Are individual bias pots per tube better or a waste compared to one bias pot per 2 tubes (1 channel)? My understanding is that the individual pots would allow greater freedom when using unmatched tubes.
The kits I've built using the Curcio driver mods use 1 pot per channel with the option to also balance the bias between the tubes using a 2nd pot.
While I haven't looked extensively I haven't seen anyone claiming their bias controls equates to better sound. Then again I tend to gloss over the marketing bs unless it's really entertaining.
Its a good question and the answer depends on several things. Although David Manley likes to have a pot for each tube this can be difficult for the user especially when installing a large set of new tubes. If you have 8 pots you should turn them down siginificantly (not necessarily all the way) to avoid overheating the new tubes which will likely be stronger as they are new or just because they are different. There is a terribly large range of bias voltages for output tubes from maker to maker, batch to batch and even within a batch. When I match up tubes I have at least 100 tested and measured with precise bias and gm values. The range from top to bottom forms a bell curve and the ends are usually 10 volts apart. At typical gm values every volt is about 6-10 milliamps of difference. This is why we match to 0.5 volts typically and we can provide matches to 0.1 volts which is a difference of say 4 mA out of 50 which is less than 10%. Read my paper on matching for more details.
http://tubeaudiostore.com/virofpowtubm.htmlThe main problem with idivudial bias pots is interaction caused by the fact that as you increase any one tube it reduces the current in the other tubes due to power supply sag. (Someone called me yesterday and said "tube sag" is all over the internet and he thinks they are talking about power supply sag}. Any links would be appreciated). This is quite normal and it forces the user to go around the circle of bias pots several times to get them all equal. What I did in my RM-9 SE was to provide one bias pot per channel and one DC balance pot so once the bias is set the balance, which is more important, can be nulled to zero. With individual bias pots this is very difficult to do as one has to learn to overbias a tube by the proper amount to compensate as the others are adjusted, just as a skilled piano tuner sets a string a little sharp to compensate for the torque of the tuning pin as he releases the tuning hammer, its a learned skill.
If you read the paper linked above you will also see other advantages of matched tubes like symmetrical clipping and reduced distortion. Bottom line, there is just no good reason to use unmatched tubes unless you can confirm that they are reasonably close. Two tubes from the ends of the spectrum would sure degrade the performance of an amplifier.
Poll