. Then play around and see what gives you your favourite combination.

So, do you use a zener in series with the cathode resitor? I've never heard of that! very interesting, one thing I donīt understand is how do you meassure the dissipation without the cathode voltage, just with the plate voltage and resitor value, or is the cathode voltage the zener voltage?
If you keep the resistor unbypassed, how does that affect the overall volume (or PA distortion) of the amp? I think itīs reduced in some way, cause you're reducing the PA gain, and in the 18 watt most of the distortion comes from the PA.

[loudthud]

You use the voltage across the resistor and the resistor value to get the current. Then you measure the votlage from cathode to plate to get the voltage. Multiply and you get the power for both tubes.

Instead of a fixed zener, a variable power zener could be constructed from a TL431, a pot and a power transistor.

[jimboyogi]

Yep, as loudthud said you calculate your dissipation from the voltage drop across your cathode resistor.

If you are interested in this technique, there is a bit of info out there, especially on the AX84 site.

Here's a bit of info -

AX84.com - The Cooperative Tube Guitar Amp Project

You can also try googling "zener assisted cathode bias"

Any specific questions, I'll do my best to answer!