Question on Biasing

[Madison]

I'm trying to bias a DSL50 to about 40/45mA (first time). At the lowest setting on the pot, the multimeter reads 59...can't get it any lower. Dropped the second EL34 down to 59, went back and re-checked the first tube, and it now reads around 70. Evidently I'm doing something wrong here. Can anyone advise me on what I'm missing? Thanks

[danieldroukas]

1) make sure the DC voltage out of the pot is maximum negative voltage. Assuming that you're reading it right, and that the voltage out of the pot is indeed negative, then -59 volts seems like a good reading.

2) are you reading the current via the voltage drop across a cathode resistor and, if so, what value resistor did you put on the cathode? What voltage are you reading on the cathode?

3) You need to aim for biasing toward a static dissipation wattage, not just by measuring current on the cathode. Multiply the current that you read from the cathode (in amps [45mA=.045A]) by the voltage you read on the plates of the tubes. For EL34s, 20 Watts is the max you'd want to aim for, anywhere below is fine.

4) If you get values greater than 20 watts, come back and we can investigate further.

Anyone else have any ideas?

[Madison]

Thanks, I appreciate the info. All I can say is I'm using a simple bias probe attached to a single EL34 tube, plugged into my multi meter...and according to the probe directions I should just adjust the pot until it reads 45. Sounds easy, but of course isn't working out that way.

[K-man]

The tubes may be bad. Do you have another set you could try?

[trdlasvegas]

Doesn't a DSL50 have the standard Marshall test points already built into the back of the amp?

-Tony