The itty-bitty TO-92 Siemens BSS101 MOSFETs used as source followers
after the line stage tube fail on rare occasion. If you cannot find
this exact part, then the Zetex ZVN4424A is a good replacement (TO-92,
N-Channel, 240V). I recommend replacing the right and left channel MOSFETs
at the same time to keep then two channels identical. www.digikey.com
carries Zetex parts.