Here's the insides
They are el34 push pull mullard topology with Edcor iron.
It is based on the Marantz 5 with some things from an Eico 86 too.
I used a jensen 10K:10K input transformer for balanced inputs feeding a 5687 grounded cathode amp which has a low sensitivity - around 10vpp for full output, as it designed to hang off a TC Finalizer which is on the outputs of my desk.
It has first tube stage direct coupled to 6sn7 phase splitter with an ac balance adjust pot and uses fixed bias with seperate adjust pots for each el34. An ammeter shows bias current for each tube on a DPDT switch.
The 100mA ammeter monitors the cathode current and is in series with a 10R 1W resistor - a little less actually. Meter resistance is some tenths of an ohm so it makes up the difference to 10R resistance.
For more accuracy I would need better low resistance measurement of the cathode resistances and a lower value bias pot on the grids - 5K instead of 10K or even lower for a finer adjustment.
When switched to each side, another 10R is substituted in opposite and vice versa.
I chose 10R resistors which were a little more than 10R.
So the monitored cathode and unmonitored cathode are still balanced as far as I can easily measure. (10ths of an ohm).
The fixed bias method and measurement also allows me to confirm that the tubes remain balanced.
Avoids the need for the original Marantz monitoring which measures on HV. Safer in a diy build not to have HV metering and switching !
I am biasing to 40mA, 460V plate (18.4W) and 45mA each tube. The whole thing uses around 12mA for the input tube, 16mA for the PI and 90mA for the finals -> 120mA of HV at B+ 466Vdc
Uses elevated heaters to 65V dc referenced from HV.
Feedback is currently around 12dB or so and still looking at that with my cap/resist sub boxes. The circuit is quite low gain overall, as you could imagine with a low senstivity to full output.
Just building some dummy load boxes for the performance bench tests.
I'm still tweaking but early indications are good.
These are my first tube amp builds where I calculated and sim'd the operating points of the circuits.