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New tube amp noise...


Jazzer2020

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OK I just finished reading a comprehensive thread here on tube amp hum and there were tons of ideas/suggestions to fix it.

 

So I will just cut right to the chase.

 

I just spent Mega bucks on a custom built boutique amp.

Received it a couple of days ago.

Numero uno of my build requests was to make it as quiet as possible (no hums/hisses).

 

While the overall sound of the amp is creamy smooth, as I wanted, it does have a hum and hiss that is

noticeable in a quiet room, where I practice 99% of the time.

 

I'm not sure what to do really?

 

So I started comparing it with my two Fender amps (Princeton II and Vibroverb).

I split both amps into head and speaker cab a couple of years ago.

 

Both are much quieter.

 

My new amp is also split into amp head and speaker cab.

 

Where do you suggest I go from here?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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I'll get some stupid questions out of the way first;

 

Are you plugging all 3 amps into the same power source?

 

 

Are you using the same guitar in all the amps?

 

Have you tried switching guitar cables?

 

As far as your "noisy" Princeton II Have you tried switching the speaker cable out from your "quiet" vibroverb?

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I'll get some stupid questions out of the way first;

 

Are you plugging all 3 amps into the same power source?

 

 

Are you using the same guitar in all the amps?

 

Have you tried switching guitar cables?

 

As far as your "noisy" Princeton II Have you tried switching the speaker cable out from your "quiet" vibroverb?

 

Thanks for helping out!

Yes all 3 in the same power source, one at a time of course.

 

Same guitar, yes. I've tried many guitars now in comparison but the hum is always there.

 

This is not a cable issue. Strictly amp noise.

The noise is there with no cables plugged in.

 

Yes I did try switching speaker cables, but no difference.

 

I've tried several different power outlets too. No change.

 

Just now I discovered something interesting.

I played with all the controls for the first time.

 

The odd man out was the reverb knob.

 

The hum is a 'B note' and can be broken down into two octaves.

I noticed when I turn the reverb up from the 9 o'clock position it gets progressively louder in the

lower B octave.

 

Louder and louder until up to full at 5 o'clock position.

 

I find this interesting, but having said that, even at 9 o'clock, where I had kept it until just now,

the hum is annoying (in a quiet room). The hum just gets much much worse the more reverb is added.

 

 

One more note that may or may not help.

When the amp volume knob is increased, only the hiss increases, not any of the humming.

 

 

 

 

 

 

 

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Does the hum go away when the reverb is at zero? Maybe a bad reverb tube? If amp is brand new it should be under warranty...

 

Fwiw I have a PRRI & it is dead quiet (although when it's cranked there is a little hiss- but I would think that the case with most any amp).

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Since this is a new amp, the problem may be power tubes. As new tubes are burned in the bias cab drift a little and have increased hum. The kind of hum they produce is a low level background hum that doesn't change in volume. When tubes are matched and the bias perfectly balanced the hum is canceled. Tubes often need a couple of hundred hours to burn in and stop drifting in value. The brand and quality of tube plays an important role too. Some tubes can be dead on and drift little from the start and some can be wickedly bad drifting all the time.

 

The cause of the low level hum can come from a number reasons. Number one is the filament voltages are likely AC. If the filament wiring isn't routed tightly against the chassis and that chassis is made of a low mu metal that doesn't shield well you can have higher levels of emf floating around in the chassis being amplified. Given this is a custom build you can have many other factors involving wire routing and component placement. If the power tubes are balanced this doesn't usually matter because the hum is canceled out. The problem could be insufficient DC filtering or over gained tubes too but I have no way of knowing at this point.

 

The reverb hum too is most likely the result of filament voltage. It can also be the shielded wire between the tank and circuit aren't the best wiring too. Filament hum can be reduced by using a spiral filament heater. The Sovtek LPS is supposed to be better at this. https://www.tubedepot.com/products/sovtek-12ax7lps-preamp-vacuum-tube. Problem is they aren't the best choice for combos because they can become microphonic from the vibration. The reverb circuit adds extra gain stages and the tank contains elements at the ends of the springs that work similar to a speaker to make the string vibrate and a microphones to receive the signal. Any added hum from the tank circuit will be added to the signal chain. Shielding the bottom of the tank may be a way of reducing hum.

