6sc7 vs. 6su7 in LTPI duty

[aletheian]

I have a few of each (GE for the 6sc7 and Tung-Sol for the 6su7) and wondered which I should tinker with in a LTPI with a FET CCS driving a class A PP el34 output section. They seem fairly similar in performance on paper, but does anyone have experience with their sonics or op points where they are most comfy?

[ray_moth]

Yes, they have almost identical mu and Rp but there is one big difference betwen them, namely, the 6SC7 has a shared cathode and the 6SU7 has separate cathodes. For LTP application this doesn't matter, of course, unless you want to use separate resistors between each cahode and the long tail, to compensate for any DC imbalance or to provide a little current NFB.

The 6SU7 is basically a 6SL7 selected for good matching. This tube makes an excellent-sounding LTP with a current of between 500uA and 1mA in each half. Use it with a CCS, as you say. 6SU7 is unsuitable for driving any kind of low impedance load or anything requiring significant voltage swing. It is far better to follow it with a 6SN7 differential driver.

PP El34s in pentode mode with cathode bias might be OK for 6SU7 to drive directly but with fixed bias or if strapped as triodes, they would impose too much of a load. I imagine a 6SC7, having similar characteristics, would be much the same.

[aletheian]

Good info. I'll consider a 6sn7 between the LTP and the el34's... shouldn't bee too much of a problem. Are you referring to a DC coupled follower setup with the bias voltage applied to the cathodes, or an AC coupled voltage amp setup (probably with a negative voltge "ground" refrerence) with the signal from the plates feeding the grids?

[ray_moth]

I'm sorry, I don't understand your question.