Not too hard a task, although it's been some time since I ran my old one. Behind the power tubes and centered in front of the tranny cover is a five (5) terminal female jack. This jack is laid out in a row similar to the layout of the four power tubes. Using a voltmeter (digital if possible) with pinned probes, your negative probe goes in the center terminal. The positive probe goes in the terminal corresponding to the power tube position (i.e., left outboard terminal = left outboard tube). Don't worry if you do it backwards, no harm will result from crossed polarity. Bias adjustment is via the small recessed pot closest to that power tube (I used a plastic TV adjuster tool from Radio Hack for safety). Nominal values for the 502 series is 50 mv (+/- 5 mv). Try to err on a lower bias voltage to extend tube life. I've found that the Jolida holds a fairly stable bias - once every six months was plenty, but YMMV. It's nice to let the amp warm for 30 or so minutes before setting bias, but if you don't where it is to start, you may want to set it initially in the 45 mv range and readjust after 30 minutes. Also, be careful how you drape the test leads - they can fall on to a hot tube if you are not careful. Some side notes: this is one amp I've found runs dramatically better plugged straight into a wall outlet with no conditioning whatsoever. It also benefits from a coupling cap and input wire/jack upgrade and replacement of the stock Chinese input tubes. I heavily modified mine several years ago, including some power supply tweaks, with very positive results. Bypass the balance pot if you can and redo the output tranny leads to the power octals (as many, including mine, had dirty solder joints). Others have gone further, with extensive outboard power supply mods. Best power tubes for me were the KT-90s. You can also easily convert this to triode mode and throw a pot on the feedback loop (or disconnect it altogether). This amp has fairly decent iron for its price point (all important), and all the rest can be improved at low cost. Great tinkerer's amp. Good luck!