 

All of these need to be tested by an expert and minimized. The problem may not be one single item but a combination of many smaller items.

 

Hiss is common with all tube amps. Its caused by two main items. One can be carbon resistors which reduce electron flow. Electrons collide with the carbon inside which reduces current but it also adds noise to the signal. The other is the tubes themselves. A beam of electrons is created as the filaments heat the cathode. They pass through a grid trying to get to the anode. This grid is a physical screen like a screen door. The electrons either pass through the holes in a straight line or hit the wire mesh and ricochet around hitting other electrons.

 

These electrons bouncing around cause hiss and there isn't a dam thing you can do about it besides minimize and mask the effect. Comparing one amp to another is pretty useless unless you compare the circuitry. Guitar amps in general have high gain staging so noise is going to be in your face. The difference in his between one amp and another may be the amount of gain staging and total wattage, its may be the tone stack that EQ's a certain amount of the hiss away. It can be the speaker itself. many speakers roll the treble off at a lower frequency then others and thereby masks the higher frequency hiss. The speaker SPL level may be high which can increase the noise floor making both the his and hum more audible.

The circuit type can be a huge factor, Class A vs AB for example. The type of power tubes used and how hot they're biased play a huge role.

 

What it all boils down to is, a trade off. If you want hot tube tone you're going to have to deal with a certain amount of hiss. You want a loud sound from a lower wattage amp a high SPL speaker with a wider frequency response will make that hiss more apparent. Vintage speakers had lower SPL levels which keep the noise floor low and they're limited frequency response rolled off the top end masking much of the his an amp normally causes.

You can do certain things when building like using metallic resistors which have lower noise levels, using tubes that are quiet, and biasing the tubes to a lower level but you will loose gain and tone when you crank the amp up.

 

For the hum I recommend running the amp for a month or two. If the hum persists, have the power tubes rebiased or replaced. If you can check the brands of tubes used I can make other suggestions. Some tubes like Electro Harmonic's are low noise high gain and can make a huge difference in noise levels. The speaker can be swapped to something else and mask a great deal of the problem too. Everything else would need to be gone over by an expert (not nessarily a builder either - it doesn't take a degree in electronics to build an amp but it does to minimize the issues with a poor build)

 

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...

 

The odd man out was the reverb knob.

 

The hum is a 'B note' and can be broken down into two octaves.

I noticed when I turn the reverb up from the 9 o'clock position it gets progressively louder in the

lower B octave.

 

Louder and louder until up to full at 5 o'clock position.

 

I find this interesting, but having said that, even at 9 o'clock, where I had kept it until just now,

the hum is annoying (in a quiet room). The hum just gets much much worse the more reverb is added.

 

 

The note 'B' is very close to 60Hz or a multiple. - it is possible part of the Reverb circuit is picking up some 60Hz radiation (which WRGKMC erroneously referred to as "emf" in the above post) from the amplifier circuit or floating around the room.

 

It could be something as simple as a a poor ground connection on one of the wires connecting the reverb tank

 

One more note that may or may not help.

When the amp volume knob is increased, only the hiss increases, not any of the humming.

 

If the problem is a filter capacitor in the power supply then that would indicate the filter for the power tube bias supply (if the amp has fixed bias).

 

 

 

 

 

 

 

 

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The note 'B' is very close to 60Hz or a multiple. - it is possible part of the Reverb circuit is picking up some 60Hz radiation (which WRGKMC erroneously referred to as "emf" in the above post) from the amplifier circuit or floating around the room.

 

It could be something as simple as a a poor ground connection on one of the wires connecting the reverb tank

 

 

 

If the problem is a filter capacitor in the power supply then that would indicate the filter for the power tube bias supply (if the amp has fixed bias).

 

 

Thanks WRG and onelife.

 

I forgot to mention one thing. Like my Fender Princeton II and Vibroverb, my custom amp consists of separate head and speaker cab.

 

I did a little more testing today. Using the Fender Princeton II head + custom speaker cab and comparing it to the custom head + custom speaker cab.

 

The overall sound was very close. A little brighter with the FPII. The noise level was close between the two amps, just slightly different characteristics. More highs (noise) with the FPII, more lows with the custom amp.

 

More twiddling with the volume knob today (custom head) revealed that the hum does in fact increase as you increase the volume.

