sailor wrote:Interesting that Carver chose the 12AX7 as a phase splitter. The impedance miss match between the split outputs is many times worse with a 12AX7 than with a 12AU7. Also there is no amplification advantage because the phase splitter will always be about .9 gain no matter what triode you use. The third problem is the 12AX7 has much less available amperage and more prone to microphonics. I think it was a bad choice all the way around.
Which brings up another point I have been harping on for quite some time. The ST and SCA both have positive feedback. This feedback comes from one side of the phase splitter. I don't like the use of positive feedback in this type of amp to begin with and Dynaco only put it in to increase the output of the driver stage. But forget that, the main problem it creates is no matter how matched your EL84s are the split driver signal is miss matched because part of the output from the cathode is being pulled off to run the positive feedback loop. The result is distortion which gives the amps a very pleasant dispersed tuby sound. Most people actually like this distortion and call it to detail.
So, could the positive feedback part of the circuit be eliminated? Looking at the stock circuit of the ST35, it looks like the .22uF cap and 150K resistor. If I'm understanding you correctly doing this would reduce output of the driver but would also eliminate some distortion?