Amp buzzing when not touching the strings

[Leevi]

I got a Matchless Spitfire clone wired. The sound is excellent and there is no hum.
The only problem is a buzz that occurs when not touching the strings.
I think the problem is mainly caused by the guitar and/or guitar cable and the Spitfire preamp is probably sensitive to that kind of a phenomenon.
Are there and good ways to prevent or decrease the buzz on the amp side?


http://el34world.com/charts/Schematics/files/Matchless/Matchless_spitfire.pdf


/Leevi

[shooter]

there is no hum.

with nothing plugged in and no hum, the "amp side" is fine, buy a better cable?
without a good cable/guitar you are putting an antenna into the amp

[Leevi]

The amp is deep quiet even if the guitar is plugged and guitar volume is off.
/Leevi

[shooter]

guitar volume is off.

my last build did just what you're saying, same with guitar volume 0, quiet,  but buzzed when I turned up, I wrote it off to my cable, an RCA cable with rca to 1/4" adapter ends, sure enough, when I picked up a $30 guitar cable, no buzz 

[jjasilli]

1.  a) Your schematic shows shorting input jacks.  Is this really really true & are they properly grounded?
     b)  If yes, when you turn on the amp with nothing plugged-in to the input jacks AND turn up the vol, is there hum, buzz or noise?  If yes, there is noise in the amp.  If no or mostly no. . .


2.  Plug a guitar cable into the amp, no guitar;  repeat 1 b).  There will probably be at least some noise. 


3.  Your guitar controls, p/u covers, and/or strings may not be properly grounded; or the control cavity shielded and grounded.


4.  There may be sources of ambient noise: fluorescent lights; light dimmers or fan speed controllers, etc.

[Leevi]

a) Your schematic shows shorting input jacks.  Is this really really true & are they properly grounded?

It must be properly grounded since it doesn't cause any buzz when the guitar is unplugged.

b)  If yes, when you turn on the amp with nothing plugged-in to the input jacks AND turn up the vol, is there hum, buzz or noise?  If yes, there is noise in the amp.  If no or mostly no. . .

No abnormal noise exists even with big volume when nothing is plugged

Plug a guitar cable into the amp, no guitar;  repeat 1 b).  There will probably be at least some noise. 

Yes since the input is open

Your guitar controls, p/u covers, and/or strings may not be properly grounded; or the control cavity shielded and grounded.

Yes, might be  a reason

There may be sources of ambient noise: fluorescent lights; light dimmers or fan speed controllers, etc.

That's true

/Leevi

[Leevi]

I changed the V1 grid resistors to metal film resistors and I think the situation improved a bit.

The root cause of the problem is on the guitar side, I'm quite sure about that.
/Leevi

[TIMBO]

Hey mate, A bit of a read here.
http://el34world.com/Forum/index.php?topic=12674.msg118757#msg118757

[jjasilli]

I changed the V1 grid resistors to metal film resistors and I think the situation improved a bit.

The root cause of the problem is on the guitar side, I'm quite sure about that.
/Leevi

In that case you should debug the guitar.

[kagliostro]

What happen with one other guitar ??

Try one other V1

Is the 68k resistor connected to the pin of the tube or to the input jack ?

Franco

[davidwpack]

I have an old '66 Fender Mustang with original single coils that's all buzzy and hummy. The amp is very nearly silent with humbuckers plugged in. Did you say that you tried more than 1 guitar?

[Leevi]

There is not a big difference between guitars with single coil pickups.
With the Humbucker guitar the buzz is remarkable lower.
The resistors are connected to the pin.


I noticed that the same phenomenon happens with other amps as well but
not so strong. Maybe the amplification of the Spitfire increases the effect since
its first stage has a big gain and then there is nothing else except volume and a poor tone control.


/Leevi

[MFowler]

I usually isolate the Switchcraft 1/4" input jack and ground to V1 cathode resistor/bypass cap ground.


With Cliff jacks your already isolated so must check ground connections.


Mark

[jjasilli]

There is not a big difference between guitars with single coil pickups.  With the Humbucker guitar the buzz is remarkable lower. . .  I noticed that the same phenomenon happens with other amps as well but not so strong.


The crux of the problem has been identified as single coil pu's.  Hence, the insistence on curing this problem solely within the amp is a misguided and ineffective effort.  Yes, different guitars and amps will react differently with one another.  That's a truism which is beside the point.  The point is that crux of the problem has been identified as single coil pu's.  That is the issue which needs to be addressed.


