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B+ Power?


unclej
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i picked up a copy of the tube amp book a few days ago and came across a phrase that they didn't explain very well. in their troubleshooting section they say the first thing to do is "check the B+ voltage--most of the troubles will be found in the power supply". they refer to B+ voltage several times and even to the B+ circuit but never explain it so i thought i'd check with you gentlemen.

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thanks..in general i'm just learning how to troubleshoot and repair tube amps..specifically i'm working on an ampeg geminii II. the slightly frustrating thing about all of the books that i've read so far is that they assume that i have knowledge that i don't. :D so when i book tells me to check that B+voltage it would be great if they had a picture of the specific place that i'm supposed to check..but they don't. i've ordered some videos and i'm hoping that they'll unravel some of the mystery.

by the way, the same books gives a step by step process of checking voltage on plates and screen grids but don't say whether the tubes should be in place or removed. which would it be?

thanks

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B+ is the supply rail voltage, usually at the first power supply stage - at any given stage in the amp, it's the voltage level before the plate resistor (or transformer, in the case of the first power supply node), generally measured at the power supply filter capacitor. Always check voltages with the tubes in place - without the current draw of the tubes, the voltages will read higher.

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  • 2 weeks later...
Always check voltages with the tubes in place - without the current draw of the tubes, the voltages will read higher.

WISE VERY WISE INFO HERE. things to consider most amps that are production models can and will jump several hundred volts if the tubes are not in place and you run it at full voltage.. i was looking at this plexi where the plates were gettting like 460vdc when the amp wasn't loaded. this could blow your 400v caps if left to prolonged exposure.. EXPLODING CAPS ARE STINKY DONT' DO IT LOL..

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