Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: Fresh_Start on June 15, 2008, 10:28:32 pm
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My Blues Jr. was driving me nuts with intermittent snap, crackle, pop noise. I couldn't track it down, and nothing looked obviously "wrong". Did all the usual tube swaps/pulling, etc.
Doug's "listening amp" seemed like a great idea, but I didn't have a spare amp lying around. So I built a small box along the lines of what Doug describes HERE (http://www.el34world.com/Hoffman/tools.htm) (scroll down to bottom).
The only difference is that I connected the ground lugs of the two input jacks together and isolated one of the jacks from the box. Then I took a crappy guitar cable, cut off one end, and attached alligator clips to both the inner lead and the shield of the cable.
Here's a pic of the insides: http://i222.photobucket.com/albums/dd298/Chipster457/Amp%20Miscellaneous/Listeningboxguts.gif?t=1213585875
The input from the sick amp is the isolated one with the cap attached to it to block any DC from reaching the pot.
Here's a pic of the box and the other end of the "probe": http://i222.photobucket.com/albums/dd298/Chipster457/Amp%20Miscellaneous/Listeningbox2.gif?t=1213586805
Couldn't think how to lable the two ends, so "B" is for the "bad" amp and "G" is for the "good" amp (i.e. the listening amp). I definitely have to make the ground lead much longer.
It worked great in terms of being able to listen to the signal at different points in the circuit. :D
Unfortunately, I still couldn't find the source of the electrical noise in the Blues Jr. :P Hopefully, I cleaned it up by re-flowing solder at all of the tubes sockets and ribbon cables.
Anyway, is there any danger from the way I wired this up versus Doug's approach with the separate grounding lead?
Cheers,
Chip