there are screwdriver reachable pots one next to each pair of tubes. there are also test lead reachable holes next to each pair of tubes. the two smaller tubes are input (taller tube) and split duo-triode(smaller one) that do not need any bias. deluxe monos have solid state recrifier that is only visible on its PCB. contact luke manley or bea from vtl to get precise voltage setting. it's been very long time since i've owned those.