Express Bias!

[HeeBGB]

or "BAEZ" as Allyn says I pronounce it!

In the T'Wreck pages KF states that to bias an express one simply needs to set the voltage at pin 5 of the power tubes to -30 volts DC.

how does this affect 6V6's as opposed to El34's? What is the difference in current draw between the two?

[krash]

-30vdc should be in the range of ok for 6V6's ... a Deluxe Reverb spec is -35V.

[rhinson]

hello, i'm sure ken was probably talking about using the el34's that came in express amps back in the day (siemens i believe). you should always adjust the bias based on the current draw of the tube relative to the plate voltage that the power trannie allows. a very small change in the neg. bias voltage may give a very large swing in current, depening on the config of the bias circuit and the individual set of tubes in question. just setting the neg. voltage at -30v without checking the current draw is not a good idea if you're going to use today's el34's or 6v6's. if you do you may find this is way too hot to run them even at the plate voltage for our express clones (in the 390-400v range i'm assuming). since no one here will probably install 1 ohm resistors on the power tube cathodes, the best way to check the current draw is the transformer shunt method which is described in various books, the web, etc. at any rate, you check the current draw and not rely on a specific neg. voltage setting. at the express plate voltage, i usually set mine in the 40-45ma range (keeping in mind that screen current is included here and el34's draw a good bit more screen current than the other tube types with which we are famliar so if you're measuring total current draw you have to take into account the 5-6ma's that el34's might draw). anyway, enough on that---just use current draw and not a specific neg. voltage--you just want to make sure your bias circuit will give you the broad neg. voltage range you need to get in the ballpark and then bias the tubes based on current draw. rh