Build A Vacuum Tube Preamplifier





words and photo by: Telegraphy

  Rise your hand if you think digital sampling is superior in sound quality to analog vacuum tubes
..........anybody? Well for those of you that didn't rise your hands continue reading, all others lower your heads back down in front of your laptops, because today we're going to build a tube microphone pre-amplifier.

  Ah yes, the old debate amongst audio engineers and audiophile's. Does analog sound better than digital. The short answer (if you were to ask the almighty) - Yes it does sound better.  But why?

 Analog is defined as ": Something that is similar to something else in design, origin" which in our case of the "great debate" - analog vacuum tubes are replicating natures sound waves precisely as it is detected by our ear's. Digital on the other hand (keep your heads down laptop people) chops up the smooth natural sound into millions of steps. These steps makes our reproduction sound rough. Just as digital sampling uses "Solid State" devices to detect and reproduce sound. Though they are highly susceptible to distortion; "hollow State" on the other hand (which is referred to as tubes) are much more forgiving towards strong harsh sounds.

 The first vacuum tubes made (around 1902) were actually intended for radio reception and not audio amplification.  It wasn't until 1925 when the Western Electric Co. developed a complete electronic recording system, where by capturing, refining, and amplifying audio. This method employing tubes, revolutionized audio recording. For the first time in history, it was possible to tailor the quality of sound.
 
 Tube preamps are great pieces of gear to have in any home recording studio. There is no better option to obtain high quality audio from low level devices such as microphones and phonographs. So when I built a plate reverb (a low level device) I realized I was in a bind. "How am I going to get enough gain out of this plate reverb and yet still retain excellent audio quality", I thought to myself as I was driving to one of those big ticket item music stores (Ehheemm!! guitar center  ) I've always dreamed of having one of those expensive one or two tube mic preamps that retails for almost a THOUSAND DOLLARS!!! Seriously, if you can show me a capacitor or resistor that cost more then $20, I see no reason why these simple circuits with the minimal amount of components being used, should cost as much as a root canal. Anyways, I digress. Strolling through our un-named music store (hee-hee) I found a rack mounted mic preamp. Now my first though was, "Wow, a rack mounted piece of equipment"
I have a strong proclivity to when it comes to rack gear. It's neat and clean, always where you want it, and it just looks cool.
Looking at the very low, low price on the tag of this mic preamp ($50) How could I not resist. Being un-wise to spotting cheap equipment (They call that being young and stupid) my stupid behind bought myself a Behringer mic2200  Yes, some of you who are familiar with this so-called "tube preamp", can attest to it's crappy noise floor. In fact, others say it has a special knob which is marked "gain" but in reality varies the amount of noise gain. Powerhouse Mega from recordingreview.com forums stated that it only makes a good serving platter. And hey, it works great as a serving plate. My dinner parties turn out great after using the mic 2200 serving platter with auto gain noise knob. My friends are happier, drunker and I almost had a chance to have sex. Thanks Behringer mic2200.

Behringer mic2200 as a serving platter.
  So at this point I've decided to gut the insides of this skin crawling, abomination of a mic preamp and build my own using it's cabinet. So enough about me. Lets learn how to build a vacuum tube preamp. Shall we?
      
The following article is a step by step description of a home built vacuum tube  microphone preamp.
Lets examine the construction of a vacuum tube. As you can see from the cut-a-way view that it is physically hollow. Hence the term "Hollow State". Your basic run of the mill tube has two elements. 1) Cathode  2)Anode. This tube is called a "Diode". It's job in life is to allow current to pass one way and not the other. Much like those gates at the train station you pass though which are eerily similar to cattle fencing at a cattle farm. (It's a conspiracy man!!)  Between these two elements is a space or a vacuum. Tubes work on the principle of charged electron amplification. A small change in electron flow at one end of the tube effects a bigger change in charged electrons on the other end. This electron flow starts at the Cathode, flies through the vacuum and hits the Anode. Now in order for this effect to happen (The Edison Effect) the Cathode needs to be heated to the point where electrons fly off. A filament much like used in incandescent lamps is placed inside of the cathode to heat it up. This is what you see glowing inside of a tube and this is way it takes a few moments for the tube to turn on. Now we have flying electrons coming off of the cathode, it would be nice if we could put'em to good use. Like those holiday meal left-overs. You hate to just through them out - but you don't want to eat another piece of ham or turkey for another year........off load'em on your in-laws!





 The Anode or plate as it's called, is placed some distance from the cathode. Made out of metal or graphite, it is the final element in our tube. Electrons flying off of the cathode are attracted to the plate. The reason why these electrons are attracted to the plate is because of the higher voltage with respect to our cathode. We're talk'en high voltage folks. Nun of this 5 volt digital sissy pules (keep your heads down computer people) At least 200 volts !!

 Now that we have a conducting tube where electrons flow from the cathode to the anode, we can reproduce sounds from it by varying the flow. This is done by placing a third element in between the cathode and plate to act as a check valve. We shall call it - The Farnsworth Circular Modulated Multiplexed Coherer  Radiant Stimulator......just kidding folks, it's simply called a "Grid". It is used to control the electron flow and can be used to  modulate the plate voltage. In other words (our dubious digital dudes) a small signal voltage from a microphone impressed on the grid will vary the electron flow in the same manner and ultimately effect the bigger plate voltage (remember....BIG plate voltage).  Without the grid, we only can use this tube as a simple "current gate" or diode.

