Scodiddly
Well-known member
OK, not DIY but I suspect this is the best place for an answer.
I've recapped a 70's Ampeg B15N, and while it works OK it's extremely microphonic (even the power tubes, and different power tubes were tried) and also (I suspect related) the plate voltages are all 30-40 volts too high. I'm stumped - the only clue is that the 5AR4 rectifier tested "really really good" on my tube tester. The bias is right where it's supposed to be, and the output tube current is right where it's supposed to be. I would have guessed that there's a missing voltage drop resistor, but the circuit matches the schematic inside the lid while the voltages are way off. So either somebody swapped in a hot-rod power transformer (unlikely), or I've got an ultra-rare rectifier tube with Kryptonite plates.
One actual change made in the recapping was to replace the 30uF/600v electrolytic with two (parallel) 20uF/600v, resulting in 40uF instead of the original 30uF. I can't see this having any affect on the voltage.
A close-enough schematic can be found here:
http://members.aol.com/portaflex/schems/b-15-n68.gif
I've recapped a 70's Ampeg B15N, and while it works OK it's extremely microphonic (even the power tubes, and different power tubes were tried) and also (I suspect related) the plate voltages are all 30-40 volts too high. I'm stumped - the only clue is that the 5AR4 rectifier tested "really really good" on my tube tester. The bias is right where it's supposed to be, and the output tube current is right where it's supposed to be. I would have guessed that there's a missing voltage drop resistor, but the circuit matches the schematic inside the lid while the voltages are way off. So either somebody swapped in a hot-rod power transformer (unlikely), or I've got an ultra-rare rectifier tube with Kryptonite plates.
One actual change made in the recapping was to replace the 30uF/600v electrolytic with two (parallel) 20uF/600v, resulting in 40uF instead of the original 30uF. I can't see this having any affect on the voltage.
A close-enough schematic can be found here:
http://members.aol.com/portaflex/schems/b-15-n68.gif