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RE: New schematics

Posted by KevinC (M) on March 6, 2015 at 05:58:17

In Reply to: RE: New schematics posted by KurtP on March 5, 2015 at 19:56:04:

I have built several PP amps where I used direct coupling between the first and second stage. The problem that I encountered was not bias instability, but the consequences of imperfect first stage tube matching. If the second stage tube had low grid bias voltage, a small plate voltage offset in the first stage due to mismatch in the input stage tubes "ate" a fair amount of the voltage swing available to drive the second stage to symmetrical clipping. It's not that I envisioned clipping the second stage in normal use, as that would be bad design, but simply want to have plenty of "headroom" in this transaction. The least fussy solution I found was to use a driver (second) stage tube that had a high enough grid bias voltage that a few volts of plate imbalance wasn't a big issue. I ultimately settled on triode-connected EL34 drivers in those amps. Of course there are other ways to deal with this as well, short of adding another IT.

Kevin Carter
K&K Audio
www.kandkaudio.com

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