------------------------------ Date: Tue, 4 Oct 94 14:23:43 EDT From: John Shriver Subject: Fisher 400 schematics The Fisher amps that use the 7591A tubes, and presumably those that use the 7868, use fixed bias for the output tubes. It's a fixed -24V (not adjustable), and is sourced from a selenium rectifier. The same supply also feeds the preamp tubes with DC filament voltages (12AX7's in series). The problem is that selenium rectifiers age most ungracefully. Their internal voltage drop slowly increases over the years, so that by now ANY of these Fisher receivers has less than -24V negative bias. (This aging is further aggravated by the noticeable current draw of the preamp tubes, which accelerates the failure of the selenium rectifier.) This low negative bias leads to the output tubes being underbiased, and thus running too near class A and their plate dissipation limit. When I got my Fisher 500B, two of the output tubes had obviously scorched plates. Most were gassy, one had near-zero emission. So, don't just replace the 7591A or 7868 tubes. Veryify, and FIX, the bias supply. Otherwise, your just throwing your expensive output tubes into a tube eating machine. (I replaced the selenium bridge with a silicon full-wave bridge. That slightly overbiases the output tubes, due to the lower drop, but I feel safer that way. One could bleed the voltage back down to 24V with a resistor if desired.)