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This amp was clearly designed by people who hate service techs. Even the schematics are nearly unreadable despite being created in the age of CAD. And I have a feeling this particular amp hates me personally as it seems to violate laws of physics.
I've got a 200 Hz signal generator plugged into the "high" Top Boost input. Measuring at the grid (pin 2) of the first preamp stage with the power off, I get 250 mV RMS. Turning on the power (but not standby) the signal level drops to zero as the tube warms up.I turn off power and as the tube cools the level slowly recovers to 250 mV. If I turn off power and pull the tube, the level immediately goes back to 250 mV. With power on and standby on there's a bit of signal, but only 30 mV RMS.
What gives? The grid electrode is supposed to be very high impedance. I've tested the passives and they're in spec. Heater voltage is +/- 3.50 V DC. I've tried a few different tubes and checked them all for shorts.
(On the schematic, tube pins 1,2,3 are on the left.)
I've got a 200 Hz signal generator plugged into the "high" Top Boost input. Measuring at the grid (pin 2) of the first preamp stage with the power off, I get 250 mV RMS. Turning on the power (but not standby) the signal level drops to zero as the tube warms up.I turn off power and as the tube cools the level slowly recovers to 250 mV. If I turn off power and pull the tube, the level immediately goes back to 250 mV. With power on and standby on there's a bit of signal, but only 30 mV RMS.
What gives? The grid electrode is supposed to be very high impedance. I've tested the passives and they're in spec. Heater voltage is +/- 3.50 V DC. I've tried a few different tubes and checked them all for shorts.
(On the schematic, tube pins 1,2,3 are on the left.)
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