I may have set the Treble and Bass knobs to 0 yesterday when I said I could only hear the hiss increase on the volume knob.

Because today when I started playing the amp, the Treble and Bass were at 0. I didn't even notice for a while.

 

I said to myself, "Wow my FPII sounds much better than this new custom build!"

When I set the Treble and Bass properly they were very similar in sound.

 

I'm not technically inclined like you guys are with amps.

But I do possess extremely sensitive hearing and can hear sounds/noises that many people can't.

 

Having said that, when I read others here saying their 'X' tube amp is dead quiet, I say to myself

shouldn't my Mega$ custom built amp also be dead quiet?

 

I mentioned that my Fender Princeton II was pretty dead quiet before I separated head and speaker cab.

That is the noise level I am expecting (pre-split).

 

 

 

 

 

 

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Not to make things too complicated, but all tube amps will emit some sort of hum/hissing, that’s why guitar tube amplifiers sound so good.

 

Some obvious things to stay away from with a guitar tube amp. CRT old style TVs will cause a 60Hz hum if present in the same room, so if you have one on in the room, get a LCD or LED to replace it. Also, make sure that your guitar pickups are far enough away from the transformer(s) in the guitar tube amp - instant 60Hz hum. You may want to get some Deoxit (deoxidizing solution) and clean your preamp tubes. Owning a guitar tube amp you should always clean your pre/power amp tubes when replacing them even if they are new they still build up a corrosive barrier that can cause a hum, hissing and/or scratchy type of irritating nuisance.

 

Good luck.

 

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The note 'B' is very close to 60Hz or a multiple. - it is possible part of the Reverb circuit is picking up some 60Hz radiation (which WRGKMC erroneously referred to as "emf" in the above post) from the amplifier circuit or floating around the room.

It could be something as simple as a a poor ground connection on one of the wires connecting the reverb tank If the problem is a filter capacitor in the power supply then that would indicate the filter for the power tube bias supply (if the amp has fixed bias).

 

Erroneous??? You really need to brush up on your abbreviations. I realize some countries use different terminologies in electronics, but this one permeates the internet so I cant see how you can possibly get this one wrong.

 

EMF has two definitions.

 

One is Electromagnetic Force = Electromagnetic force is the force present between electrically charged particles such as electrons and protons. It has the ability to repel and attract charges. The electromagnetic force is attractive for unlike charges( electron and proton) and repulsive for like charges (two electrons or protons). Electromagnetic force is responsible for the binding of atoms.

 

http://www.physics-and-radio-electronics.com/physics/natural-forces/electromagnetic-force.html

 

The second definition - the one I was obviously referring to -

 

Electromagnetic Field = An electromagnetic field (also EMF or EM field) is a physical field produced by electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction. It is one of the four fundamental forces of nature (the others are gravitation, weak interaction and strong interaction).

https://en.wikipedia.org/wiki/Electromagnetic_field

 

Everyone is exposed to electromagnetic fields (EMF) of different frequencies that permeate our environment. Exposures to many EMF frequencies are increasing significantly as technology advances unabated and new applications are found. While the enormous benefits of using electricity in everyday life and health care are unquestioned, during the past 20 years the general public has become increasingly concerned about potential adverse health effects of exposure to electric and magnetic fields at extremely low frequencies (ELF). Such exposures arise mainly from the transmission and use of electrical energy at the power frequencies of 50/60 Hz.

 

Low Frequency Radiation/Power Lines (ELF fields) are defined as those having frequencies up to 300 Hz and can produce electromagnetic interference (EMI). At frequencies this low, the wavelengths in air are very long (6,000 km at 50 Hz and 5,000 km at 60 Hz) Magnetic fields come from the motion of electric current. Their strength is measured in units of tesla (T) In some countries another unit, called the gauss (G), is commonly used for measuring magnetic induction. Any device connected to an electrical outlet, when the device is switched on and a current is flowing, will have an associated magnetic field, the strength of which is directly related to the current drawn from the source. Magnetic fields are strongest close to the device and get lower with distance. Most common materials do not shield them.

 

 

http://hps.org/hpspublications/articles/elfinfosheet.html

 

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Erroneous??? You really need to brush up on your abbreviations. I realize some countries use different terminologies in electronics, but this one permeates the internet so I cant see how you can possibly get this one wrong.

