Mystery Musicmaster Bass Hum

[Randall]

I recapped a '74 Fender Musicmaster Bass combo that came in for being too noisy. Sprayed it, cleaned it, and put it in the cabinet. Upon first power up it made a few pops and random noises, and then a loud hum appeared. I left it on just long enough to learn the amp worked with a guitar, and the hum went away as the volume was turned down. I shut it off, pulled the chassis and put it back on the bench, figuring it is before the volume control, so how hard can it be to figure out? Connected my shop speaker, and... quiet. The amp worked just fine. I probed and wiggled, etc, but the hum was gone.

I put it back in the cabinet, and since then it has been working as it should. I can't imagine what happened here. I ruled out the input jack shunts and the preamp tube. Anyone want to take a swing at this?

[Enzo]

A wire could have been pinched between chassis and cab. A ground connection could have failed. The jacks and controls rely on good contact with the chassis. Especially on the jacks, that is why they add the star washer. Loosen each jack enough to push the star washer out of position a little. That makes the points of the washer hit fresh metal. The tighten. I also loosen then tighten all the control nuts.

And don't forget your fist. Whack the thing to see if it reacts or if the hum comes or goes.

[Randall]

I did all those things straight away. There is not much to this amp, two pots, two jacks, and tightening jacks is part of my normal routine. If it were a loose anything, I have to believe I would have found it when I was back in there looking for just that.

No, it was something else.

[Bob M.]

This is one of the few Fender tube amps that doesn't use a jack/plug to connect the speaker but the secondary of the output transformer is directly connected to the speaker. Recently, I had another amp in for repair with this method of speaker connection and the transformer wires were badly crimped going thru the chassis and the plastic/cloth covering was compromised creating a hard to see problem. However, you seem to have a pre-volume control problem so I'd touch-solder each ground connection and try replacing the 1st preamp tube. You say the ground problem is not present on your bench w/ test speaker but is present when chassis is returned to it's cab with it's own speaker??? Maybe, try running a ground wire from the speaker frame to the chassis temporarily and see if that helps. The only variable seems to be the speaker. This is a good one - please let us all know what it turned out to be when you finally solve it.

Bob M.

[Randall]

Bob M. The loud hum only did it that one time when it was in the cab. It stopped when I took it out, and never did it again no matter how much I tried to induce it. I did try a new preamp tube, BTW. It's back in the cab now and seems to be fine. The only thing I can guess at is perhaps a clipped lead from the recap was touching something, I did shake a small one out of the chassis when I pulled it the first time, but it was hiding in a crease so I don't know.

And it was a loud hum that tracked the volume control, so I doubt it has anything to do with the speaker connection.