Thanks Tom, I try re-adjusting the OD trimmer.
I used my Fluke 77 to measure the ac voltage on the PI and adjusted the trimmer to get a matched output from the PI stage.
I seemed to have glossed over things here. I put a 1Khz signal into the PI, I then measured the ac voltage on either side of the PI. I did this as it is relatively quick compared to using the CRO. I thought the meter measuring the ac voltage would be doing it the same way for both sides of the PI.
I should also point out, this isn't the end of the process, it is the beginning, the way the output tubes were any adjustment to the PI was futile.
I needed to reach a point where everything is balanced again, I wondered whether the transconductance of the tube was as big a deal as we'd made it out to be?
I'm not sure, but I seem to recall output tubes don't have a lot of gain, but the function of the output tube is to pull current through the output
transformer thus the significance of transconductance.
As I mentioned earlier I will have to see if the same bias current means a similar transconductance, and when does it all go pear shaped?
The bias does set the operating point which is in effect the plate characteristics, does the different forward conduction points really matter, I'm thinking that you shouldn't really be driving the output stage all that hard in a Dumble?
Who knows I might learn something.
I believe adjusting the symmetry of the PI makes no difference to the PI itself, it is about how it drives the output stage.
I also wondered why HAD didn't use a 12AT7 which could provide better drive to the output stage if biased correctly.
So progress for me it to check the OD stage again, measuring the transconductance of the tubes (in a primitive way) and then try adjusting the PI by ear to hear the bloom.
I hope I've covered it all a bit better?