All tube types, brands etc. will react differently to the amount of bias needed to be in range.
They all draw current at different amounts due to the tube type and natural current draw.
Most tube vendors sell sets, quads or sextets in a range that matches as closely as possible to
the current draw of each tube. For instance, a Groove Tube EL34 rated 1 will bias much quicker on
the pot than a 10. For other vendors, they label the tubes from 20-50 etc. These numbers are the tubes
natural plate current at a specific voltage and bias from their testers. A 20 will bias slower on the pot
than a 50 because the 50 is 30mA hotter in what it draws naturally, or how it came out of the process to
manufacture it. When dealing with a six pair wise power amp, or just a four tube type the closer the tubes
are matched in natural plate current, the more each tube is running at it's optimal bias. In a pair wise like
Herbert you don't want one tube to have a natural plate current of 50 and the other to be 20. The 50 will require
less bias and the 20 will not even be near the setting you want. Duets of any tube type you want within 5mA of each other
should be perfect for your Herbert so it has each tube working with it's partner and will reduce any hum in the power amp to a
minimum. They will also wear more closely over time.
Don't forget that each pairs draw while biasing effects the other two. Keep checking them all until you have all
three sets balanced and set to your target number.