  Alright ! We know how a tube works. Now designing a accompanying circuit to make the tube work the way we want is a whole different ball game. Briefly put....

                                              Ionsonde Recordings presents..,..
                                          How to design a vacuum tube circuit.
                                             
( the analog deficient's version)

  This exclusive guide will teach the digital junky in four easy to follow steps, how to design and modify your pre-virtual reality analog vacuum tube microphone pre-amp. Learn how to....calculate bias, plate loading, cathode resistance and much MUCH MORE!!!  We're so confident that you will learn the art of designing a vacuum tube pre-amp's that we gave it a 30 day money back guarantee. If at anytime you feel that you haven't grasped the concept of analog reality, just simply send back your guide and 12ax7 tubes via particle beam transporter too...port 80 ip address 127.0.0.1    



Step 1) Choose your tube. The most common audio tube is the 12ax7. So we will use this one. Now look up the spec's for this tube and you will find a really, really scary looking plotted graph. Yes the  type you dreaded seeing first thing in the morning on the chalkboard in physics class. This graph is a analog computer (Do I have your attention computer people?). Believe it or not this graph, which is called a "load line graph" is able to calculate grid voltage vs plate voltage, grid and cathode bias residences. Basically you can design a complete circuit just by drawing a few straight lines on this ghastly of a graph.
     



Step 2) Choose a plate load resister value. A good starting value is around 100,000 ohms. We will use 150 k ohm. Using simple Ohm's law, we figure out the idle plate current. This means the amount of current draw of our tube at rest - No amplification. Calculations are.... now computer people can follow along using their Windows Calculator application. All others use your abacus or slide rule. 200 volts / 150,000 ohms = 1.33 milliamps. Hey! I see 1.33 milliamps on the right side of this graph! Hey!!! I see 200 volt down at the bottom......Well, draw a line connecting this two points. This is our load line of a 12AX7 tube. That wasn't so bad.     


Step 3) Now that we have formed a plate load line, we can use it to calculate values of other components or figure out different voltages and currents throughout our tube.  Most importantly we will calculate the cathode load resistance but first I have a secret to tell. I think you've noticed all of the animosity toward you computer people. Yes, I'm a little biased to when it comes to digital vs analog. And I'm sorry for all of the joke cracking. Us analog folks love your virtual ways (even though you can only count to 1). Yes I am biased. Biased to covering our next subject which is biasing our tube. It's alright, this will be just as easy as figuring out the loadline.

 Cathode bias - is the voltage present at the cathode which allows the tube to conduct and amplify in a set fashion. Picking any old biasing voltage from the plotted graph would work but our tube would run the chance of distorting our audio signal. We want the best audio quality. To do this, an amplification class known as "Class A" is to be used. Simply put "Class A" amplification in a tube has the best reproduction quality with the least amount of distortion. But, like all things in life there is a trade off. "Class A" isn't very efficient. Yes it's like driving a Bentley because you want to look good but when you want to race that annoying prick with his souped-up ghetto cruiser, blasting heavy base from their rattling trunk lid, your not going to do it! So we will sacrifice speed for class. For a 12AX7, a good all around cathode bias voltage (for Class A, don't forget) is 1.5 volts. Now back to our graph - look up the -1.5 volt curve and place a dot where the plate load line intersects. This is the biasing point. Now pay attention computer people. Draw or read across to the left hand side. You should read 0.5 mA. We have volt and now we have current. Both of these readings can be used just like our calculation of the plate load line. 1.5V/0.0005 A =  3000 ohms. This is the value of the resister off of the cathode. Draw a straight line from 0 volts and 0 mA through the bias point. This is our tube's cathode load line.

Step 4) PUT AWAY YOUR DAMN SLIDE RULER!! We are done. Now it's time to stop number crunching and start constructing.

 OK step back, take a breather. Drink some tea. That intense coarse study in analog computation is like a acid flash back to calculus class in high school. We're all done. Now it is time to have some fun with electronic components, wires and tubes.


The lone 12ax7 with
back lit LED's
Are you kidding me?
The design of this preamp is by Norman L. Koren  His "Spice preamp" is the design I choose for this project. Now, first thing on the list is build a power supply.

 Before I do that - a little bit more ranting about the Behringer mic2200 preamp. After discovering it's eccentricities, I opened up the cabinet only to find one (count'em one)12ax7 tube. I've also notice how the designers included a handy see through window on the front panel mounted just in front of the tube just to show off the fact that this is a "tube preamp". With only one resister and one capacitor on a printed circuit sub board where the tube is mounted, I was a bit concerned. Looking at the schematic diagram, I found a disturbing sight. The one 12ax7 tube in question, was only designed to be used as a cathode follower. What that means (computer people) that it doesn't amplify the audio signal what so ever. All it dose is transform a high impedances down to a low impedances. The amplifying is all done by cheap solid state op-amps. What a joke!


