How about using the phantom power from a mixer to power a 12au7 preamp for acc guit?

[Wynand]

Hi everybody, im new here and also green in the electronics field, however i think im onto something. would it be possible to build a preamp for an acoustic guitar using the 48v phantom power supply from the desk to power a 12au7 tube? im looking to breathe life into the bluuuaaeggg sound the average pickup preamp provides.

The project needs to be simple as i have limited electronic knowledge, and also cost effective for obvious reasons.
I would also like to later explore the possibility to miniaturise the preamp to build it into acoustic guitars.

[Koreth]

The immediate concern that comes to mind is the current requirements of the tube's heater and the current limits of the phantom power specification. Per spec, phantom power can only supply a maximum of 10mA per microphone. A 12AU7 requires 150mA of power when powered by 12.6V heaters or 300mA at 6.3V -- 15 or 30 times the current limit of a phantom power supply. Looking through datasheets, this appears to hold true for all the members of the 12A*7 family of twin triodes. Even similar subminiature twin triodes such at the 6112, 6111, and 6021 require 300mA to power their heaters. There's a reason transistors have taken over many fields where tubes once held sway, including audio preamps -- they require far less power to operate.

Truth be told, I'm not certain how to overcome the current requirement hurdle for what you want to do. But there's people on this board much smarter than me, so maybe they know something I don't.

[km6xz]

There are a number of preamps which use phantom power but a separate heater supply is needed. Portable radios in the 50s used 1 volt heater tubes that were intended for battery operation. The radios used a C or D cell 1.5 volt carbon zinc cell for the heaters(a common flashlight battery) and 1 or more high voltage series wired cells for B+. The high voltage batteries were expensive but lasted a long time. The heater D cell would last 10 hours or so but were cheap.
A preamp requires a lot less current than a broadcast radio so if you can get the HV batteries, a moderate gain preamp could be feasible.

But why? A well designed solid state preamp has lower noise, works on easier to supply power sources, has lower distortion, wider frequency response and does not need expensive transformers. Tubes have the one advantage of being bad at amplifying, they introduce distortion, that bad generally, happens to be pleasant for generating music. There is nothing magical about a tube preamp, if it is intended to be clean, either solid state or tube will be indistinguishable from each other. But what you probably want is distortion and non-linear operation which tubes work fine for. The tube distortion does not have to be inside the guitar, it can be anyplace in the signal chain so boost the level with a linear and clean preamp and somewhere in the chain add a tube effects unit.
You mention the average pickup preamp sounds bad. Actually the sound fine but it just is not what you want. Generating additional and stronger upper harmonics(harmonic distortion) is not the limit of what you can to to create a more interesting sound, you can also, at the same time adjust the dynamic range of your signal chain so the sound seems to be fuller and punchier. That comes from altering the linearity of gain of the signal chain to compress, say, 90 db dynamic range into a PA or recording system's 60 db of effective dynamic range. The listener will not hear it as the thin sound that guitarists describe a low distortion hi-fi signal chain. With out much effort and money just about any sound characteristic can be simulated if you know exactly what your goal is. But people don't know, but are sure what they have is not it. Of course much of the sound character comes from playing style, not electronics.

[Wynand]

There are a number of preamps which use phantom power but a separate heater supply is needed. Portable radios in the 50s used 1 volt heater tubes that were intended for battery operation. The radios used a C or D cell 1.5 volt carbon zinc cell for the heaters(a common flashlight battery) and 1 or more high voltage series wired cells for B+. The high voltage batteries were expensive but lasted a long time. The heater D cell would last 10 hours or so but were cheap.
A preamp requires a lot less current than a broadcast radio so if you can get the HV batteries, a moderate gain preamp could be feasible.

But why? A well designed solid state preamp has lower noise, works on easier to supply power sources, has lower distortion, wider frequency response and does not need expensive transformers. Tubes have the one advantage of being bad at amplifying, they introduce distortion, that bad generally, happens to be pleasant for generating music. There is nothing magical about a tube preamp, if it is intended to be clean, either solid state or tube will be indistinguishable from each other. But what you probably want is distortion and non-linear operation which tubes work fine for. The tube distortion does not have to be inside the guitar, it can be anyplace in the signal chain so boost the level with a linear and clean preamp and somewhere in the chain add a tube effects unit.
You mention the average pickup preamp sounds bad. Actually the sound fine but it just is not what you want. Generating additional and stronger upper harmonics(harmonic distortion) is not the limit of what you can to to create a more interesting sound, you can also, at the same time adjust the dynamic range of your signal chain so the sound seems to be fuller and punchier. That comes from altering the linearity of gain of the signal chain to compress, say, 90 db dynamic range into a PA or recording system's 60 db of effective dynamic range. The listener will not hear it as the thin sound that guitarists describe a low distortion hi-fi signal chain. With out much effort and money just about any sound characteristic can be simulated if you know exactly what your goal is. But people don't know, but are sure what they have is not it. Of course much of the sound character comes from playing style, not electronics.

