08-06-2014, 08:47 PM
I just restored a B15-NF, installed a 3-wire plug, new caps, new weber speaker. Fired it up and with an MP3 player it sounded perfect. I plugged in my bass and low E to A sounds like "farting" blocking distortion on both channels. I replaced all the coupling caps, no improvement. Then I replaced all the resistors with new carbon film, still no improvement. I put a 50hz sine wave on the input and scoped the phase inverter and the output tube grids checking for blocking distortion. The sine wave is perfect on the scope but the amp is still farting. I start scoping around the NFB resistor. Sine wave is still ok. Then I scope the other side of the "external speaker" jack on the green wire of the speaker cable. While I'm connecting the probe the Farting goes crazy. The jack switch was bad, I replaced the jack and the amp is now solid and totally quite. I've had plenty of the jacks be bad on the inputs but this is my first intermittent external speaker jack. I guess sometimes I can't see the trees for the forest! Hope this helps someone else sometime.