Dual Triodes with Common Cathode

[drewspriggs]

Howdy,

Working on another miniature, and the original called for a pair of 6N1's (or equivalent) - one tube acting as two successive gain stages, and one in self-split PP for the output to get around a watt from 300v. As I've got a shitload of Belton octal sockets floating around, I thought I'd do something slightly different and do it from octals.

As the original design of the amp came out of a design challenge to get something around a watt for under $100 in total, I want to stick to the general design philosophy so avoiding all the ridiculously overpriced octals - 6SL7/6SN7/etc is out (especially in Australia, where you're looking at minimum $40-50 per tube). I came across a quad of 6N7S's which looked like they would fit the bill and after ordering them (for a whopping total of $25) realised they are common cathode.

From all the reading I could see it looks like they were originally designed to run class B but apparently run class A they put about a watt - perfect for me.

My question relates to the common cathode design:
*can I treat them as triodes sharing a single cathode resistor/bypass cap for the gain stages?
*as the original output stage called for connected cathodes, can I just leave this how it is?

Finally, if these tubes aren't going to be suitable, what other reasonably common, reasonably cheap octals would you suggest? I've seen 6N8S for ballpark same dollarydoos, which are apparently equivalent to a 6SN7.

Thanks

[sluckey]

You cannot share cathodes of series gain stages w/o creating a feedback loop.

Read this discussion. Listen to what PRR has to say.

[drewspriggs]

You cannot share cathodes of series gain stages w/o creating a feedback loop.

Read this discussion. Listen to what PRR has to say.

Well that answers that then.

Guess it's time to go a-browsing for more weird Soviet tubes