18Watt.com

I've come across older posts suggesting that unmatched tubes can produce a sweeter tone, and I've noticed this in my own experience a few years ago, particularly with my 18 where the tubes are approximately 8mA apart, resulting in a pleasing tone. Since my PT lacks CT for heating, I used two 100-ohm resistors. My question is, would a linear pot with 250 ohms help mitigate the hum from unmatched tubes? Also, is there an upper limit to using unmatched tubes?

What reason do you have to suspect the mismatch is causing the hum?

Hum cancelling is lost when power tubes are very far from matched. What's considered matched or unmatched is a matter of opinion anyway.

To learn more, read this thread, and especially pay attention to what Phil says: viewtopic.php?t=23653

How far apart is the plate dissipation? I’ve got a 5f11 clone with a mismatched PD of 8 watts and no added noise. The current is about 10 mA apart if I recall.