Fender amp hum?

[Tom Mc1]

I have owned a Fender Hot Rod and a Blues Jr. Both had a pretty loud hum. I took both amps to a recomended shop in my city. They said that ALL Fender amps hum/buzz and that is normal and acceptable with Fender tube amps. None of my other amps have this issue. Budda, Peavey, Ampeg, Genz Benz.

 

Do Fender amps buzz?

[WRGKMC]

Baloney. My fender amps are dead quiet and have no hum. Whoever told you that was an idiot.

 

The question does come down to the specific model amp and weather the hum is actually coming from the amp or is being fed in from the line input. If the tech did know what he was doing then the amps may be fine and the problem is originating before the amp. Even cheap wall warst can be a source of hum which sound identical to power amp hum.

 

If the hum is internal you should have the hum with nothing plugged into the amp, and it usually makes no difference is the channel volume is turned up or not. If there's a master volume that would need to be turned up.

 

If you have this idol hum, and this is a tube amp it can come from either unbalanced tube bias, an unmatched set of power tubes or inadequate power supply filtering. If this is solid sate amp, then the power caps are inadequate.

 

Both of these can be easily fixed and aren't too expensive.

 

If you get the hum when you plug in or kick on stomp boxes than this hum is coming in through the input. Its likely heard more on the fender then your other amps because of the kind of gain staging and tone stacks the fender uses, but the hum is coming in on the line and can be minimized before it ever gets to the amp. You need to remember that Leo Fender designed his amps to produce clean tones with no stomp boxes. Any hum would come from his crappy Fender guitar wiring with single coils and unshielded wires which can be vastly improved by shielding the guitar cavity and even changing the pickup wires to shielded like Gibson uses.

 

If you plug a guitar straight in and the hum is there when you turn the channel volume up, your issue is either the cable or the guitar shielding. Budget cables are often 70% shielded and will hum. You better cables have 90% shielding and shouldn't leak in AC hum. The guitar cavity can be grounded with copper foil or conductive paint. If its got single coils you can even use metallic covers and ground them.

 

Your other amps likely have different gain staging and the hum frequencies aren't in your face because of the amps tone stack. Fenders will amplify any incoming hum and be heard easily.

 

Troubleshoot it down as I suggested. If these are fairly new amps, the caps should be good. If its over 5 years old you may need new caps. Some of the budget parts they stick in these reissues are crap. Same thing with tubes. Always use a matched pair and have them properly biased.

 

Oh and there a quick way to test if you have AC leakage in the power amp. Connect an AC volt meter to the speaker connections, have the power amp turned up with no instruments connected and nothing in any effects loops. The most you should get on the meter is less than 1V ac. If the tubes are out of balance or the power caps are getting weak, you will show a higher voltage on the outputs because the AC is leaking into the DC and making it through the power amp section.

[Tom Mc1]

I not trying to bag on Fender amps in any way. I really like the sound of both of the ones that I mentioned. After looking deeper into the Blues Jr,, I discovered that one of the ribbon strips that connect the two pc boards may have a bad solder connection. By bending the strip differently the hum was almost, but not quite, elimanated.

I will be taking this amp back to the shop and ask the tech to look harder and not just blow it off to "acceptable Fender hum".

 

I'll also try retubing the Hot Rod. At least the power tubes. Should I have the tubes tested or just bite the bullet and buy new ones? Shielding is also an excellent idea. I think Stewmac carries the tape and it doesn't look to expensive. Could be enough to do a couple of guitars.

[onelife]

Was your HRD always noisy?

 

I use the term 'noisy' because quite often the word 'hum' is used to describe the idling sound of a high gain tube circuit.

 

The problem with the Hot Rod series is that the Master control is positioned in the early stages so it does not turn down the noise generated and amplified in the later stages. With the Master turned dow the signal to noise ratio (S/N) is very poor. If you turn the Master up to get a better S/N then then the HRD will probably be too loud for most situations.

 

Because the effects loop is designed to accommodated stomp boxes there is a still a considerable amount of gain between the effects return and the poweramp/speaker.

 

You can actually plug your guitar into the Power Amp In jack and get significant volume out of the speaker. Try plugging your guitar in to the Power Amp In and see how much noise there is coming from the later stages. If it still 'hums' then you may have an issue with the power tubes as WRGKMC alluded to earlier.

 

The solution I use for those amplifiers is to put a compressor in the effects loop and turn the output level of the compressor down. The compressor then functions as a true master volume control for the amplifier allowing me to turn up the clean channel volume control as well as the Master control for the drive channel. This gives me a usable S/N and allows me to use the compressor's output level control to turn down the volume of both the guitar sound and the inherent noise generated by Hot Rod series of Fender amps.

 

If you don't have a compressor you could use any sort of pedal that has an output level control. Some people use EQ pedals or overdrive pedals with the gain turned down and adjust the output level as needed.

 

[WRGKMC]

Bending the ribbon strip may be an indication that that strip must be close to the chassis to eliminate hum. The strip likely carries signals and with it being suspended away from the chassis its susceptible to EMF. This is why well made point to point amps always have their wires routed along the chassis as close as possible. That metal chassis is the key to keeping things quiet inside.

 

Many fender amps usually have some shielding that the chassis mounts against that can be improved. On solid state amps they often use a large piece of aluminum foil which doesn't block hum very well. You can improve it by either using copper foil or getting a piece of copper screen when you can by in a hardware store and staple it down over the aluminum foil. This beefs up the bottom chassis shield and blocks the hum much better.

 

I had to do this on a solid state Fender red knob M80 head when I rehoused it in a combo cab. Without that extra shielding for the chassis bottom that head would hum badly which I quickly discovered. The foil in the head cab was just minimally good enough in that cab. I used similar aluminum foil when I first installed it in the combo cab and it was no go. After I used copper foil it was dead quiet. Aluminum sucks for shielding audio frequencies and needs to be at least 10X the thickness of copper to get the same results. Magnetic waves will pass right through it instead of being collected and carried to ground.

 

[Tom Mc1]

These are all excellent ideas. Yes I so have a compression pedal and will try your effects loop suggestion onelife. As far as the EMF goes, I think you're on to something. Shielding tape just might do the trick and it's not that expensive of a fix. I'll try the copper screen first because the tape will have to be ordered. I think I'll order it anyway and use it on the guitar build I'm working on.