Throwing metal film resistors in Fender amps

[Wil]

Hi everyone,

Despite the fact that English is not my mothertongue, I try to read as many articles as I can in English in order to improve my modest knowledge in guitar amps and lead my experiments in a better way. But I still get a little confused with all the specific component names.

I currently own a DRRI which is a amp I truly love (even more since I put a WGS G12C in it, such a perfect match) and which as been modded a little :
Middle pot, Bright switch, NFB resistor on-off switch, all Mallory 150s caps, ... very versatile and great sounding amp.
But I experiment some hiss (which is louder with the less efficient G12C) that I want to reduce as much as I can.
From what I've read, the easiest solutions seem to lean toward a smart wiring and metal film resistors.
That's where I need your tremendous help.
For the first one, the inside of my DRRI is quite a mess and might cause some unwanted noises, if anyone has a guide or link to how to properly place the wires in a Fender amp, that'd be great.

For the second one, I'm a little confused in all the resistors name but I mainly understood that high-wattage metal films were the quietest and that they're very useful for reducing noise in the input stages and power stages (complete this if it's wrong). For the moment my amp is running on stock carbon film resistors (it's a 1999 DRRI, for a 22 year-old dude like me, I consider it vintage piece, haha!). Resistors that drifted have been replaced. I'm now looking to replace the ones who are prone to make indesirable noises.

Based on this schematic, can you please point out the resistors or various components I should replace to minimise noises (Hiss in particular) without loosing the "Mojo" factor of carbon comp and carbon film resistors which for unknown reasons affects me quite a lot ...
And if you have the time, that would be great if you could write the specific names (fonction) of these components so I can improve my knowledge and impress my mom who thinks I'm crafting bombs in the cellar... haha



Thank you very very very much !
Wil from Belgium.

[R.G.]

Despite the fact that English is not my mothertongue, I try to read as many articles as I can in English in order to improve my modest knowledge in guitar amps and lead my experiments in a better way. But I still get a little confused with all the specific component names.

Hi, Wil. I admire how very well you do in English. I wish my abilities in any other language were as good.

Resistors that drifted have been replaced. I'm now looking to replace the ones who are prone to make indesirable noises.

Based on this schematic, can you please point out the resistors or various components I should replace to minimise noises (Hiss in particular)

All of them, with only two exceptions, those two being R54 and R55. But there is a middle ground that works almost as well and is less work.

The nature of noise is that the noise/hiss of the first amplifier stage almost totally determines the hiss. So the other middle ground way is to start with the resistors at the input jack of the amplifier, and replace those first. In your schematic, this approach would start with R1, 2, 3, 4, 13, 14, 15, 16, 28 and 29. These resistors are all involved in amplifying up a quite-small voltage, so any hiss from them gets amplified with the signal.

without loosing the "Mojo" factor of carbon comp and carbon film resistors

Read http://www.geofex.com/article_folder...carboncomp.htm

which for unknown reasons affects me quite a lot ...

It's human physiology and psychology. Humans simply like soft even-order distortion.

[Wil]

Hi, Wil. I admire how very well you do in English. I wish my abilities in any other language were as good.


All of them, with only two exceptions, those two being R54 and R55. But there is a middle ground that works almost as well and is less work.

The nature of noise is that the noise/hiss of the first amplifier stage almost totally determines the hiss. So the other middle ground way is to start with the resistors at the input jack of the amplifier, and replace those first. In your schematic, this approach would start with R1, 2, 3, 4, 13, 14, 15, 16, 28 and 29. These resistors are all involved in amplifying up a quite-small voltage, so any hiss from them gets amplified with the signal.

Read http://www.geofex.com/article_folder...carboncomp.htm

It's human physiology and psychology. Humans simply like soft even-order distortion.

Thank you for the flash fast answer ! One off-topic question remains in my head : Are you R.G. Keen, webmaster of Geofex.com ??

[Mick Bailey]

It's a good idea to narrow down where any hiss is coming from to save unnecessary work.

Firstly, does it hiss with no guitar plugged in and the volume and tone controls set to where you'd usually have them? (reverb off) If it does, do either (or both) of the volume controls affect the hiss?

Is it worse with your guitar plugged in?

Does the reverb control affect it?

