Guitar buzzing through Express

[Littlewyan]

When playing through my Express I sometimes get issues with buzzing. If I touch the strings it stops. Its not the guitar as I've tried different guitars and the strings are grounded on all of them. So far the buzzing has occurred in my bedroom and in a practice studio.

In my room its my wireless printer so I just turn it off but I couldn't see what it was at the practice studio. Its only my amp thats affected as well. Added to which when I turn the amp on/off we hear a pop through the PA. Any ideas? I haven't got any grid stoppers but can't see how they'd help if the buzz stops when I touch the strings. I don't mind it, its not overbearing, I'm just worried that it could happen at a gig.

[M Fowler]

I use at least 33k grid stopper and isolate the input jack from chassis.

[Bob S]

Glen uses 2 cords to get the capacitance up on the input signal.
Grid stopper & miller effect definitely help.
Hopefully a quick fix.
Good luck.

[Colossal]

Its not the guitar as I've tried different guitars and the strings are grounded on all of them. So far the buzzing has occurred in my bedroom and in a practice studio.

Have you tried a different instrument cable? Sometimes it's the little things that cause the most hair-pulling.

[Littlewyan]

Colossol - O dear, someone forgot to go back to basics. I will try a different instrument cable.

Bob S - Increased capacitance will mean I'll need a smaller grid stopper to achieve the same roll off frequency, is that right?

M Fowler - If none of the simpler fixes resolve the issue I'll try a grid stopper. Probably start with 10K and work my way up to 33K if needed.

[geetarpicker]

Most likely it is your guitar picking up air born noise, from things such as light dimmers, florescent lighting, or other odd electronic items. The high gain (and hi frequency openness) of these amps makes noise pickup simply more problematic but even an old Plexi will do this if dimed. Still when you ground the strings with your hands your body then acts as a temporary shield knocking most of the noise out. Hmmm....wear a velco ground strap on your wrist! (probably available from Newegg.com, in the computer parts dept.)

That said, check around the area (even downstairs, upstairs, next door) for anything like what I mentioned before. I used to live in a townhouse many years ago, and every day at dinner time they would dim their dining room lights and I could count on noise pickup until they were done. Sadly in my experience any sort of AC filter devices will have little to no effect as it is air born transmission, and not really through the AC line. Humbuckers typically will help, and these days I run a Suhr hum cancelling back plate (dummy coil) on my Strat to attain some noise cancelling with my Fralin single coils.

Another way to test things is the next time you have the noise crop up stand up and rotate your guitar/body around slowly and see if the noise fades in and out as you aim the guitar in different directions. If this is the case it is definitely air born interference and you simply need to go find the source and shut it down...

On a side note these amps can even make their own RF noises. Many years ago my recording studio was at a relatives house. They had a satellite dish for the TV that had an RF remote control, so that you didn't have to aim the remote all the time. That said, it became apparent that my Express when cranked into distortion during recording sessions put out enough RF noise that the satellite dish remote would pick up a signal and start turning the dish on the roof randomly! The fix believe it or not was to put one tight wrap of aluminum foil around each EL34 held with too loops of solid core wire and grounded. Basically shielding the plates. This stopped the RF field (and stopped the crazy satellite dish from turning) but luckily didn't actually effect the sound as far as I could tell. Now I know that distorting output tubes actually generate frequencies way UP there, that can transmit at least 50' or more...

Hope that helps!
GK

[Littlewyan]

Thats brilliant! Your relatives were picking up Glen K TV!

The practice room is in a big old industrial building so may not be much I can do. However I think it was my synth player's bass amp that popped when I turned on the TW. So could be that causing interference and at the same time picking up interference. And yes my synth player uses a bass amp for his keyboards and synths, hey it works.

I've also heard the radio through the amp once or twice, not sure I'll be able to shut that down......or could I. Anyway won't adding grid stoppers suppress it a bit?

[geetarpicker]

I used to pick up AM radio now and then, but not since I started using Mogami cables.

[Littlewyan]

I have got a coupler so I could connect a 30ft Cable up to my Bullet Cable. Thought about doing that just for safety in case the Bullet Cable gets pulled.

[martin manning]

A 10k grid stopper will give a corner frequency of 150kHz, so it's not likely to be noticed.

[Littlewyan]

Well I just did a test. Plugged a small patch cord into the amp, turned it up and got really horrible buzzing. Plugged my guitar in using my normal bullet cord and the buzzing was much quieter. However the wireless printer interference stayed at the same level so its just air born interference being picked up by the guitar or cord like Glen said and there ain't much I can do about it except switch off the printer.

I did find however that a valve shield on V1 helps with hiss with all of the knobs on 10. When I have all of the knobs set to 5 with the bright switch on it doesn't make a difference.

I also tested my JTM1 which already has a 10K grid stopper and the printer interference and buzzing was the same if not worse on that amp. Ah well I'll just live with it.

[martin manning]

The grid stopper is more about the RF interference and hiss, it won't help with a LF buzz.

[bcmatt]

I thought I had a quick idea for a second... but grid stop resisters need to be right on the tube socket, don't they?

Anyways, the idea was to just wire up a tiny box with 1/4" in and out jacks and a 33K resister wired between the tips (could actually put a switch in with a few different values). Then, without modifying the amp, you could have a grid stopper for certain rooms. Anyways, I forget what happens as you get physically further from the tube socket with that resister, but I'm pretty sure there's a reason why these resisters are not on the inputs or the turret board most of the time.....

[geetarpicker]

I think you still need to kill the noise at the sources, and leave the amp alone. I highly doubt grid stoppers are going to make much of a dent in the noise. The grid stoppers are sometimes added to reduce just a hint of high end response which can kill oscillations. By the time you use resistors big enough to cut down the apparent noise issues you will probably have a pretty dead sounding amp IHMO.

In other words keep looking for the noise sources and turn off the printer. You have a Ferrari there in a box, but you need to make a clear path for it!

[Littlewyan]

If the grid resistors are on the turret board then the wire between the resistor and tube socket could pick up interference. Thats why its best that they are on the tube sockets.

I'll have a hunt around at my next band practice for the source of noise.