Phew, allot of terms to wrap my head around but i got your general point. Im no expert at electronics that's for sure. I don't know the technical terms here but ill try my best, here goes:
I owned a boss gt10 multi effects unit that emulates all these old tube stuff, and to the majority it is the benchmark of emulation. however, to my ears, and obviously some will agree some will disagree i HATED it. i ended up trading it in for analog pedals, they look bad, they are heavy, expensive and not practicle at all and i love the sound. its like dating a super model, awesome to look at, terrible to maintain.It could be in my head, it could be in my ears, who knows? Since then analog and tubes has been the answer to me.
I have since built the Valvecaster (12au7 booster of a 9v supply) but only with the wurlitzer 12fq8 tube and loved it.
Now im looking at building a tube preamp for acoustic guitars, im looking for those rich harmonic overtones. for some reason the tube warms up the sound with bass without going farty and at the same time ads a shimmer the the highs without it sounding treble.
it was ambitious to want a built in unit and in retrospect a pedal unit is just as good of an idea and solves the power supply problem.
i could not agree more about tone being in the guitarist fingers, however this is for my church and i dont know what your experience with worship musos are but some of them need all the help they can get! here in south africa we also tend to play with entry level guitars due to the fact that by the time they reach our shores and filtered thru all the middle men they are unaffordable to the majority.

Ps. thanx for the reply, i feel honored that the gurus would take a look at my ambitions.

[Wynand]

try this video, those speak more than words:
YouTube - Brad Davis Demonstrates Takamine Cool Tube Preamp

[Steve Conner]

As others have said, it's a good idea except the phantom power isn't powerful enough to run the tube heater. Maybe you can add a 6v battery.

[Koreth]

While I was quick the point out the issue with phantom power not being able to drive the tube heater, I must confess, the idea intrigues me.

There are CK5971 subminiature tubes for sale on eBay right now. Their heaters are designed to run from 1.25V +/-20% (1-1.5V), which is within the capability of a standard AAA or AA battery. The heater only draws 80 mA, which, with the typical 1200 mAH capacity of an alkaline AAA battery, should get you several hours of tube heater life, up to 15. Maximum plate current is 5mA, only half of the 10mA limit of phantom power, so in theory you could use this tube with a battery to make a tube preamp uses phantom power for plate voltage. Finally the maximum grid leak resistor is 5 MegΩ, which I'm guessing should be high enough input impedance for a piezo pickup. I am assuming the acoustic guitar in question is using a piezo pickup -- I've not heard of magnetic pickups for acoustic guitars.

If you wanted to gang 4 AAA or AA batteries in series, you could drive the heater(s) of one of the 6.3V heater submini tubes, or the more common 12A*7 family tubes. However, at 300mA per tube you'll get much less battery life, 4 hours at best.

I still wonder if it would be worth it or not. The pleasing warmth in the bass and sparkle in the top end comes from the harmonic distortion that a tube causes, even when a tube is not being overdriven and is running 'clean'. You'll get more of said pleasing harmonic distortion the harder you drive the tube. Again, I assume the pickup on the acoustic guitar in question is a piezo pickup. A Google search finds pages that say the average piezo pickup puts out signal voltage of about -30 dBu(about 0.024V rms or .069 V peak to peak). Assuming you were able to get all the gain from available from a 5971 (which tends not to happen with real circuits), the output voltage would only be swinging about 1.6Vpp on the output. 0.069Vpp in and 1.6Vpp out isn't driving the tube very hard at all when you consider that the theoretical max voltage swing you could get from a tube with a plate fed from 48V phantom power is 48Vpp. Driving the tube that lightly, just how much pretty harmonic distortion is being added to the signal? Now, if you were to cascade a couple of those tubes, the 1st one could drive the 2nd hard enough to get some nice harmonic distortion to get some warm bass and high end sparkle, and if the circuit was done right, without any overdriving the 2nd tube or the input on the mixing console supplying phantom power.

[Wynand]

Onit again

Ok guys, this is for the people that would like an out the box challenge.

I see in the above mentioned youtube vid that they push +3v thru the tube. on a low voltage like that, wont the current be lower to? They run that pre amp from a battery yes, but to the gurus out there, +48 phantom down to 3v? another idea, isnt it possible to boost current with a second tube?

bare in mind, i only have ideas so far, and very very little electronic knowledge im hoping to learn from you guys, and really appreciate your effort,but please only answer this thread if you have an idea that will keep the tube in the preamp. i am not interested in solid state amps regardless of your opinion on solid vs tube.

[Guitarist]

I just received a pre from this Ozzy: Electric acoustic guitar tube valve battery preamp kit | eBay But still have to install it.

I agree solid state is rarely my first pic.

[Wynand]

I just received a pre from this Ozzy: Electric acoustic guitar tube valve battery preamp kit | eBay But still have to install it.

I agree solid state is rarely my first pic.

that looks cool, which tube is that and would you be able to draw a schematic of the circuit? and finally how does it sound? do you have a sound clip?

