Bias question once more ...

[Samycasanova]

My first 18watt project is growing every day ... I´ll post some pics someday ...

I don´t have experience with valve amps , so , there is a sesitive question about Bias ... , do i need to check the bias everytime i change valves ? I´m not speaking only in the 18watt , but all valve amps ...
Can you explain to me why ?

P.S. Sorry my English.

[cberry]

It's not necessary to do so with the 18W to a degree. You should check that they're not running too hot but the circuit is ideal for combo amps in terms of it's simplicity in that area.
With some valve amps, you do need to check the bias where there's a separate bias circuit.
The reason is that not all valves are the same and they perform quite differently - even over time it's not a stupid thing to do to check the bias once in a while.
Bias adjustment for EL34's in a Marshall is set to about 70% dissipation or where it sounds best taking valve life into account. with separate bias controls for each valve it's easier to bias each valve more precisely (a matched pair often has mor than a 1mA difference between the two sides of a PP amp!).
Just substituting valves (even ones preselected for the same characteristics etc.) *can* be OK but you'd probably prefer to check the bias anyway so that you can a) get the most out of your amp and b) make sure the valves are running safetly and run less risk of damaging the valves and the transformer.
cb

[zaphod_phil]

With 18Ws it's just a matter of what the cathode resistor for the EL84s is. The standard 125 ohm usually runs them slightly too hot, so you could use a 150 ohm if you want to keep them within their ratings. That's about all there is to it.

[danelectro]

yeh i'm using a 150ohm'er now standard. seems to be working great