Tracking Down Hum in a Blues Junior
Blues Juniors are not inherently hum-prone, but tube amps in general are more likely to hum than solid-state amps. Getting hum down to the bare minimum can be a challenge. Here are some steps to take to find the cause and reduce or eliminate it.
The likeliest cause of hum is 60Hz hum that’s induced from the heater (filaments) to the cathodes in the preamplifier. Does the hum go away when you press the Fat switch? That’s a sure sign that the tube is worn out. Replace it.
You can also move your 12AX7s around; put the quietest one in V1.
Note: If you have no idea what you’re doing and you can’t tell one tube from another, the short ones are 12AX7s, commonly called preamp tubes, although only one of them (V1) is actually a preamp. The two tall ones are the output tubes, EL84s. If you put an EL84 into a 12AX7 socket, there’s a very good chance that you will fry the circuit board. You’ll know because of the bright blue flash and The Bad Smell, followed by a blown fuse and other collateral damage to the amp.
V2 and V3 are not preamplifiers and they are less likely to induce hum into the amp. So you put a hummy tube from V1 into V2 or V3 and it may work OK.
You can perform the following process of elimination:
1. Remove V1. Turn the amp on. Did the hum go away? If yes, see above. If not,
2. Remove V2. Turn the amp on. Did the hum go away? If yes, replace V2.
The amp will almost surely be silent if you remove V3. It’s seldom the source of hum, but try a fresh tube in there anyway to see if it helps. You should always have at least one known-good 12AX7 and a matched pair of EL84s in your kit.
Your output tubes are a matched pair for an important reason. They’re arranged in a “push-pull” configuration, which allows each tube to rest a bit while the other works. The phase inverter (V3) makes mirror images of the signal, the music you’re playing, and sends it to each tube. The push-pull arrangement causes any noise or hum that appears in both tubes at the same time (not mirror images) to be canceled out–a pretty neat trick. So cathode hum in the output tubes is generally canceled pretty effectively. The more closely balanced the tubes are, the better the hum canceling works. Tubes wear differently as they age, so old tubes may not be as effective in canceling hum.
Hum can come from many other places. Bad filter capacitors (those big, gray things) are a source of hum. If your amp is going on 20 years old, you should probably replace them. And if they are leaking, like the ones shown on this page, http://billmaudio.com/wp/?page_id=267, replace them. If one is bad, replace them all; they’re from the same batch.
Hum can also be induced. Your power transformer converts the AC line or mains voltage into the higher and lower voltages needed by the amplifier. The line voltage constantly magnetizes and demagnetizes the steel core of the transformer. A significant amount of magnetism leaks out, which is why you hear a loud, low hum when you bring your guitar close to the amp. The pickups respond to the 60Hz field around the power transformer.
The output transformer works on the same principle–a high voltage from the output tubes is converted into a low voltage suitable for the speaker. The output transformer sits inside the large magnetic field around the power transformer, but it’s oriented in a different direction and shielded with a core band (around the laminated steel plates) to prevent the 60Hz AC from “infecting” the output transformer and coming out the speaker.
Sometimes, however, the PT’s field overwhelms the OT’s defenses and some hum gets in. Amazingly enough, the push-pull configuration of the output tubes helps to fight some of this hum. Try this: Turn on a cold amp with your ear right up against the speaker. You will hear the 60Hz hum of the power transformer through the output transformer and into the speaker. As the amp warms up, however, the low hum should be devoured by the push-pull of the output tubes and will be replaced by a faint buzz that’s actually twice the pitch, 120Hz. You’re hearing the “sound” of the high voltage supply, which has a volt or two of 120Hz buzz on top of the 320 volts being fed to the output tubes. The tubes cancel most of it, and the Billm power supply stiffening mod reduces it further.
In my experiments, I’ve found that the amount of hum that gets coupled from the power transformer to the output transformer can be greatly reduced with a simple sheet metal shield:
The shield sets up a counter-polarized magnetic field that actually cancels the emanations from the power transformer. You adjust the angle for minimum hum. The shield is about 6 inches by 2 inches. The metal does not touch either transformer. Using a wider piece of metal does not work any better. The vertical portion should be as high as the output transformer; higher doesn’t help. Drill a hole or cut a notch for the screw. This works for either the stock transformer or upgrade transformers.
That big magnetic field from the power transformer can actually induce a 60Hz vibration in the chassis itself. If you can feel the chassis or the whole cabinet vibrating when the amp is on, sorry! You got a noisy one. You might be able to remove it and tighten the bolts, but it’ll probably be just as noisy when you reinstall it. Mojo Musical Supply makes a Blues Junior replacement transformer that’s slightly higher quality than the stock one. You could replace your noisy, vibration-prone PT with a better one, but that’s a lot of money for a rather minor hum. Play louder!
Bad grounds in the amp, ungrounded or miswired electrical outlets, and other problems can also cause hum. You’ll probably need an experienced technician to track down grounding problems in the amp. Get an outlet tester from the hardware store or Radio Shack to tell you whether your outlets are properly wired and grounded.
Offsetting the filament voltage with a positive DC voltage often helps, too. It prevents the negative swings from the heaters from impressing hum on the cathodes. It’s easier to steal a DC voltage or build a small DC supply than to rectify and filter the heaters. In my experiments, I was surprised to find that bridge rectifiers that can handle the current run pretty hot and place an additional load on the power transformer. I’ll add some details about this when I get a chance.