One thing to make sure of is that the preamp tubes are as quiet as they can be. A surprising amount of noise can be attributed to a good-but-noisy tube. Auditioning different tubes, or installing ones which have been tested for noise is a good bet. Out of any batch of preamp tubes there will be a small number that are quieter than the rest, given that other characteristics remain the same.

[Wil]

It's a good idea to narrow down where any hiss is coming from to save unnecessary work.

Firstly, does it hiss with no guitar plugged in and the volume and tone controls set to where you'd usually have them? (reverb off) If it does, do either (or both) of the volume controls affect the hiss?

Is it worse with your guitar plugged in?

Does the reverb control affect it?

One thing to make sure of is that the preamp tubes are as quiet as they can be. A surprising amount of noise can be attributed to a good-but-noisy tube. Auditioning different tubes, or installing ones which have been tested for noise is a good bet. Out of any batch of preamp tubes there will be a small number that are quieter than the rest, given that other characteristics remain the same.

Thanks for the answer.
Well, actually, that amp is magical. I played it 2hours ago and it was hissy but very open and articulate.
Now, to experiment what you just wrote, I lit it up and it was kind of silent.
Volume on 10, all other controls on 0, very low noise, near no noise at all.
Volume on 10, Treble only on 10, hissy as hell, horrible. (Middle control does the same thing in a less loud way)
Volume on 10, Bass only on 10 adds a noise that sounds like a big industrial fan.
Volume on 10, Reverb on 10 adds some sort of 60 cycle hum.

It's noisier with my guitar plugged it. It adds a low background noise and low 60 cycle hum which I assume is produced by the P90s on my guitar. (I have no humbucker at all actually).

I believe I already replaced R18 with a Metal film equivalent. I might go with a higher wattage like 1-2watts.

EDIT : The magical thing I'm talking about is actually real. I wanted to check the controls again and this time, the amp made an extra constant white noise... I did not open the amp, just turn it off and light it up an hour later... Defective switches ? But with the hiss, the amp sounds more open and clear. Funny.

[Alan0354]

It is normal to have more noise when volume is up even without guitar. Check the reverb dial, see how much noise it goes up when you crank the reverb high. R.G got most of the resistors already. The preamp tubes V1 and V2 also can be problem as Mick Bailey said.

There is no difference in Thermal noise between MF and carbon comp resistors. The main difference is the 1/f noise at low frequency. Carbon comp resistor may have more low frequency noise ( not necessary all of them, you can even hand pick ones that don't). MF resistors have no 1/f noise.

Before you do anything, I would get a lower noise 12AX7 ( or 7025 of Fender) and try that first, because it's the easiest to do. Then worry about the resistors. I would isolate exactly where the noise is from before blanket replacing all the resistors. It's a big job to change all those resistors on a pcb.

Let me raise another question to the knowledgeable people here. Does it make a difference in sound between MF and carbon comp resistors? If so, how much and why. I never A/B compare as it is hard because of all the other variables. But I personally do not believe it makes a difference. I understand carbon comp has voltage coef that it's not totally linear with voltage. But I feel the curve ( distortion) of the triode way dominates the linearity. Please correct me if I am wrong.

[Mick Bailey]

The Fender tone stack will cut out most of your signal when the three controls are turned down, that's why the hiss almost disappears, even with the volume on 10. Because the volume control for each channel is connected off the tone stack, there's not much difference between all tone controls on zero/volume on 10, or all tone controls on 10/volume on zero.

From what you describe the hiss is mainly coming from V1a and V2a, certainly prior to the tone stacks, and you need to get the right tubes in those early positions before deciding to replace resistors. Don't forget that even if you replaced all the resistors with MF, the pots will be left. Each time you work on the PCB you risk lifting a track or pad.

A few years back I built two amps and I still have both of them. The first was with carbon film built as a head. I liked it so much I built another as a combo with metal film. No difference at all with hiss or any other noise, which leads me to consider that resistor type isn't the major contributor to hiss in a tube amp, but preamp tube quality is.

Plenty of amps have wires all over the place and nothing special in terms of components - just industry standard stuff at the time they were built, and they're as quiet as you could ever want.

[R.G.]

One off-topic question remains in my head : Are you R.G. Keen, webmaster of Geofex.com ??

Yep, that's me.

