Grover,
Thanks for the link to the Keroes article, I was trying to find it to sent you with respect to my bias/balance scheme, but could not find it (should have bookmarked). I have been using that article as well as the Hafler Modified Williamson as the basis.
With respect to your comments/questions,
I am using a 10K input resistor to the input stage (grid stopper). No pot.
I checked the transformer primary windings, and they are correctly tapped.
My bias arrangement is similar to the Keroes, only I used as 25K pot with the wiper tied to one end to work as a variable resistor instead of the fixed resistor. Balance pot arrangement same as Keroes (10K pot), however, using two 47K resistors to ground instead of 100K resistors in the Keroes circuit. This is to increase current through the bias network since I included a 150H/8mA choke inline with the bias pot (between the two bias supply caps) to improve filtering. (Note to self, I need to try bypassing this to ensure it is not inductively coupled and injecting noise into the bias supply). The two resistors above are effective only in DC, since the last bias capacitor bypasses these, I calculate a corner frequency of 0.14 Hz. From an AC perspective, it should have no effect on the voltage divider formed by the resistor bypass of the coupling cap and grid leak resistor.
I did some quick calculations of the effect of the coupling cap values and voltage divider values, compared with the Keroes and Hafler Williamson variants.
This circuit is a LF step (cut) circuit (Grover, you are right about that, I mistakenly thought the effect was at high frequencies). It basically limits the LF rolloff of the coupling cap, wiht the cap acting into the grid leak resistor defining the bass cut frequency, and the cap in parallel with the bypass resistor defining the stop.
The Hafler circuit used 0.22uF || 1M -> 100K, which gives a 7.2 Hz corner for bass roll off, stopped at 0.72 Hz
The Keroes circult used a 0.22uF || 470K -> 47K, which gives a 15.4 Hz corner for bass roll off, stopped at 1.54 Hz
My substitution of the 0.47uF cap results in the same bass roll off and stop frequency as the Hafler circuit. Therefore, I believe I am OK with respect to the original Hafler circuit (using the Dynaco A-430/A-431 iron, whereas the Keroes is using Acro TO-330).
So I am pretty sure that my modifications are technically sound, still, the amp is oscillating...
David