Blues Junior Hum Problem

[dmart]

I have a strange problem with my newly acquired 1 ˝ year old, tan PCB, Blues Jr. There is a low 60 cycle hum that is intermittent at first but becomes more constant as it warms up. I say “low” because it is both louder and more bassy than the very slight “normal” hum that is always there. OK, here’s the strange part. When I take the back cover off, I can make the hum go away simply by placing my hand within a few inches of the V3 ribbon cable. I don’t even have to touch anything. I can move my hand from side to side (parallel to the back of the amp) and each time I get close to the V3 ribbon, the hum stops. Eventually thought, after about 10 minutes, I have to just barely touch the V3 ribbon to stop the hum. I’ve checked both ends of all of the ribbons and I don’t see any problems with the solder joints. And it doesn’t seem to be related to vibration. Also, no burn marks or other indications in the PCB. Everything looks pristine.

I also must add that the reverb tank is dead due to a dead output coil. (I determined that by detecting “1” when touching both prongs of the coil with my test set – am I correct?). So I definitely need a new tank. But I’m wondering if whatever is causing the mystery hum might have fried the reverb tank. Has anyone ever heard of anything like this? Any ideas and/or help would be greatly appreciated.

[52 Bill]

Welcome to the board.

Have you tried to change the pre-amp tubes? Just try swapping the pre-amp tubes around and see if the problem persists.

...I can make the hum go away simply by placing my hand within a few inches of the V3 ribbon cable.

You can try and fold the ribbon cable away from the tube and circuit board.

...I also must add that the reverb tank is dead due to a dead output coil.

Remove the tank from the cab and check the small wires that connect the coil to the RCA jack. This is a very common problem area.

[dmart]

Thanks Bill. Yes I did move the 12AX7's around and in fact had a few spare "good" tubes to substitute as well - no difference in the hum.

I did carefully bend the V3 ribbon forward and backward but that didn't seem to affect anything, except that while my hand was on the ribbon, the hum went away. Why would the hum vanish just by placing my hand near the ribbon without actually touching it? Is there some sort of electromagnetic field that shouldn't be there?

Regarding the reverb tank, I do have continuity in both sets of wires between the RCA jacks and the respective coil connectors - just not between the output coil leads. So all of the tiny little wires are fine.

[52 Bill]

...Why would the hum vanish just by placing my hand near the ribbon without actually touching it? Is there some sort of electromagnetic field that shouldn't be there?

V3 is the driver tube, so I wouldn't think that there was anything weird going on there. When you bring your hand near the ribbon cable you could be either causing a capacitance or an inductance change in the circuit. Have you checked the driver circuit for bad connections, loose components, cold solder joints, etc.

...just not between the output coil leads.

Then I'd guess that the coil wire is broken. They usually break at the ends where they solder to the connector pins. Sometimes if you inspect them with a magnifing lens you can find the broken end.

[Enzo]

It is normal to have some hum develop when you bring your hand up to those ribbons. If you already have some hum from another source in the signal path, the following can happen. The amp is passing some hum along, then you introduce some additional hum into the ribbon. It happens that the hum from the ribbon is out of phase with the other hum, so they cancel. The new ribbon hum counteracts the existing hum.

This is the same thing as an amplifier that hums least when the volume control is set to like 2 or 3 on the dial.

Does disconnecting the dead reverb at the chassis have any effect?

Do any of the controls affect the hum in ANY way?

[Mark Black]

Those Blues Juniors are a little prone to oscillation due to lead dress at the V3 ribbon cables and their proximity to the output transformer primary wires (specifically the blue wire that crosses over or under the ribbon cable).

This site has a nice explanation of his version of a fix: Billm Audio Cream Board Phase Inverter Oscillation

I've had pretty good results just moving that blue wire as far as possible away from the ribbon cable, sometimes by routing it under the ribbon & pushing it back against the chassis.

When you have those amps on a scope you can really see the oscillation effects of getting your hand or a test probe anywhere near that blue wire.

Of course the oscillation is usually super high frequency (beyond the abilities of the speaker and/or our ears) so all we hear is increased hum due to the power section being heavily loaded by amplifying the oscillation at something near full power and increasing the ripple in the power supply filter caps.

Good luck on yours!

[Enzo]

I route the blue wire right along the board, not so much to be near the board, but to make sure it is as close to right angles to the ribbon as possible. Wires are right angles do not couple like wires more parallel can.

[dmart]

Bill - good idea with the mag glass at the connector pin - those wires are indeed tiny. I'll check it out tonight.

Enzo - Interesting about hum cancellation from two different sources. Makes sense. The hum is the same whether the "dead" reverb is connected or not. And turning any or all of the controls in any combination doesn't seem to make any difference either.

Mark & Enzo- I wondered about that blue wire that seemed to be resting right on top of the EL84 ribbons so I pulled it away a little bit. I'll check out BillM's explanation (great site there). Tonight I will disconnect the blue wire and pass it underneath the EL84 ribbons so that it rests between the board and ribbons, making sure that it's at right angles to the ribbons.

Thank you all for your valuable insights!!