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I've got heater current, sort of, but the tubes won't light

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  • I've got heater current, sort of, but the tubes won't light

    I finished up a 5E3 build the other day and the amp sounded great, was quiet in a good way, and worked fine last night. When I got home from work this evening and plugged it in I was getting nothing.

    I didn't see the power or preamp tubes lighting even though the pilot light is on, and they weren't getting warm. I pulled out the DMM and I have 7.2 VAC across the wires where they solder to the lamp, and about 3.55 VAC from each of those wires to ground. I tried measuring across the filament pair where they first connect to the power tube sockets and got nothing so I figured I needed to resolder. The odd thing is that I can go along the entire string of heater wires all the way to V1 and I have 3.55 VAC to ground, but I never get the 6+ volts across any pair.

    Does anybody have any ideas before I go back and start hitting the solder joints? If I have correct voltage to ground at each wire I don't see how I can only have the full voltage across the pair at the lamp. Am I missing something here?

    Thanks.

  • #2
    There is an open connection between the pilot light and the first tube on one side. Since the cold heaters look almost like a short, you get the same voltage on each side to ground at the tubes but no current is going through the heaters. Pull all the tubes and one side will show zero volts.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    • #3
      Thanks a bunch. It's nice to know why I was getting the readings that I was getting. Basically I was reading through the tubes to the other side of the transformer, right? I must not have tested the voltages with all the tubes out even though I tested the resistance. After taking into account what you said I'm assuming I never got an open reading when testing ohms because I was reading back through the pilot lamp.

      I found the bad solder joint even though it passed the chopstick test and I tested voltages without the tubes before taking the iron to it so that I knew what I was fixing and didn't just find it blindly.

      Thanks again.

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