cGil

That amp is cathode biased, and simply adding a pot to R44 to screw around with extreme bias settings is just a bad idea all around when only adding 10 or 20 ohms can make a huge difference. Besides, R44 is a 250r/25watt resistor. Seen any 500r/25watt pots lately? If so, it's gonna be a bit large. And expensive! And converting the amp to fixed bias, whether actually "fixed" with a resistor or variable with pot, is not an easy undertaking even for old guys much less new guys. Better work on your calculator chops before you tackle that mod. There's gonna be a whole lotta math and load lines involved. http://www.aikenamps.com/WhatIsBiasing.htm Gil...

The KT88, 6550, KT77, KT66, EL34 all fall under the "more" category. I'm afraid you're pretty much stuck with 6L6, 5881, and 6V6 on the low end. http://tdsl.duncanamps.com/tubesearch.php As far as adding an FX loop goes, there really isn't a good place to throw in a passive loop like we did on the VJr, and there isn't a typical cathode follower stage for properly driving a loop unless you want to make your own. If you want to add the tube (or 3SB board) and circuitry for a driver and recovery stage, I'd recommend inserting it before the reverb send. A Vibrato circuit is gonna be yet another tube. Honestly, if you want an AB763, ya might as well just gut it and start fresh. Or keep it for what it is and get to butchering a fresh chassis. Gil...

That is indeed a good question. The OT still isn't designed for the 3.2k impedance EL34's like. Probably closer to 4k. The optimal bias for EL34 might require resistor R44 swapped for 120R, 25W, which would potentially draw 50ma more current through the OT. That extra 50ma could overheat the OT and eventually kill it. Another big question is how long would the PT last supplying an extra 1A of heater current? A day? A week? Forever? Can't say till somebody does it and reports back. If the amp had started with EL34's, it'd be easy to go down the pecking order of tubes as it were. But by going uphill from 5881/6L6 to EL34, you may be forcing yourself into a corner that's gonna require buying all new iron for that thing. Is it worth it? Gil...