[Koreth]

Judging from the pics it is some kind of sub-miniature tube. As the description says it can be ordered with either a triode or a pentode, which specific tube is being used is anyone's guess.

[km6xz]

The sub-miniature tubes where created for battery operation and came in a couple different design classifications. The first were miniature versions of existing types, mostly for the military in WWII. These were scaled down versions of RF and AF tubes used in radios and pack sets.. Later, in the 1950s lower voltage submiature tubes were developed for hearing aids and other low voltage applications. The tubes were usually 1.2-1.5volt DC heaters(no separate cathode) and some hearing aids used them in what was called "starved circuits" which ran on much lower than optimum anode voltage and due to extremely high impedance, developed amazing voltage gain.
One thing all the sub-miniature tubes had in common was being noisy. Most did not have much linearity to their curves. Later, another small tube was created that would operate at much higher frequencies, into the UHF region, called the Nuvistor. They were used primarily in TV sets in tuners and preamps. Some were used in instruments like FairChild wide bandwidth oscilliscopes. The whole vertical amp section was various Nuvistors and the low frequency portions were solid state.
The problem with Nuvistors, even though tiny, 1/2 inch high. although the high voltage versions had plate caps, was that they required conventional heater voltages. They were tried in several pro condenser microphones including the C-12's that I had. The most common one that can be found today is the 6CW4, and would make a tiny mic or guitar preamp if the power supply issue was resolved using an in-line power supply like tube mics used. There was a low voltage version that is not too common called the 8056. If you like the triode sound, NuVistors would be a better sounding choice than the hearing aid tubes which never performed well in for quality audio.
There are lots of ways of getting heater current and plate supplies from batteries now, since small switching supplies are possible, just add a secondary for anode voltage on the high frequency transformer to get high voltage(with new ultra-fast diodes and low ESR higher voltage caps and 6.3 DC. I have a little switcher on my bench in front of me now, that could not be more than 4 inches long 1.5 wide and 1 in tall that could be modified to provide all that was needed to run an optimum tube in a well designed circuit if someone was interested in quality as opposed to just small.

Some commerical preamps run tubes on 9 volts or some other ridiculous anode potential. Takamine has one built into one of their acoustic guitars, as an option. The standard is an FET version. The FET version sounds much better as would be expected from 9volts on the plates of a 12AX7. As in most cases like this, it is a gimmick for sales rather than something for serious musicians.
One of the problems is the distortion created is not what people think, very high inter-modulation distortion that results from running in such a non-linear mode is not pretty sounding at all.

[Wynand]

. I have a little switcher on my bench in front of me now, that could not be more than 4 inches long 1.5 wide and 1 in tall that could be modified to provide all that was needed to run an optimum tube in a well designed circuit if someone was interested in quality as opposed to just small.

Some commerical preamps run tubes on 9 volts or some other ridiculous anode potential. Takamine has one built into one of their acoustic guitars, as an option. The standard is an FET version. The FET version sounds much better as would be expected from 9volts on the plates of a 12AX7. As in most cases like this, it is a gimmick for sales rather than something for serious musicians.
One of the problems is the distortion created is not what people think, very high inter-modulation distortion that results from running in such a non-linear mode is not pretty sounding at all.

So cooltube is a sales gimmick for non serious musicians. (like me) i gotta tell you, the sales gimmick works quite well on me.

tell me more about the " I have a little switcher on my bench in front of me now, that could not be more than 4 inches long 1.5 wide and 1 in tall that could be modified to provide all that was needed to run an optimum tube in a well designed circuit if someone was interested in quality as opposed to just small." Could you be so kind as to provide me with a schematic? ive finally given up the idea of using phantom power, so an external unit only makes sense now. im specifically looking for a preamp for acoustic guitars

[J M Fahey]

*If* the cool tube approach works for you, fine.
Just build a (working) SS circuit, your choice, mount a tube (your choice too, even a 6L6 if you wish), connect its filament in series with an amber Led to make it believable or a blue one to look cool and the extra series resistance so it does not run your battery down too fast and you are set up.
If you want Phantom power you can even use 5 of the 10 mA available for it, and the other 5mA to run the "real" circuit.
The Tube *will* run cool and definitely *look* cool too.

[Wynand]

*If* the cool tube approach works for you, fine.
Just build a (working) SS circuit, your choice, mount a tube (your choice too, even a 6L6 if you wish), connect its filament in series with an amber Led to make it believable or a blue one to look cool and the extra series resistance so it does not run your battery down too fast and you are set up.
If you want Phantom power you can even use 5 of the 10 mA available for it, and the other 5mA to run the "real" circuit.
The Tube *will* run cool and definitely *look* cool too.

wow, its getting hot in here, it seems like im rubbing you guys the wrong way. but thanx anyway, i see your point.

However, nobody seems to read the parts where i ask for a schematic on a good tube preamp for a EXTERNAL UNIT for ACOUSTIC guitars. if you feel like helping a beginner, PLEASE ANYONE help and guide me to a nice circuit. i am not capable of designing my own amp.