by dcgillespie » Tue May 17, 2011 7:37 am
Geezer (Bottlehead) -- You've got the sum of the modifications required down perfectly. However, let me point out that even in this case, the vast majority of bias is still introduced at cathodes. With the two channel version, all that is being done is to allow a slight adjustment of potential to be applied to the output stage grid return (470K) resistors in each channel. When either of these individual adjustment pots are adjusted so that these resistors are referred directly to ground, maximum bias is created for that set of tubes for any given setting of the main bias control -- now call a bias "range" control. As these individual controls lift the 470K grid return resistors away from ground, they start to apply a small positive voltage onto the grids, causing the tubes to draw more current. Since each channel has its own adjustment in this scenario, each channel then becomes independently adjustable relative to each other, while the original bias control simply acts to bring the individual controls in range for the particular tubes employed.
In using this option, the original EFB circuit action is still fully in place, since bias for the tubes is still being established independently of current flow through them. This is the essence of fixed bias operation by definition, and as such, distinguishes itself from traditional cathode bias operation, since in that mode, bias is always determined by current flowing through the tubes.
Regarding your other questions:
1. The value of the 470K resistors does remain the same. Some have expressed concern that this value is now somewhat higher than is recommended for 6BQ5/EL84 tubes when used in fixed bias operation. That is true, but in this case, it is of little concern, since the maximum recommendation is based on a tube operating at maximum dissipation levels, and the EFB modification causes them to operate efficiently at well below maximum levels -- hence the improvement in tube life with EFB operation. Still, if this is of concern to you, you can resolve this issue by replacing the original 470K grid return resistors with 220K resistors, and then replacing the original coupling cap value with .22 mfd caps. This will bring the grid return resistor values in line with published maximums, while causing no change to the overall operation of the amplifier -- but as I said, the necessity to make such changes is more based on personal preference rather than any real circuit need.
2. The 100 ohm screen stability resistors do not affect the NFB levels in any way. These resistors are required due to changing the output stage operating conditions from cathode bias operation to that of fixed bias. This causes the tubes to operate at a higher effective levels of Gm. Since they are already high transconductance (Gm) tubes to begin with, this along with the operating conditions can promote output tube arcing if not properly addressed. The addition of the 100 ohm screen stability resistors removes any concerns in this regard. If you are interested in this more, please click on the link provided in the EFB article to see an additional article I wrote, "Techniques to Maximize Power Tube Life". In it, the use of screen stability resistors is discussed in greater length.
Good luck with your amp!
Dave