Can someone look over my biasing data to see if I've got it right.

[wozt]

Just want to check I'm calculating the bias / dissipation stuff out right. I'm using ecll800's with are a compactron with PI and two pentodes in one envelope.
Data sheet states 6w max dissipation per pentode. I have re-biased on advice here as I was hoping to get more output.

So, I have replaced the cathode resistor to 220ohm (from360). Measurements all with tubes in.

HT 280 Bias 9.4v = 42.7ma through cathode.

Screen 1X5ma internal Pi 1.5ma therefore Pentodes= 36.2ma? subtracting cathode voltage drop leaves Pentode voltage drop of 270.6v. Power disipated at idle = 270.6x0.0355= 9.6w? which is around 75% of max according to the data sheet.

Is this right? Thanks.

[Chuck H]

That about right. I only say "about" because actual screen and triode draw are untested, but for this calculation the figures plugged in should be close enough. At 75% you're still pretty cool for cathode bias. That's not a bad thing if the amp sounds good. If you play mostly clean and want a lot of dynamic response then it's actually better. If you like to crank the amp you may experience crossover distortion due to the bias drifting colder with current. Cathode bias will do this. So your 75% would be a lot less with the amp clipping and a colder bias can hold the tubes in cutoff too long to make the push/pull cycle seamless. So, Your well within safe limits. If you experience a buzzy/wha sort of sound that increases with clipping, it may be crossover distortion. You can try a hotter bias to reduce it a bit. Remember that even if you bias at something like 90 or even 100% the cathode resistance will still cool the bias as current is drawn so the tubes should never over dissipate. As long as the standing idle isn't so hot as to have the tube red plating it should be fine. I just want to reiterate that if it sounds good now there's no need to adjust. I only ran down the situation so you would know your options depending on your playing style and what you're hearing in the tone.

[wozt]

Thanks m8, penny is dropping. I am going to start raising the bias a bit this week. Thanks.