Trying to understand biasing

[ipetruk]

Hi everyone,

I've built a Mainline today, pretty nice device. Learning the basics of operating it.

Biasing made me curious. Why does Mainline need biasing? How some amps are autobiasing, like Crack? Is it Crack's simplicity that makes it not require this, basically it's autobiasing comes from NOT having something that Mainline has? Could Mainline be hypothetically upgraded to have autobiasing?

Does biasing allows you to use "unmatched" pairs? Many online shops sell matched pairs, is it not important anymore since we do it ourselves?

Interesting, with manual bias in amps like Mainline would one have to re-bias theoretically each time when they roll 12AU7 or 6c45pi's? I know there is not much difference, but I have become curious of the procedure itself.

Thank you,
Igor

[Paul Birkeland]

Rolling the 12AU7 does not require rebiasing the amp.

In order to make the rated power, the 6C45 needs to make its full voltage swing symmetrically, hence the plate voltage is dialed in with relative precision.  You could definitely redesign the Mainline and add another 100V of B+ voltage and eliminate the adjustable biasing in the process, but you'd have some substantial thermal issues to deal with in the process!

[ipetruk]

Thank you. What about matched tubes? Are those required for Mainline, or not because of biasing (e.g. biasing makes any two tubes matched)?

[Paul Birkeland]

No, matched tubes are not required.