Calculating screen resistor dissipitation

[passfan]

Into a discussion on screen resistor heat dissipitation and want to clarify the calculation. With a 450 volt plate and 425 volt screen would the power be calculated based on the drop across the resistor or the full screen voltage ? The resistor in the circuit is 600 ohm 1/2 watt dale rn65d.
I'm seeing roughly 10 ma's x 600 ohms for 6 volts x 10 ma's for a .o6 watts of dissipitation , which would be why the 1/2 watt resistor has not burned up in 5 years of use. Am I wrong , is my question ?

[martin manning]

It's the voltage drop across the resustor that determines the dissipation required. I understand RN65's are conservatively rated.

[jazbo8]

I'm seeing roughly 10 ma's x 600 ohms for 6 volts x 10 ma's for a .o6 watts of dissipitation...

What condition was the measurement taken? Did you just measure the voltage across the resistor at idle?

[R.G.]

There is a problem lurking there for guitar amps.

Screen grid resistors live a fairly easy and easy-to-calculate life until the tubes are pushed hard. Then screen grid current skyrockets, as the screen grid starts acting like a second plate - the real plate is driven very low in voltage, so more electrons are attracted to the still-highly-positive screen.

The screen grid resistor is supposed to help with this by dropping the screen voltage when screen current rises. But power is proportional to the square of the current, so some values of "skyrocketing" can put a big dissipation load on this resistor.

And "pushed hard" is what guitarists like to do to their amps, right?

[martin manning]

Fender historically used 1W screen resistors on 6L6 amps and still does on the reissues, so I can see where a robust 1/2W RN65 could hold up for a good long while under moderate playing. I don't know what output tubes you have here, but an upgrade would be the prudent thing to do.

[passfan]

No measurements taken as the amp is not in my possession. The current is a rough estimate. The voltage is a guess using ohms law. The owner was curious as to why the 1/2 watters didnt burn up. The thread is at the tube amp builders facebook site

[pdf64]

It will all depend on the tubes used and the specifics of their operating conditions, eg EL34 or 6V6, saggy or stiff B+.
The fact they've lasted 6 years kinda demonstrates they're fit for the application and usage.