Biasing the Express.

[leadfootdriver]

I plan on using the the Cathode Resistor method, and installing the external test points on the back of the amp along with a pot on the top. .

So, what percentage of dissipation is everyone biasing their Express at? Or, even better, what did Ken bias the amps at? I read in the TW catalog that the amp is advertised at 36 true watts.

I plan on using JJ's. So, when using the resistor method, do I still factor the published power rating of the JJ EL34L which is 25 watts (50 for the pair)?

[martin manning]

I believe you are comparing apples and oranges. The 25W max plate dissipation is not the same thing as the 36W output power. The first is the maximum power that the plates can handle (current flow from cathode to plate times plate voltage), and the second is the output power at the speaker, which depends upon the HT supply voltage and the load impedance, and the losses in the OT.

I'd bias them at 70% of the 25W maximum, based on a calculated plate dissipation at idle, plate voltage times cathode current. This will actually be a little less than 70% plate dissipation due to the screen current being included in the cathode current you measure.

[leadfootdriver]

I believe you are comparing apples and oranges. The 25W max plate dissipation is not the same thing as the 36W output power. The first is the maximum power that the plates can handle (current flow from cathode to plate times plate voltage), and the second is the output power at the speaker, which depends upon the HT supply voltage and the load impedance, and the losses in the OT.

I'd bias them at 70% of the 25W maximum, based on a calculated plate dissipation at idle, plate voltage times cathode current. This will actually be a little less than 70% plate dissipation due to the screen current being included in the cathode current you measure.

70%! Thanks.

I plan on using 1 pot for the pair. Should I wire the test points up with both cathodes in parallel, so I can do my calculation using the total cathode current of both cathodes?

[hans-jörg]

I plan on using the the Cathode Resistor method, and installing the external test points on the back of the amp along with a pot on the top. .

So, what percentage of dissipation is everyone biasing their Express at? Or, even better, what did Ken bias the amps at? I read in the TW catalog that the amp is advertised at 36 true watts.

I plan on using JJ's. So, when using the resistor method, do I still factor the published power rating of the JJ EL34L which is 25 watts (50 for the pair)?

Hello,
you mixed the bias method: cathode Res. is only with "Cathode Resistor",
but "fixed bias" is that wath you intend to do, with Test points and bias pot.
The biasing by cathode resister includes not a bias pot.

Hans-Jörg

[martin manning]

Usually one 1-ohm resistor is used per tube, with a test point for each to monitor individual tube current.

Hans-Jorg, this is a fixed-bias amp, the cathode resistor being spoken of here is a 1-ohm current-sensing resistor.