5e3 biasing advice

[ernest]

Could I get some advice on if you would adjust the cathode resistor based on these readings?  This is a 5e3 with a Hoffman circuit.  New JJ 6v6 tubes and new GT  5y3.


Cathode resistor measures 245.5 ohm with a 21.64 voltage drop.
379v Plate


379 - 21.64 =357.36 v


21.64/245.5=.088 ampere


357.36*.088= 31.45/2= 15.725 watts dissipation


Should I be concerned that I'm not getting a reading closer to 12 watts?  Or is this reading at "idle" normal and will be lower at playing volume? 
The amp sounds fine, I'm just educating myself and would like some input from people with more experience. 


Thanks!

[PRR]

> 15.725 watts

That's P+G2. P diss alone is near 15W.

12W 6V6 is the target number when you are not going to measure every radio you build. If you do check for part variations, 14W is perfectly legal. I would not worry about 15W in non-life-critical applications; at least one modern product idles its 6V6 at 17W.

Note that if you add an R-C filter (lower buzz) to get to 311V, you hit 12W Pdiss. OTOH some folks insist you don't get "the sound" until 360V, 380V, even 440V(!). Do you want to live forever or die beautiful?

[ernest]

thanks, I didn't understand all of what you're saying...P+G2?  but I get the gist, die beautiful will work just fine.  I'll leave the cathode resistor where it's at then.

[HotBluePlates]

The 6V6 has plate current (10's of mA's) and screen (G2) grid current (few mA's). All of that current is emitted from the cathode, so cathode current = plate current + screen current.

After moving beyond triodes, tubes have more than 1 grid so they get numbered. The control grid (where you apply signal and sometimes a negative bias voltage) is "Grid 1" or G1. It's also the 1st grid the electron stream encounters after leaving the cathode.

The next grid is the screen grid, or G2. This is your last grid before the plate if the tube is a tetrode.

Pentodes have a 3rd grid on the way to the plate, the suppressor grid (G3).

Beam power tubes (like the 6V6 & 6L6) have beam forming plates (along with other features) which perform the function of the suppressor grid, so even though there is no physical grid, these often get the "G3" reference as well.