cathode biasing 6V6s - is this right?

[HTH]

I've dug out an old build of mine (2+ years ago) and I knew even less back then. Anyway, it's basically an AC15 with a cathode biased 6V6 output stage.

- I've got 370v on the plates
- seperate 680ohm cathode resistors bypassed with 220uF caps
- 23v on each cathode

does that sound about right?

V=IR
I=V/R
370/680
I=0.54mA

P=IV
0.54 x 23
P=12w per tube

is that ok or is my maths WAY off like usual?

thanks... Ian

[stokes]

To get the current draw you divide the volts across the cathode resistor,by the resistor,not the plate volts,so you have more like 33ma per tube.This is high.For 370 plate volts you want 29ma for Class A.If you are going for Class AB it would be about 22ma.

[HTH]

Stokes,

thanks for the help. I'm already quite high on the cathode resistors from what I can see on other cathode biased schematics - should I go higher than 680ohms?

could probably get 820ohm wirewounds or use a shared 470ohm wirewound (?)

[Enzo]

And remember it is the voltage across the tube, not simply the plate voltage. So 370 on the plate and 23 on the cathode means there is 347 volts across the tube.

When you change resistors, check the new plate and cathode voltages to determine the new voltage across the tube.

And as he said, determine current through the tube by cathode voltage divided by cathode resistance. Then multiply THAT current by the tube voltage to get tube dissipation.

[stokes]

I would try the 470 shared.And do the math as Enzo pointed out.Those plate will change with different cathode resistor values.

[HTH]

Stokes & Enzo - thanks for the help, much appreciated.

The amp sounds glorious at the minute - one AC30 'normal' channel and one EF86 channel.

The normal channel is very receptive to OD pedals - I'm getting a great rock tone with my modded SD-1 into the front end.

These pair of 6V6s are louder than I thought too. I'll see how the amp hangs with a loud rock drummer this weekend.

[HTH]

ok, here's an update - am I doing this right?...

cathode resistor= shared 470ohm (bypassed with 220uF cap)
plate= 372v
cathode= 28v

that's 344v accross the tube. I then divide the cathode voltage by the cathode resistance right?... 28/470= 0.059 (59mA for both tubes?, i.e. 29.5mA per tube?)

how do I then calculate the wattage dissipated?

[stokes]

Yes that is correct.To calculate the idle watts disipated you take 344X.029=9.9 watts.I would use a 50uf-100uf bypass cap,tho.When you had the seperate cathode resistors the 220uf was okay,since they are in parallel,you end up with about 110uf bypass value.It wont do any harm but it will change the tone somewhat.Remember,too,that when you figure your cathode current it is including the screen current as well,most likely around 1 or 2ma's.

[HTH]

thanks Stokes, just one more question though... I've got 10w per tube there, should I be looking to run these at their max or just run them around 70% like when using fixed bias?

Thanks for the tip on the bypass cap too - I was thinking of knocking that down a bit as the bass end was compressing a fair bit at rehearsals today.

[stokes]

I would say it would be safer to run in Class AB at 70%.If you want to try to bias it into Class A you have to go about 90%,but I wouldnt recomend biasing them at 100%.You dont have to be exact here,anything below 90% that sounds best to you is good.Just be aware the hotter you bias them the quicker they die.

[HTH]

cheers Stokes,

so with 344 accross the tube using a 470ohm resistor I'm running them at around 85% max dissipation... 0.85 x 12 / 344 = 0.029 (per tube)