Hoping to spur some movement toward firuring this out. Here is what I have compiled about what is going on.
The (2) 10K resistors at the output (R32 & R44) are in parallel. The question was raised earlier about which to remove, one or both. I have also seen a post about raising the coupling 4.7uF cap (C13) to 20uF to extend the low end knee.
http://www.groupdiy.com/index.php?topic=16699
In looking at Jakob's description of the output:
The output trafo is wired 2:1 to get the 2400 Ohms Zout of the SRPP stage down to around 600 Ohms. The 2:1 ratio means that when using the OEP 1+1:2+2 transformer, it has to be reversed. I've had very good results using this output stage topology in all sorts of designs - it's simply my favourite tube circuit. Also used in the Gyraf Pultec amplifier.
The output of the transformer is taken to SW4, the phase reverse switch, that has 10K resistors across it to control transformer load when switching. And then to the output XLR.....
A 4.7uF cap with a 600 ohms gives a knee of 56Hz. In calculating what resistance gives you a knee of around 450Hz, I get about 75 ohms. A high pass filter with 75 ohms and a 20uF cap gives a knee of 100Hz. It seems that taking out the 10K resistors, or raising the coupling C13 to 20uF is basically going after the same goal of lowering the bass cut off.
I am not sure about the variables, as my design skills are minimal at best, although my troubleshooting skills are respectable. Are the 10K output resistors really needed? Seems the easier solution (if this is really fixing it). What is lowering the resistance at this point in the circuit? I measure 23 ohms off of pins 2 & 3 on the XLR output while power is off. Taking out the 10K resistors seems it would do the opposite of what is needed, you want to increase the resistance to lower the low end cut off. In looking at it from a troubleshooting perspective, what is really causing this to happen, or is this the designed specs for the G9? Are the common variables OEP output transformers and Gustav boards? Is there a common mistake in assembly here? The phase switch is right in the middle of all of this and one of the 10K resistors is on the main board, while the other is next to the phase sw on the front panel PCB.
Part of me wants to figure it out, and the other is very happy to get it working and move on. I am going to try taking out the resistors to see what happens as that is the quickest and easiest thing to try.