I address the guitar issue with the guitarnutz method:  Shielded guitar cavity grounded to amp chassis through guitar chord; No ground loops in the guitar; Separate signal ground from chassis ground in the guitar.  This makes a noticeable and worthwhile improvement.  But single coils will never be dead silent.

[HotBluePlates]

... The only problem is a buzz that occurs when not touching the strings. ... Are there and good ways to prevent or decrease the buzz on the amp side? ...

I think the idea of grounding the input jack through a small cap is at least partly about addressing this issue.

However, I've been thinking about getting the pickguard version of the Ilitch System for my Tele to eliminate buzz.  I already only have a little buzz as long as I'm touching the metal bridgeplate, but it would be nice to not have to worry about it at all.

I probably won't move on it until I decide whether to change pickups in my Tele.

[jjasilli]

Input (no pun intended ) is welcome. But, to me a small cap to ground at the input is to kill RF interference - i.e., radio broadcasts from coming through with guitar signal. 

Here we seem to have AF noise.  Guitar p/u coils are known to serve as antennas for ambient AF noise.  Humbuckers have inherent noise cancellation, while single coils don't.  Single coil noise can be obnoxious.  If so, the guitarnutz method will reduce it; at very little cost; and is good practice anyway.

EDIT:  I think a shunt cap at the amp's input jack is unlikely to effectively reduce AF noise.  It will likely reduce as much guitar signal as it does noise.  Hence, the signal to noise ratio will remain the same, with no benefit. So, the first step should be to reduce the noise at its source. 

Have you taken any grounding / shielding steps in your guitars?

[Colas LeGrippa]

check the continuity between your guitar bridge and the amplifier chassis.


By touching the guitar strings, you act as a ground. There should be a wire that connects the bridge of your guitar to the shield of your cable tp your amp. Even a cheap cable doesnt make that much noise so don't spend money on a good cable if you don't have the budget for it.


Colas

[ginger]

Keep your hands on the strings...that is the point

[ginger]

What I said was true... but it has bothered me too.  I can overlook more things now... the fix is.. hire someone to always have their hand on your guitar

[Colas LeGrippa]

a topless  groupie by example

[ginger]

No... noise bugs me of course... but hum can be overcome, but small noises don't matter , if the noise of other instruments  are louder... if keeping your hand on the strings works, ( and i totally understand your complaint ).. then keep your hand on the guitar

[ginger]

reminds me of the Black Crows... maybe " Sometime Salvation "... where the Marshallls are humming before they begin..... then.. they start playing.. and the noise goes away

[ginger]

I always kind of like the players who really play out a lot...they like a good guitar.. but seem unconcerned about the amp..

[jojokeo]

Small cap from input jack's ground to chassis via short lead length = RF shunted to ground that might be riding along your cables. No affect on tone since it's not touching/connected to signal wire.

No noise when guitar vol is on 0 = signal is shunted to ground via cable to amp. Should be quiet like when shorting input jack is used.

Cliff jack's ground going straight to V1a's filter cap negative side is best practice (not cathode resistor & cap unless these also go directly to filter cap ground?).

TEST: turn amp up off standby, plug in guitar and volume up fully. Now stand facing amp > then turn 90º > then turn 180º > then walk further away. You will notice changes in your "floor noise" that you hear based on proximity and direction. This is normal. Many guys - especially amp building tweakers like us will often sit right in front of the amp and play and tweak and adjust. This isn't the best position to be in regarding the noise you hear from being in this position.

The reason the Spitfire is a little worse than other amps you may have??? The first tube has both stages wired in parallel which gives a little more gain than a single stage. More gain equals a bit more "floor noise".

I agree with JJ that any and all things you can do such as shield you pu and controls compartment helps. BUT, you must remember to ground any and all copper foil areas to the guitar's output jack ground or it is rather pointless. A screw or sometimes soldering carefully to it will do the trick. I have also folded a little extra foil over to the top of a strat's body and then copper foil under the pick guard and when you do this at one or more screw connection points they get sandwiched together nicely when mounting the pick guard.

[Leevi]

Small cap from input jack's ground to chassis

I don't think this is a solution in my case since the preamp is grounded via a metal input jack?


/Leevi

[jojokeo]

Leevi, no it wasn’t for you rather clarifying/confirming it’s use for shunting any RF to ground before it could make it into an amp’s input.

[MFowler]

So did you cure the problem yet?


Mark

[Leevi]

So did you cure the problem yet?

If you are asking me, no, there is not much to do in that simple circuit.
Actions on the guitar side might improve the situation.


/Leevi