High frequency buzz when guitar is plugged in.

[evirob]

I've got this problem with a noise in my amps that I just can't chase down. I'll call it a buzz although opinion seems divided on precise terminology of various unwanted amp noises, and therefore the subsequent diagnosis.

From an objective point of view it's a fairly high frequency noise. I don't have a scope but using a spectrum analyser app on my phone held up to the speaker, it showed a peak around ~1.2khz. It sounds like a buzzing fluorescent light. This lead me to suspect switching noise from the rectifier diodes, but there was no change when I tried ultra fast diodes or snubbing capacitors.

Both amps are built by me: a #183 Overdrive Special and a Trainwreck Express. In both cases they are quiet when no guitar is plugged in, and remain quiet even with the gain/volume/presence dimed except some hiss which I consider acceptable.

If you plug in a short patch cable and you get the buzz which increases in volume as the amp is turned up. With a guitar plugged in, the buzz is slightly quieter than just the cable, but it still increases with the volume of the amp. The buzz does not change depending where the guitar is positioned or oriented relative to the amp, nor is it affected by single coils vs humbuckers. I do get the usual 60 cycle type hum with single coils, but that comes on top of the buzz I am trying to diagnose.

As a side note, one strange behaviour I have observed is that if you turn the amp up fairly loud and back off the guitar volume (as you would to control the Express for example) the buzz actually gets slightly louder as you back off the volume, then gradually decreases the closer you get to zero.

I have tried just about everything I can think of: tube rolling (pre and power amp), transformer orientation, shielded cables on all signal paths, star grounds (and just about every other permeation of grounding scheme), add/remove grid stoppers, chop-sticking all the wiring, snubbers on the rectifier diodes, even a stand alone bias transformer. I've tried various power outlets around my home, turned off all the lights, unplugged all my appliances. None of this made any difference.

There was only one modification that had any observable effect, a humdinger pot on the heater wires of the Express. I tried this because the Express (as opposed to the Overdrive Special) had the buzz even without anything plugged in. The humdinger amazed me by getting the amp near silent when nothing is plugged in. It just buzzes when the guitar is plugged in, which is a shame because that's how I like to use the amps.

Having said all this, there is also one other thing I have discovered that cures the buzz completely, but it feels like a band-aid solution. If I use a buffered pedal in front of the amp (e.g. Klon or Bad Monkey) the buzz is totally killed (man!). This is extra confusing because usually people have trouble with pedals introducing noise.

I know how hard it is to diagnose amp problems remotely, but I thought since I get the same issue on two completely different amps using three different guitars, someone might recognise it as a general problem to which there is a proper fix. Sorry for the essay and thanks for reading!

-Chris

[Malcolm Irving]

The symptoms you describe are all consistent with the buzz going away whenever there is a low impedance connected between signal and ground at the input. That includes the buffered pedal, which will have a low output impedance.

But the question is then: why is the buzz there when you have a guitar connected?

I had an amp which intermittently did something similar and I discovered that what looked like a very good solder joint on the ground terminal of the input jack socket was intermittent.

I would suggest using a multi-meter to measure resistance and check all the following (with power off and caps drained):
With a patch lead plugged in: jack-body to amp signal-ground 0 ohms, jack-tip to amp signal-ground 1M (or what you have for a grid leak), jack-tip to first tube grid 68k (or what you have for a grid stopper). By checking all the ‘obvious’ things like this, you will hopefully find a bad-connection or a miss-wiring.

Good luck.

[evirob]

Thanks! I'll try that this evening.

[brewdude]

Does this problem occur with other commercial amps?
Does it occur in other locations? Within your home or elsewhere?

I have had amps that seemed noisie at my house, but silent at my friend's house--my home is old and the wiring archaic.

[evirob]

Unfortunately I don't have any commercial amps. My house is fairly new so I guess I automatically ruled that out, but you're right - never rule anything out without trying it first. I'll bring them in to my workplace tomorrow. I'd be delighted if that was the case! (and after that, worried about the wiring in my home!)

[pompeiisneaks]

Also rule out any CFL bulbs or dimmers in the area, those can add mad noise.

I'd suggest using a chopstick and pushing the leads that are soldered to a ground, adding slight pressure to them can cause either the sound to dissipate or get much worse, reflow the joints where this happens, maybe even add some flux to get a better joint. I've chased and fixed these demons quite a bit as my earlier soldering skills kinda sucked I also found I was chasing a ghost for a long time on one amp when I realized it was just noise coming from the guitar in my noisy house. I now don't trust certain regions of my house due to the massive dimmer/cfl/flourescent noise. I've removed all of them I could and replaced them with LED bulbs and am hoping to replace my older dimmers with new, less noisy ones at some point.

~Phil

[tubeswell]

How old is the guitar cable? Old cables tend to develop capacitance issues.

[evirob]

Thanks for all the suggestions. Tubeswell, the cable is a couple of years old, still in pretty good shape. Don't think that's the root, but I'll try another just to eliminate the variable.

[brewdude]

I once chased a tremolo sort of oscillation that was driving me nuts for days... Then I realized it was the echo from the ceiling fan.

I also was once at a friends house and was experiencing a new buzz/rattle/hiss... Then I found a small bowl full of change on hi shelf.

I apologize for the derailment.

[evirob]

Yeah I've had really irritating ghost notes on an amp in the past that I assumed were electrical, only to discover it was mechanical vibration in the speaker cabinet. But this issue shows up with multiple speakers in multiple locations.

Resistance readings on the input jack were fine and I tried a different cable to no effect. Trying the amp at work suggests my house's wiring is not the issue either (or my work has the exact same bad wiring). Poking around with a chopstick also yielded nothing.

Since the only thing that made a difference so far was the hum-dinger, I'm wondering if it has to do with the heater supply so tonight I'll try running the heaters from a battery. I've got a 7.2v RC car battery pack which I can drop down to 6-ish volts with a resistor.

[pompeiisneaks]

Sounds like a great next step.

[tubeswell]

Not the cable, not the speakers. So far so good.

How many input jacks does your amp have? And if the answer is 'two' (or more), do you get the same problem when plugging into either jack?

[sluckey]

Plug in your short patch cable. Now touch the sleeve (not the tip) of the dangling plug to chassis. Does this quiet the buzz?

If so, you may have a wiring error on your input jack. If using a cliff style jack I would suspect the wiring to the ground lug and the ground switch lug is mixed up. Maybe post a close-up hi-rez pic of your input jack(s).

[evirob]

With a short patch cable, only touching the tip produced a loud buzz. Nothing when touching the sleeve. There is only one input.

I'm pretty convinced this is something to do with the heater supply at this point. I didn't get round to trying a battery yet (didn't have the right dropping resistors for my battery pack) but when I put the wiper of the hum-dinger to ground instead of an elevated supply, the buzz got far worse. My heater elevation is about 30v, maybe I'll bump it up a tad and see if that helps.

[sluckey]

With a short patch cable, only touching the tip produced a loud buzz. Nothing when touching the sleeve. There is only one input.

I'm not talking about "you" touching the tip or sleeve. The experiment I want you to try is for you to touch the sleeve of the dangling plug to the chassis. Or use a gator clip lead to ground the sleeve of the dangling plug to chassis ground. Does the buzz go away when you do that?