Raising the Bias


When adjusting the bias on my 300B tubes (amplifier recommendation 85MA) I noticed I could not raise the bias beyond 45 to 50 MA. The manual adjustment knob in the amp was turned to the highest point. Previously the tubes could eaisly be raised to 90MA range. Does this mean the tubes are bad or have got weaker? Am I close to a tube burning out?

Good bet it's just the tubes. If you can obtain a known good pair and test them, you will know the answer by the readings.

If these weak 300B's sound good, simply bias them the same current reading and enjoy.
Mcintech, Thanks for the reponse.

I did try another pair & was able to raise the bias to 85MA, so the problem is not with the amps.

I actully like the sound on the older pair but will this be hazardous to the amps in anyway?
Old, weak tubes will not harm the amp. If anything you'll save a bit in electricity usage!


My 1964 HH Scott 222D integrated still has its original output tubes, and with careful bias matching, I get the sweetest, most accurate sound (albeit at a lower power). Hell, it's only 22W to begin with...