211 and 845
These two tubes look similar, but are very different. The 845 has a Mu of 5.3 and the 211 has a Mu of 12. What this means is that the 211 has over twice the gain of the 845. Therefore the 845 requires much more negative bias and much more drive voltage than the 211. The two tubes have a definitely different sound. There is a similar tube, the 838, that has a high Mu, and is made to run at near zero bias.
It is possible to design an amplifier that can accept both types of tubes, but that is not usually the case. The amplifier would need a bias that was adjustable over a wide voltage range, depending upon the plate voltage. Plugging 845's into an amplifier made for 211's would probably lead to a bright red glow, followed by blown parts.
My 845SE amplifier can accept both types. the bias is adjustable from -200 volts to -35 volts. I have used both types of tubes. For my tastes the 845 has far better punch, bass and dynamics. I have also tried 838 tubes, but I wasn't impressed with the sound.
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4th August 2005, 10:30 PM