 

EMF has two definitions.

 

The second definition - the one I was obviously referring to -

 

 

 

In your own words...

 

I always get a laugh when people blame hum on AC. What do you think an outlet is supposed to provide DC?

 

AC Alternates in current and as it does it generates and external magnetic field around all wires. This electromagnetic force (EMF) is the same as any other radio wave except its 60 cycles falls within the same frequency range as the audio spectrum or 20~20Khz.

 

 

Although electromagnetic field is sometimes abbreviated as EMF it is more commonly referred to using EM field to avoid confusion.

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In your own words...

 

 

 

 

Although electromagnetic field is sometimes abbreviated as EMF it is more commonly referred to using EM field to avoid confusion.

 

What about "Chocolate Fudge Sundae"?

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All right then, the force produces a field. My mistake, and No, EM is not used more then EMF to reduce confusion. EM is normally associated with a nuclear blast producing an EM pulse. It's still a magnetic field but its not used more in technical discussions, at least not here in the US.

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My amp builder will be contacting me shortly to advise on next steps.

 

In the meantime I have a question for you guys.

What is your opinion on the tremolo circuit? In the initial discussion for the amp build I asked if omitting the tremolo

would make the amp quieter (I never use it). He said it wouldn't so I decided to leave it in.

Your thoughts on it?

 

 

 

 

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My amp builder will be contacting me shortly to advise on next steps.

 

In the meantime I have a question for you guys.

What is your opinion on the tremolo circuit? In the initial discussion for the amp build I asked if omitting the tremolo

would make the amp quieter (I never use it). He said it wouldn't so I decided to leave it in.

Your thoughts on it?

 

 

 

 

In engineering, we have a very applicable phrase we abide by: "If it ain't broke, don't fix it".

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In engineering, we have a very applicable phrase we abide by: "If it ain't broke, don't fix it".

 

 

I understand that in principle and in general use. And also agree with it.

 

I don't have electronic expertise at all. Just know the very basics.

I just assumed that the fewer parts introduced into a circuit, the less potential

noise is created. Maybe that's not true?

 

Now if the case is that this builder likes to go by a stock wiring diagram/schematic

for convenience sake, and making alterations that may help lessen noise

is not recommended by them (because of the additional work involved ),

then I'd like to know about it.

 

 

 

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A Tremolo has a depth knob that can be turned off and bypass it all together so if there's noise in that loop its bypassed all together.

 

Whether it adds noise just by having the extra tubes in the circuit is questionable. Tubes do have heaters and require bias voltage. Extra wires inside the chassis running AC filament voltages increases the likelihood they may radiate some of that AC into the amplification path. Pots designed to bypass the circuit can bleed small amounts of signal.

 

If they used high quality pots, properly routed wires tight to the chassis there shouldn't be extra noise with the effect bypassed.

 

What I used to use to find hum issues is a tape head demagnetizer. A Demagnetizer uses an electromagnet to produce an AC field around its probe tip. I'd use a scope to monitor the background noise then probe around the circuitry wiring to find wires and/or components that are most susceptible to hum and then move them around, usually closer to the chassis to minimize the amount of noise they can generate.

 

For hiss I'd again monitor the noise levels with a scope and try different tubes to find ones that produced minimal background noise. You can do the same for resistors and caps in key areas like coupling caps and bias resistors if needed. All this can be time consuming but it is what's needed sometimes when an amp misbehaves. Small tweaks that cant be heard clearly with the ears are cumulative so finding several may help to improve things.

 

You will eventually hit a floor and cant improve it beyond that point simply because God doesn't make components any better.

You can design something mathematically perfect but when it comes to building, every part has an imperfect tolerance range. In most amps caps can be 20% and resistors 1, 5, 10 & 20% out of specs. Budge builds will use high tolerance components and quality builds low tolerance. You pay for it however. 1% tolerance resistors are military grade and very expensive. I had a friend who made them for Precision resistor. They were wire would resistors which were wound by hand. I'd get a few pennies per resistor winding them in his spare time. He gave me a batch for my Fender amp and I can say they did even up the bias circuit in that amp.

