Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: labb on October 26, 2015, 12:05:19 pm
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I am working on a Crate Blue Voodoo amp, BV-60 schematic that is eating my lunch. It has a loud hum with nothing plugged into the amp. By pulling pre amp tubes I think that I have the problem isolated around the second pre tube. Can I use the hoffman listening amp tool and technique to pin down exactly which component is causing the problem. Thanks. (Don't have a scope)
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Well, you can determine where the problem is in the signal path, but not necessarily which component is the problem. You put a clean signal into the amp and walk forward along the signal path until you find out where the noise starts.
Let's say it starts after the coupling cap coming off V2's plate. Signal is clean on V2's grid. (Remember: you do NOT want to connect the listening amp anywhere there's DC voltage. BTW that's a really good test - is there DC voltage anywhere there shouldn't be? You should check before and after V2. If you find DC voltage where it shouldn't be, there's a bad capacitor.)
Back to my scenario: the problem could be the tube itself, the tube socket (you've cleaned it, checked pins, and swapped in a known-good tube, right?), the resistor/cathode network on the cathode or the power supply for V2. I'm not familiar with the amp so you need to study the circuit for potential areas.
/Captain Random mode off/
Sorry for the disjointed brain dump. Hope something in there helps.
Chip