As promised, here is a schematic for the final version of the modified 8800, that includes the Z-565 4/8 transformer.
Final thoughts: I know for me -- and I suspect others as well -- the 6V6 represents as much of an emotional throwback to the early days of economical high fidelity, as it does in continuing to offer good performance today. Throughout the early to mid 50s, the audio magazines of the day were filled with circuits, reviews, and projects using this tube, from the likes of Grommes, Arkay, and transformer manufacturers. In all of it however, I'm struggling to recall a commercial UL product (other than White) that operated these tubes in UL. Besides the aforementioned DIY article of Hafler, another notable DIYer of the day (Marshall) also experimented with the tubes in a tapped screen circuit -- but again, there was no discussion in his articles regarding performance as the onset of full power output was reached.
By the mid 50s however, the EL84 was starting to make itself known, and with its power output advantage -- as well as (we now know) its advantage when operated in a tapped screen grid circuit -- the poor 6V6 fell by the wayside rather quickly (except with manufacturers like Magnavox who could scoop up the vast supplies rather cheaply), so that ultimately, there were just very few if hardly any commercial examples of equipment operating the 6V6 in UL mode -- certainly not like there were EL84/6BQ5 examples -- which themselves were fairly rare as well. But all else being equal, in pure pentode fixed bias mode (as is used here in this final draft), the 6V6 will give the newcomer tube a serious run for its money in terms of performance delivered -- and, deliver all the nostalgia that a tube that helped build America can along with it. As far as 10 watt 6V6 amplifiers go, it will be tough to surpass this one, so for those with a fond spot for the 6V6, who want maximum performance from it, this should be on your short list. That it delivers the performance it does, with just good classic design implementation, is a testimony as to just how good its Beam Power configuration really is. It may not have the Gm or the diminutive size of the EL84, and it does have a a rather sharp clip (as compared to the EL84) in the circuits discussed in this thread, but if you can see your way beyond those drawbacks through proper design, it's minimized screen grid current and reduced heater current draw represent their own advantages, which has been put to good use here. Not bad for a tube that turned 80 years old last year!
With that then, that makes a trio of Magnavox amplifiers visited, with the bigger units significantly improved even without a transformer change. When they are changed however, it really moves them into an entirely different performance category. If you're serious about your Magnavox 8800/9300 amplifier, the Z-565 transformer is well worth the money they cost -- compared to other replacement offerings.
These amplifiers are each unique, with each having their own character. All are extremely enjoyable to listen to. I hope they've offered all of you some ideas for your own DIY projects, and shown that with a little re-engineering of the original designs, a lot more performance can be coaxed out of these chassis.
Happy listening!
Dave