Self Biasing circiutry and the Crate BV6212?

[dugdiamond]

29601SCH_2.pdf296-11PL_PWA_SCH_1[1].pdf
Hello Everyone,
I've been lurking around these forums for awhile, and have found it very interesting, informative and at times entertaining.
Especially useful was reading all the Blue Voodoo posts while I was restoring the function of a defunct BV60 chassis I had EBAY'ed for real cheap.
I am surprised nobody has addressed the theory of operation on the "auto biasing" circuitry.
According to the Crate owners manual they state that the end-user is able to replace the stock 6L6 power tubes with EL-34 because its "engineered with automatic biasing to facilitate easy powertube replacement".
Is there anyone out here that would be willing to share with us how this is accomplished? Walk us through the circuit, what does what and how it does it? What are the factory designed operating specifications, limits or parameters. How is this measured or set.
Specifically, is it practical to make adjustments within the circuitry to fine tune the bias or basic range of the circuit? For example, suppose the stock range is a bit too conservative and you would like to run your output a bit higher? Or what if it's pushing the dissapation a bit much and you experience premature tube failure, how would this be compensated for? Or for instance,,, ya wanna get all experimental and try out another tube style all together , such as a 6V6?
(BTW- I'm running NOS Sylvania 6CA7's) It seems that they're pretty stout tubes and seem to be holding up well after about 20hrs of hard playing. Strangely, it seems that various different flavours of EL-34's , EL-34eh, EL-34g, EL-34l , EL-34wxt ect.... have all seemed to be running quite hot and at the very edge of discoloring the plates! I haven't had any 6L6's to compare with.
I'm getting a bit wary of continuing with my NOS 6CA7's until I can measure the static dissapation.
Not having a bias probe, would it be correct for me to measure the voltage drop across the R86 and/or R87 and divide the voltage by the resistance and arrive at the current, factor the calculated current against the voltage reading of the plate and get a rough idea for power dissapation? I realize that this doesn't account for the screen, but will this correctly get me into the ballpark of where I am at regarding the power being dissapated?
Am I correct in assuming that Q16 is somehow regulating the bias voltage through it's junction relative to the current reaching it's gate? (I'm not too sure of my technical saavvy?!? Simple self-edjucation) (learned it all from reading and trying to understand enzo...hi-hi).... .. .... ..
Also..... would the value of R96 determine your bias voltage or would R111 be where you would want to manipulate the bias from? Or more specifically,,,, where could a resistor be replaced with a variable resistor/trimpot for bias adjustment???
I look forward to hearing from you guys. I can't tell you guys enough what a valuable service you do for this community. This is what the internet is all about!!!! Thanks

[octal]

I can't read the transistor schematic designation, but the transistor located beneath the output transformer in the schematic is the bias regulator. It appears to function as a shunt regulator for the bias voltage based on the voltage drop on the cathode resistors for the output tubes.

You can measure and calculate bias based on the voltage across R90 and R96(?) (can't read last schematic designation.) They appear to be 12 ohm resistors, so via ohm's law, V/R=I

Divide the voltage you measure across each resistor by 12 and that'll be the current flowing through each tube.

Nathan

[dugdiamond]

Thanks Nathan. I just reviewed the Schematic and noticed that I had made a major error in which resistors that I wanted to measure the voltage drop across. I actually was thinking R90 and R91 which traces to the cathode !!!!!! OOPS!