Originally Posted by KimLaroux
I've actually been curious about building a beefed-up version of the SSMH with 12AX7 tubes and use it to power close-field monitors. Great to see someone else thought about it. I'm not that crazy, after all.
Gain is not the problem.
Stick with the 12au7 for speakers. How many vrms do you put into your nearfeield monitors? Like 2vrms? So you still have a decent bit of voltage headroom. Not an awful lot, but at the same time not an awful little ;)
You can probably find a few more vrms if you go around and realllllly optimize the circuit you have now. Using *just* the heater as the source resistor is a great way to save a few bucks, but not necessarily the best way to get the most voltage swing out the amp. Try to get the source of the mosfet around the middle of the voltage supply +/-a few Vdc. Add a resistor or CCS in series with the heater to make it happen. I would lean towards a resistor if you want absolute maximum voltage swing (despite my love of CCS's) since resistors behave better when you drive them into the rails. CCS's often work better under "ideal" conditions, but if you are really pushing an amp to its absolute limits things quickly become non-ideal. Its something else to experiment with anyways :)
You may also want to replace the plate resistors for the tubes with a CCS - this should buy you some basically free gain and improve the linearity of the tube a bit too.
If your interested in more current (have you hit that limit with your monitors? Yea, um no, but since I'm already here) try folding the heater so its 6.3V@300mA rather than 12V@150mA and adding a resistor or CCS to make up for the voltage difference. Ooh, you will need a PSU good for at least 1A and heatsinks like whoa.
Is now the right time to say that the 12ax7 is a sucky tube? Perhaps not the best time, but its never really a bad time to call a sucky tube a sucky tube. In the case of the MHSS or a variant thereof the very high plate impedance will not react well to the input capacitance of the mosfet and unless you have a very low output source you will almost certainly not need the gain after you change the plate resistors to CCS. Just stick with the 12au7, or one of its low gain, low Rp cousins.