Reducing Blues Junior Hum

Ever listen closely to your Blues Junior with everything turned to 1? It hums. Not badly, but more than you'd want if you were using it in a recording studio and playing softly. Ever pull the power tubes and listen to it? It still hums, even with no power tubes. In fact, it probably hums a bit more without them. Here's why:

The power transformer broadcasts 60Hz AC. You know that, because every time you lean close to the amp with your guitar, your pickups hear it. Some of that AC couples directly into the output transformer. It's only a couple of millivolts, but it's audible. When you have the power tubes in there, the actual amount of hum is often reduced because of the nature of push-pull amplifiers--they reject common noise, signals that are present on both sides (both tubes) of the transformer, just like a humbucker does. The noise signals, including hum, tend to cancel each other out. "Tend to" is the operative word--nothing's perfect.

I was working on an amp that was destined for a recording studio recently, and I decided to do some testing to see how much of this hum could be eliminated. I did some research on transformer design to see what I could learn. That copper band that you see from the bottom of the chassis is called a flux band, and it's designed to keep the electromagnetic flux inside the transformer, to limit the size of the field. The flux band on the amp in question was very loose, looser than I'd seen in any other Blues Junior, so I decided to tighten it.

 

I unsoldered the flux band with the mother of all soldering irons, this 310-watt monster. I use it for printed circuit boards, too--I can do all of the connections at once! Seriously, this thing radiates so much heat that it warms the area around my workbench. With about two pounds of solid copper at the tip, the iron melts the entire band of solder instantly.

I re-bent the band, clamped it, and resoldered it so that it was tight across the windings and the core. I couldn't make a direct before/after comparison, but I think this helped the hum somewhat.

I also tightened the nuts holding the core together. This one was a little looser than I thought it should be.

In my research on transformers, I came across the the concept of a core band, a steel band that goes around the outside edges of the transformer core. Most power transformers don't have core bands, but they're often used when the transformer has to be close to a magnetically sensitive device, like a cathode ray tube. I figured I would try to make one.

The literature said the core band should be grain-oriented steel. I couldn't find any in my shop--I must have left it in the trunk of my Ferrari. So out came the tin snips and some galvanized duct scrap.

The core band goes around the transformer core ends. It's held together with a screw and nut at one of the corners. I had to pad the inside of the core band so it wouldn't hum or buzz in the magnetic field.

The core band was successful--it reduced audible hum noticeably. But when I placed a piece of angled sheet metal between the power transformer and the output transformer, it got even quieter.

So the next question was, what's more effective, a core band, or an angled piece of metal? And what about shielding the OT instead of the PT? Or both?

One of the things we used to do back in the hi-fi days was rotate the output transformer a few degrees either way, to change how it sits in the PT's magnetic field. This was completely ineffective with the Blues Junior.

I tested many, many configurations and materials. In this case, a thick aluminum cylinder around the OT was very effective. Aluminum is actually good at turning magnetic fields into eddy currents. But sticking a slanted piece of steel between the two transformers was even better.

Since the flux band is made of copper, I thought that copper might be a good material for the core band or the shield. It wasn't.

This heavy-walled stainless steel cup was utterly useless. It didn't affect hum level at all. The aluminum cylinder was far more effective, but neither could match a simple piece of angled sheet metal between the trannies.

A mild steel can over the OT reduced hum somewhat, but the aluminum cylinder was more effective. It's weird to think that a magnetically transparent material like aluminum would work, but the eddy currents must have created an opposing field.

So the next order of business was to cut pieces of metal smaller and smaller, and try different mounting techniques--from the bottom of the PT, from the top of the OT, grounded, floating, etc., to see what worked best. It turned out that the angle was more important than the size of the piece.

Here's my final solution, and I can't for the life of me figure out why it works so well. It's about 2 inches wide and 5 1/2 to 6 inches long overall, and I bent it for minimum hum after I installed it. It can be wider, but the hum doesn't decrease significantly. It doesn't touch either transformer. It appears to be important that the shield come up to the top of the OT, but going higher doesn't help. The angle of around 40 degrees seems to be most effective.

From what I can figure out, the power transformer induces a bucking field in the sheet metal, which then cancels the electromagnetic waves that would otherwise infect the output transformer, but I have no way to prove this other than my ears.

 

Still to be determined: whether replacement output transformers such as this Gibson/Trace Elliott OT are quieter than the stock transformer. Since the hum is down in the millivolt range, it's hard to A/B test, given all of the AC noise that suffuses our homes and workplaces. The steel bell ends may help, but the core edges are exposed.

I've used this transformer in a number of modded BJrs, and it has cleaner, stronger bass response. I've also used the Hammond 1650E, which dwarfs the Gibson OT the way the Gibson dwarfs the stock OT.

PS: Please don't write and ask me where you can get a gold panel for your Blues Junior! This chassis is from a Woody Blues Junior, which came from the factory with a gold panel..

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