Hi Stuart,
SY said:
Tom, why such a high pre-bias on g1?
You should ask Bob D. about that
Well, using considerably lower pre-bias, statical Eg2 must be much lower, too, limiting the possible Eg2 swing range and upping Ig2 and Pg2. Also, the more (negativer) Eg1, the higher is statical stability due to automatic bias self-regulation, just as with any other grounded cathode auto-bias arrangement. This makes sense with such high gm & perveance tubes. You may remember, at ETF06, Tim de Paravincini mentioned, that at least _some_ auto bias is required to keep xL5y9 from runnaway even in plain old BPT mode.
Also, the negativer the pre-bias, the wider the "channel" between max Pd hyperbole and the area of spoiled impedance behaviour you can lay the loadline into, as you can see from this series of plots for SD EL36:
http://www.tubes.mynetcologne.de/roehren/daten/el36pentode_sd_15v.pdf
http://www.tubes.mynetcologne.de/roehren/daten/el36pentode_sd_20v.pdf
http://www.tubes.mynetcologne.de/roehren/daten/el36pentode_sd_25v.pdf
As you can see, it doesn´t make sense to use less than, say, -20V Eg1 with SD EL36, since the spoiled impedance area extends so much.
There is no reason why xL5x9 should behave differently in this regard: Below a certain Eg1 prebias, the spoiled impedance area will overlap the Pd hyperbole, so there is no chance whatsoever to get stable and safe operation for _any_ loadline.
Hence practice confirms that Professor Barkhausen was right.
Regards,
Tom