Whether it’s a vintage amp or a recent model such as this Fender Pro Reverb, hum can have several causes. Possible culprits include the preamp tubes, the power tubes, the hum balance resistors, and the power supply caps.
I was just reading some of your responses about Jonny Lang’s Deluxe Reverb amps. Thanks for your input.
I have a 1965 Deluxe Reverb and am trying to figure out if it has a Utah or some other kind of speaker. I was thinking of trying a Celestion Gold 50, but I usually only play in my bedroom, and I don't want to increase the amp’s volume. I’d rather decrease it. Any speaker ideas on that front?
I also have one of the newer Fender Pro Reverb amps with an effects loop and a silverface Twin Reverb with a volume control. Both amps produce very loud hum whether or not a guitar is plugged in. Could this just be bad caps? Or is there some other possible cause you can point me to?
Thanks, Jeff. Keep up the good work!
Steve Goldner San Diego
Thanks for your questions. Let’s start with the Deluxe Reverb.
Figuring out which manufacturer’s speaker is in your amp shouldn’t pose a problem unless it’s some aftermarket mystery speaker with no markings. Most factory speakers in Fender amplifiers have what is known as an EIA code that specifies their manufacturer. You should be able to find a number stamped somewhere on the speaker’s frame. The format may look like this:
Here the number 220 designates the speaker as a Jensen, and 637 indicates a production date of the 37th week of 1956 or 1966.
Fender has used only a handful of different speaker types over the years. Here’s a list of brands along with their EIA codes. (I’ve also included a few others brands you might encounter as aftermarket installations.) This should help you identify your speaker.
- Jensen 220
- Utah 328
- Oxford 465
- JBL 73
- CTS 137
- Altec Lansing 391
- Electro-Voice 649
- Pyle 1098
- Weber VST 1279
You also mentioned that you’re looking for a speaker option that decreases the overall volume of your amp. You need a speaker with a lower audio output, so you should consider the spec known as SPL, or sound pressure level (sometimes called sensitivity). The SPL rating specifies how loud a speaker is at a distance of one meter when driven by one watt. The higher the number, the louder the speaker.
I don’t know what speaker is in your amp, and even if I did, its specs might not be available. That’s probably a moot point, though—experience tells me that most original speakers in amps of that age are substantially fatigued and generally sound very weak.
You mention the Celestion Gold 50. It's a great-sounding speaker, but with a sensitivity rating of 100 dB, it’s relatively efficient and would probably be pretty loud in that amp.
Let’s take a look at a couple speakers from Warehouse Guitar Speakers (wgs4.com) so I can better explain this. Their G12C/S speaker is listed as 99.79 dB, while their G12C is listed at 96.10 dB. The G12C is more than three dB lower, which equates to approximately half the loudness of the C/S. If you want the least volume from your amp, search for a speaker in the style you like with the lowest SPL rating.
Another option you might consider: Eminence (eminence.com) makes a speaker called the FDM, which stands for “flux density modulation.” (Yeah, it sounds a little Back to the Future.) It comes in both American- and British-voiced versions and has a very unique feature: a large knob on the rear of the speaker that allows you to adjust the strength of the speaker’s magnetic field, effectively changing its sensitivity. It might be worth checking out.
Let’s move on to your Pro Reverb and Twin Reverb amps with hum issues. There are quite a few potential causes of this symptom, and it’s impossible to diagnose your amps without having them on the bench. I’ll list a few possible causes, but be aware that the cause of the hum in one amp isn’t necessarily the cause in another one. You have to troubleshoot each one individually.
- If the hum level varies with the volume control settings, check the preamp tubes by substituting replacements one at a time and listening for whether this changes the symptom. Some types of preamp tubes hum more than others in some amps. Also, if the amp has a hum (or hum balance) control on the rear panel, try adjusting that to minimize the hum.
- If the hum is independent of volume settings, install a new set of matched output tubes. Mismatched output tubes can create hum in the output stage, and the more mismatched they are, the greater the hum. (Remember, if you need to install a new set of output tubes, they should be properly biased.)
- If the problem is not tube-related, the electronics need to be examined. This should be done by a qualified tech familiar with tube amps. If the amplifier has hum balance resistors (as opposed to a center tap on the filament winding), the tech should verify that they aren’t burnt. Also, if the amp has a hum or hum balance control, make sure it hasn’t been damaged.
- The next thing to check is the integrity of the power supply caps. Bad or weak caps here can definitely cause hum.
- One more thing (and an often overlooked one): the capacitors in the bias supply. A noisy bias supply causes output stage hum.
I hope these tips help get your Fenders into phenomenal form!
Warning: All tube amplifiers contain lethal voltages. The most dangerous voltages are stored in electrolytic capacitors, even after the amp has been unplugged from the wall. Before you touch anything inside the amp chassis, it’s imperative that these capacitors are discharged. If you are unsure of this procedure, consult your local amp tech.