1976 Fender Vibrolux Hum after minor repair.

[gtrplayr1976]

Hey Folks,

I'm baffled. I just replaced the speakers in my 76 Vibrolux. It was working fine before this. I had a bit of an issue getting the rectifier tube back in ,but finally went in. When I fired it up there was a BAD hum. The first thing I checked was that tube ,and the socket. I found that the socket had a bent contact inside. I straightened it out ,but didn't solve the hum issue. I tried a different cab, same thing. I was then lead to the bias board ,and found a broken lead on the cap. I thought for sure I had THAT had to be it. Nope. I have replaced power tubes. The first time I checked voltages it seemed the plate voltages were erratic ,but it hasn't changed the last two times I've checked. The voltages I have checked to this point seem to be ok. I just can't figure why it was fine ,and now it isn't.
Any thoughts ?

Thank you !

[The Dude]

What about bias voltage? Do you now have bias voltage on the output tube sockets? Pull the output tubes and check all of your socket voltages.

[gtrplayr1976]

What about bias voltage? Do you now have bias voltage on the output tube sockets? Pull the output tubes and check all of your socket voltages.

I do have bias voltage ,but I haven't looked for it on the tubes yet. I'll do that. Thank you !

[Enzo]

You bent a rectifier tube socket pin, and "fixed" it, but is it making good contact? In other words, maybe one half of the recto tube is not happening. That would result in larger amounts of ripple (hum) on the B+ supply. measure your ripple.

Pull the power tubes, do you have good solid, free of ripple B+ on both pins 3 and 4 of the power tube sockets? And good free of ripple bias voltage on all pins 5?

Don't try to fix it from the point of view that you did something to it. We know you did, it happens to us all. Just pretend the amp came to you this way, and just fix it.

[gtrplayr1976]

You bent a rectifier tube socket pin, and "fixed" it, but is it making good contact? In other words, maybe one half of the recto tube is not happening. That would result in larger amounts of ripple (hum) on the B+ supply. measure your ripple.

Pull the power tubes, do you have good solid, free of ripple B+ on both pins 3 and 4 of the power tube sockets? And good free of ripple bias voltage on all pins 5?

Don't try to fix it from the point of view that you did something to it. We know you did, it happens to us all. Just pretend the amp came to you this way, and just fix it.

I replaced that tube socket Enzo ,but it didn't help.

[Chuck H]

Looking at the photos I don't see a ground lead on the bias supply filter cap (+ terminal should be grounded). It may be under the board, but I don't see it. Double check this.

EDIT: I see it now. Sandwiched between the boards. Double check that the + terminal of the bias cap is grounded though with your meter.

[gtrplayr1976]

Looking at the photos I don't see a ground lead on the bias supply filter cap (+ terminal should be grounded). It may be under the board, but I don't see it. Double check this.

EDIT: I see it now. Sandwiched between the boards. Double check that the + terminal of the bias cap is grounded though with your meter.

Thanks Chuck. I just replaced everything on that board yesterday. I'll check the ground again though.

[gtrplayr1976]

What about bias voltage? Do you now have bias voltage on the output tube sockets? Pull the output tubes and check all of your socket voltages.

I am getting bias voltage -46. I'm posting the readings. I definitely have questions about some of them. These are with the power tubes pulled. Granted some of the voltages are probably due to the differences in the two layouts. I modded this to be more like AA964 from AB568 however some of the Ab568 resistors ,and such were left. Like the ones in the can. Just that some voltages are showing where it doesn't show any on the layout ,or schematics ?
After reinstalling the power tubes I noticed a couple of things. Turning the Trem on ( I have a switch pot mod) seemed to reduce the hum a little. Increasing the Reverb pot (even though it's not connected to the tank) seemed to increase the hum ,and it seems V3 was VERY warm compared to the other preamp tubes after I shut it down. I could easily put my hands on the others ,but this one was pretty warm. It's an old Conn tube.

[52 Bill]

...and it seems V3 was VERY warm compared to the other preamp tubes after I shut it down. I could easily put my hands on the others ,but this one was pretty warm. It's an old Conn tube.

That is the driver tube for the reverb tank. Just like an output tube it gets hot, and with the tank unplugged you are basically running the tube into a unloaded transformer. If you are going to run the amp without the tank connected, pull out the 12AT7 or you may damage the tube of the reverb transformer.

Just in general, it seems that all of the plate voltages are too high.

[gtrplayr1976]

That is the driver tube for the reverb tank. Just like an output tube it gets hot, and with the tank unplugged you are basically running the tube into a unloaded transformer. If you are going to run the amp without the tank connected, pull out the 12AT7 or you may damage the tube of the reverb transformer.

Just in general, it seems that all of the plate voltages are too high.

I didn't know that about the driver. I'll remember that now. I know the plate voltages are high ,and even higher depending on which schematic you look at. Some look close to spec ,and some off spec.

[gtrplayr1976]

You bent a rectifier tube socket pin, and "fixed" it, but is it making good contact? In other words, maybe one half of the recto tube is not happening. That would result in larger amounts of ripple (hum) on the B+ supply. measure your ripple.

Pull the power tubes, do you have good solid, free of ripple B+ on both pins 3 and 4 of the power tube sockets? And good free of ripple bias voltage on all pins 5?

Don't try to fix it from the point of view that you did something to it. We know you did, it happens to us all. Just pretend the amp came to you this way, and just fix it.

Enzo, you were correct. Had nothing to do with what I did. I don't know why all of a sudden it happened ,but V4b grid was picking up noise. I moved the wire ,and hum dissipated. I changed it to shielded ,and all is good for now.