[soundguruman]

Hi everyone,

Despite the fact that English is not my mothertongue, I try to read as many articles as I can in English in order to improve my modest knowledge in guitar amps and lead my experiments in a better way. But I still get a little confused with all the specific component names.

I currently own a DRRI which is a amp I truly love (even more since I put a WGS G12C in it, such a perfect match) and which as been modded a little :
Middle pot, Bright switch, NFB resistor on-off switch, all Mallory 150s caps, ... very versatile and great sounding amp.
But I experiment some hiss (which is louder with the less efficient G12C) that I want to reduce as much as I can.
From what I've read, the easiest solutions seem to lean toward a smart wiring and metal film resistors.
That's where I need your tremendous help.
For the first one, the inside of my DRRI is quite a mess and might cause some unwanted noises, if anyone has a guide or link to how to properly place the wires in a Fender amp, that'd be great.

For the second one, I'm a little confused in all the resistors name but I mainly understood that high-wattage metal films were the quietest and that they're very useful for reducing noise in the input stages and power stages (complete this if it's wrong). For the moment my amp is running on stock carbon film resistors (it's a 1999 DRRI, for a 22 year-old dude like me, I consider it vintage piece, haha!). Resistors that drifted have been replaced. I'm now looking to replace the ones who are prone to make indesirable noises.

Based on this schematic, can you please point out the resistors or various components I should replace to minimise noises (Hiss in particular) without loosing the "Mojo" factor of carbon comp and carbon film resistors which for unknown reasons affects me quite a lot ...
And if you have the time, that would be great if you could write the specific names (fonction) of these components so I can improve my knowledge and impress my mom who thinks I'm crafting bombs in the cellar... haha

[ATTACH=CONFIG]26898[/ATTACH]

Thank you very very very much !
Wil from Belgium.

Yes metal film resistor is less noisy. That is proven fact.
Even better is Bulk Metal Foil Resistor. That is well documented.

And you can see that many manufacturers have started using Metal Film in production amp / audio equipment.

BUT in tube amp, you can also use snubber capacitor to reduce the noise. This is used widely.
3-7 pf between grid and plate is my preferred method. It works very well.
There are several other accepted methods also.

[Alan0354]

Far as I know there are three type of noise:

1) current noise ( shot noise) where it generates as current passed through a device.
2) Thermal noise where it only depends on the resistance.
3) 1/f noise that is device dependent. It is mainly lower frequency noise. You can hand pick carbon resistor that don't have this noise.

Of cause all of them depends on the BW in their definition. MF resistor don't have 1/f noise, only the other two type.

[Alan0354]

The Fender tone stack will cut out most of your signal when the three controls are turned down, that's why the hiss almost disappears, even with the volume on 10. Because the volume control for each channel is connected off the tone stack, there's not much difference between all tone controls on zero/volume on 10, or all tone controls on 10/volume on zero.

From what you describe the hiss is mainly coming from V1a and V2a, certainly prior to the tone stacks, and you need to get the right tubes in those early positions before deciding to replace resistors. Don't forget that even if you replaced all the resistors with MF, the pots will be left. Each time you work on the PCB you risk lifting a track or pad.

A few years back I built two amps and I still have both of them. The first was with carbon film built as a head. I liked it so much I built another as a combo with metal film. No difference at all with hiss or any other noise, which leads me to consider that resistor type isn't the major contributor to hiss in a tube amp, but preamp tube quality is.

Plenty of amps have wires all over the place and nothing special in terms of components - just industry standard stuff at the time they were built, and they're as quiet as you could ever want.

Can you hear any difference in sound quality between the MF and carbon in the two amps?

[Wil]

What do you mean by 1/f noise ?

Regarding the tubes, I tried swapping with no improvment, unfortunately.

[Wil]

Yep, that's me.

Wow. Actually, you were basically the brain of my wah build and I was the sloppy hands. I simply experiment relying on what was written on your site.
I really admire your knowledge and am thankful for your tremendous contribution to building my own.

[Jazz P Bass]

I Googled '1/f' noise & it came with Wiki: pink noise.

[Enzo]

Alan presented a short list of self-noise sources for the component. But a carbon comp resistor has a granular interior, which can become mechanically unsound, and that will cause noise due to vibration and even thermal expansion of the body. Also the wire leads have to bond to the resistive part, and that can become loose.