First Build Blues: Mojo JTM45

[big_teee]

I'm still working on this, just slowly. I was able to make the noise change by moving the wires around that lead to the bright volume cap, so i shortened those as much as i could. I also removed one leg of the bright cap from the volume control and that really settled the noise down. I'd like to keep the bright cap, so i am going to work on the lead dress on the input side of the amp tonight and see if i can improve this further.

Don't forget the Lead dress on V1.
Use a ChopStick and move the leads around, if you haven't already done so.
T

[jazisded]

I shortened all of the leads on the input side of the board as much as I could and I removed the bright cap. The amp is very playable now. As soon as i put that bright cap back in though - it's buzz saw city. Is it possible i just have a bad cap?

The voicing of the amp actually seems ok this way - apparently people seem to like this channel without that cap? I'm going to play it for a little while and order a new tube socket for v1 so i can use a tube shield. Maybe that will quiet it some more.

[EFK]

You can very quickly determine whether a tube shield will make a difference or not. (IMHO, it definitely does make a difference on V1 only, especially if there is a lot of interference floating around.) Just take some aluminum foil and roll it around the V1 tube a few times like a... well, a tube, no pun intended, and either make sure it is contacting the chassis well or clip an alligator lead to the chassis and the other end to the aluminum "tube."

[jmaf]

I shortened all of the leads on the input side of the board as much as I could and I removed the bright cap. The amp is very playable now. As soon as i put that bright cap back in though - it's buzz saw city. Is it possible i just have a bad cap?

The voicing of the amp actually seems ok this way - apparently people seem to like this channel without that cap? I'm going to play it for a little while and order a new tube socket for v1 so i can use a tube shield. Maybe that will quiet it some more.

If the cap is shorted then you've simply grounded the cathode, which leads to an uncontrollable high gain mu-amp added with the probable inconsistencies in the damaged capacitor(maybe not entirely shorted, the electrolyte is doing funny stuff, etc).

If the cap is fine then you oughta look at lead dress. Since adding the cap is increasing the gain of the stage, if the noise level increases it means the noise was already there but from another source.

Check resistors as well, plate and cathode resistors ....especially carbon comp if high humidity is present, they'll will crackle, buzz and sometimes sound like an AM radio.