dhuebert wrote:To me
screen feedback caps
means caps on the screen grid in parallel with the UL taps?
Sorry I got sidetracked last week, Don!
Yes, the screen feedback caps are going from the 'lower screen' to the cathode of the 12AX7. This feedback is in parallel with the negative feedback from the output transformer secondary. This is used in several of Hafler's designs, mostly a tweak due to the way those outputs were wound (my guess).
In other words, I think we can change the whole feedback stability design when using non-Hafler outputs. We can omit, or lessen, the input low pass filter. We can omit the screen cap. At that point we can do open loop testing (to see where the upper frequency pole is at so that we achieve less than unity). Then add a feedback resistor for the amount of dB of NFB we want and close the loop. Then shoot a square wave through the DUT and tweak with our cap in parallel with the feedback resistor.
dhuebert wrote:Twice now I have been told these tubes are supposed to run @ 40-50 mA, the data sheets I have seen indicate 100mA, whats the deal here? I will try and turn down the current and see if it makes a difference to output.
I sure appreciate the help, I'm the kind of guy who has to know why
Don, I'm wondering if that 100mA is the rated current for a
pair of PP tubes. Anyhow, now that you are using 6L6GC's, I wouldn't worry much.
dhuebert wrote:Oh yeah, is the ST-35 schematic posted here somewhere?
I just upped a copy here:
http://www.diytube.com/ST35_REVD_circuit.pdf
Shannon