 

You can go through a batch of higher tolerance components measuring each one to find the cream of the crop but its still no guarantee they will work noise free. Components can change value with age and temp and the purity of the materials used to make the parts is not perfect. Never has been, but your best gear will start with finest components made, then they are hand tested to weed out duds then they are tested within a circuit at critical test points to confirm they are doing what they are supposed to. The whole goal is to start as close to the original design as possible so when the circuits do drift (and they will) they will still be better then a budget build using higher tolerance components.

 

As far as having fewer components, its a dual edged sword. Less components in the signals path may reduces the amount of self noise that can be caused but it can also reduce the options to control how well components may operate and limit the amps flexibility as a user.

A super basic amp can be stripped down to nothing and only have a preamp volume control. Even that could be eliminated because its a redundant circuit - a guitar will have a volume and tone control and you could simply run the amp wide open.

 

What you wind up with however is a one trick pony. Guitar pickups have limitations so having some basic gain and tone controls are essential in adapting the amp to all the various pickup and guitar types so you can express the tones you want and get the kind of string touch to suit the player.

There is much to be said about using a minimalist rig. Too often we use electronics as a crutch to support our playing or as a mask to cover up what we aren't playing. It reminds me of what the first electric players had to use when performing with a big band. The amp quality couldn't have been very good but the players were superb well trained and the noise of horns covered up many of the flaws including they're noise levels.

 

Add to they there are no new tube types being designed. The days of R&D are long gone. I'd love to see some of it come back and even have some of the obsolete designs revived, but its unlikely to happen based on cost vs profitability. What we have is the remaining tubes and the circuits that best support them. You can only tweak the design so far before they either suck for sound quality or simply fail. That's still allot to work with and we should be glad we still have tubes around for guitar amps. The next generation my not be so lucky to experience they're beautiful tones, noise and all if the few remaining companies who build them can no longer turn a profit.

 

 

 

 

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Thanks guys I will pass along your tips.

 

One more thing. While I was doing some research on this I found this one guy who sounded like he had a really good grip on this issue.

 

He did a lot of research on the subject and claims to have come up with a solution.

 

He is quite long-winded in the build-up, but his solution is quite simple and comes near the end of the video.

Have a look and tell me what you think about it?

 

 

 

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The best thing to do, is to first narrow down which end of the amp the hum/noise is coming from. If the hum increases as you turn the volume up, the problem is in the preamp... If it does not, it is in the power section. More than likely, it's a tube issue.

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Thanks MacFangus. The hum increases as the volume is turned up. It also increases when the reverb is turned up.

So I don't know if that means two separate problems?

 

It could be a filter capacitor in the power supply.

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I just spent Mega bucks on a custom built boutique amp.

Received it a couple of days ago.

Numero uno of my build requests was to make it as quiet as possible (no hums/hisses).

 

A few random thoughts:

1) "Lead dress" is supposedly an important factor in minimizing amplifier noise. Leads from the turret board to tube sockets/pots/jacks should be as short as practical. The 12V wires for the preamp filament heaters should be twisted together to reduce induced hum, like the green wires are in this photo.

 

1423027923229

 

2) Is the top of your amp's chassis shielded? My combo amp has metal window screen stapled to the top inside of the cabinet, to shield the amp circuitry from interference. When the chassis is installed and the mounting screws tightened, it pressed the metal rim of the chassis against the metal screening insuring a good ground contact.

 

3) Yes it IS possible to add additional components to reduce an amp's hiss, but of course there is a potential trade off. Capacitors can be added between gain stages to bleed off white noise (hiss) because it's at frequencies well above guitar frequencies. Too much noise filtering can reduce the treble response (brightness) of the amp.

 

4) Hum from the reverb? It's possible the reverb's pickup coil is picking up magentic interference from the amp's power transformer. Turning the reverb pan around in the chassis may help.

 

In any case, if you payed "mega bucks" for a custom, boutique build, the BUILDER should be the one you are having this conversation with.

 

We would LOVE to see a picture of the amp's circuitry. We'd also like to know the name of the builder and how much you paid. :p

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FYI, the 12 Volt filaments, in tubes like the 12AX7, have a centre tap. In amplifiers like the Fender Princeton pictured above, the two halves of the filament are wired in parallel and run off the same 6.3 Volt secondary winding of the power transformer as the filaments of the 6V6 power tubes.

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