[Diabolical Artificer]

Joe's humbucker circuit - see attached - is really interesting and has got me thinking about phase, in relation to the AC heaters of the OP stage. PJL also makes a good point about the naff EMF properties of PSU plate.

Regarding the latter, I'll see if I can get some thin steel plate, it will be relatively straight forward to pop this on top of the existing plate.

Re phase, at present there are two separate 18.9v AC heater windings, each is referenced to ground on the amp chassis, if they are referenced to ground on the same phase, could this be causing ground/amp chassis to pulse at 50hz? If I were to ground them out of phase, would this cancel?

This brings me to a topic I've often thought about. It's easy to think of ground, be that a ground bus or chassis, as some sort of rock or magic thing that sits at zero volts with not a lot happening, but obviously there are currents of various sorts flowing through it, in my case HT current, 6.3v DC current, AC OP stage HTR current, etc, but how would one check or monitor it? You can't swop a scope probe around and stick the probe black wire on HT say, probe tip to ground or any of the other separate voltage/current sources above. I'll admit to being a bit foggy on the use of differential scoping and what have you, that might help in cases like this, something I need to read up on.

It's an idea Andy to convert the OP HTR's to DC, there might be enough room. I'm not sure the winding's will be beefy enough though, although I did overwind them. Your right about series HTR's, but like a lot of design descisions this was one I took way back at the start of the amp build and would do differently now.

This brings us to some of your other suggestion's, which are always welcome BTW. As regards running the OP stage cathode biased I'm reticent to make too many changes as the ****** has already taken me two years to build and every change means undoing hours of work, not to mention causing damage in some cases to the existing structure of the amp. I do have an extra 20-30v for the HT if needed, when I wound the mains tfmr I wound a few more turn's on the HT secondary, just in case.

Regards the power OP, your right, it is daft, 70w is more than sufficient and there is no big difference in SPL between 70w and 120w. The reasons why I'm a bit set on squeezing out every last watt is 1) because already the amp is inefficient with over 56w to just run the OP stage HTR's, seems a good idea to ensure every watt counts 2) I have this silly preoccupation with big power OP amplifiers, despite there being several good engineering reasons why it doesn't matter 3) I'm stubborn and illogical : )

So at present I just want to get the amp finished and although there are several things that arn't perfect, if I can end up with an amp that has low THD, good BW and looks good, I'll settle at that. Else I'll be here till doomsday or have to start from scratch again.

Next amp though using Joe's OPT's will be reet, I'll be able to apply all I've learned on this one and hopefully do the job correctly from day one. Hope some of that makes sense; every suggestion and idea you put forward Andy, gets absorbed and logged away ready for the next amp build.

Andy.