Musicians Roadhouse

I tried to bias my old Marshall tubes in my Ma50C. So I could get them at 53mA and not at the recommended 35mA to 40mA. I put some NOS siemens EL34 tubes in there and biassing at 35mA was piece of cake.

On the feet of these Marshall tubes I read 49mA 8/37. Does that mean they were once biassed at 49 mA?

Wich surprises me?

I changed out the Marshall tubes for the Siemens. Are these Marshall tubes toast? Or is it ok for them to be biassed so out of spec? They redplated at 70mA. But at 53mA they did sound allright and did not redplate....

Just an observation but these arbitrary bias numbers dont give a good reference of the actual dissipation of the tube, measuring plate voltage gives you a more accurate number.

For example, if your plate voltage is 460 and your ma is 35 ( 460 x 035 = 16.1 watt) That is fine for an EL34, but if your plate voltage is 460 and your ma is 53 ( 460 x .053 = 24.38w) that is right on the borderline for an EL34. Even though the plate voltage drops somewhat as current increases it still will be high.

Of course the tubes will redplate at 70ma, even if you drop 20 volts off the plate voltage the tube is still way too hot. ( 440v x .070 = 30w) The MAXIMUM idle dissipation for an EL34 tube is 24 watts and for most applications you're better off to be lower than that.

I replaced the tubes with the Siemens ones. I'll measure plate voltage when I get the chance. Thank you.... But since the Siemens were fine at 35mA.... And sound nice... Nothing else was changed on the amp.