Hey Tom, I would love to help you with this question... but I'm not 100% sure of the answer. If you are interested in learning the inner workings of a tube amplifier I would recommend " The tube amp Handbook" Buy Dave Hunter. It's a very good read. I'm at work now... but I will do some research tonight and try to get you an answer to you question. There is a ton of info to absorb about these amps and sometime's it can be overwhelming (IMHO) I am just scratching the surface of it myself... but it is very interesting and addicting.
Good luck.