 Tubes as you know, require high voltages. To obtain several hundred volts means we need to build our own power supply. Running to your local radio shack isn't going to help or will it? Radio Shack offers an old school power transformer but it's output voltage is only 12 volts. That's OK, we can get around that. By using two of them connected back to back and using a few diodes and capacitors as "voltage doublers", we can obtain the required 250 volts.
                 
by: dogstar.dantimax.dk
Ripping out the Behringers power supply which only puts out 48 volts (way to low  to power a decent tube) I managed to fit my own built power supply in the space left by the old one. I mounted the two Radio Shack 12 volt power transformers, which are coincidentally enough the right height to fit perfectly inside of the chassis. A small perf board where all the components of the power supply are mounted on, is just to the left.

The old power supply
 Warning:  This supply is capable of producing lethal high voltages. So be cautious when building and testing this power supply. (DO NOT POKE OR PROD WITH A SCREW DRIVER or paper clips for that matter) We know how computer people love using paper clips to hack things.

My own power supply (note the two Radio Shack power transformers)







 "Telegraphy"? "I know how Optimus Prime transforms. Is this the same as the Radio Shack 12 volt transforms?"...........silly computer people..........

  No that isn't how it works. Think of it as a voltage multiplier and divider. Inside of our 12 volt transformer is two coils of wire wound around a central metal core. One coil connects to the house hold mains or 120 volts A.C. This is the input or "Primary winding". The second coil is the output or "Secondary winding". This secondary has a unique property where by however many winding's there are, determines the amount of voltage present. The more winding's - the more voltage. The least amount of winding's - the lower the output voltage will be. It steps up or down the house main voltage, all done by the ratio of primary winding's too secondary winding's. So the first 12 volt transformer; the primary connects to the 120 v house mains and we get 12 v on the secondary where it is sent to a circuit to convert it from 12 volts A.C  too 12 volts D.C.. The second transformer's secondary winding (remember - back to back) is connected to same secondary winding of the first transformer. So through ratio and proportions, the primary voltage of the second transformer goes back up to 120 volts. Hard to follow and strange in design but it works. If it's ugly or atypical and it works - just have faith in it.  So now we have 120 volts A.C. coming out of the primary winding of our second transformer. At this point we can not simply apply that to our tubes. Like a small child  tasting Castoria for the first time, our tubes won't take well to that. They need D.C. voltage. So the power supply is fitted with a circuit that will convert the 120 v A.C. to D.C. (no not the band!..go back to your onion routers) A few silicon diodes does this job perfectly. They only let the alternating current pass through one way to produce a pulsating D.C.. Next some capacitors are needed to filter out the bumps and ridges left by the diodes to turn it into pure D.C.. At this point we have 120 v D.C. Just about enough voltage required for our tubes but we want more power! The voltage can be doubled by using a special circuit called a "voltage doubler". This circuit uses a combination of silicon diodes and capacitors to double the voltage to a higher level. But of course with everything in electronics, there's a trade off. Sure the amount of voltage goes up but that the same time the current handling capability goes down. In other words, our power supply puts out 120 volts D.C. with 500 milliamp current handling capability before using the voltage doubler. After the voltage doubler, the voltage goes up to 240 volts but the current drops down to 250 milliamps.  This means we have to be careful how many tubes we can use with this power supply. 

  With 240 volts ready to use, we can now start on the tube amplifier itself.  As you can see from the schematic diagram below, the pre-amp I decided to build has quite a few components. Most of them are used has ether filtering out high frequencies generated by the pre-amp itself, or used as a feed back network to control the amount of amplification. But keep in mind, the basic theory I covered earlier on how to design a tube using a loadline is the same and can be seen here. You probably noticed by now that there is one input and one output - you've guessed it! This a mono pre-amp. So If you want stereo, you need to build two.



 
 Taking the grand tour of the pre-amp; our first stop(if you please direct your attention to the left, ladies and gentlemen)  is the phono input. R1s, R1g, and C1g make up a filter network, while R1gs is the grid stopping resistor. What is a "Grid Stop Resistor" you should ask? Well, Johny a grid stop resistor works in conjunction with the input filter network to stop any high frequencies parasitic oscillations. Ha, ha! In other words Johny, our tube likes to oscillate on it's own. The grid stop - stops it from self oscillating. As you can see connected to the bottom  and top of the first tube is the familiar cathode bias resistor R1c and plate load resistor R1p. C1m is a negative feed back capacitor. This also controls the amount of amplification and insures the tube doesn't break into self oscillation. (Now if you shall follow me over to the next capacitor. Watch your step, please) C1p is called a coupling  capacitor. It's job is to transfer the amplified audio energy from the first tube, over too the second tubes grid.....continuing on!.... The second tube operates just like the first, it just amplifies the signal even more. (And finally, we conclude our tour with) The third and final tube. This called a"Cathode Follower".  Transforming impedances is the name of it's game. It takes the high  impedance from the plate of the second tube and transforms it to a lower impedance without sacrificing gain. You might notice the output is coming out of the cathode instead of the plate. That's why it's called a cathode follower. The last components are feed back and filtering circuits. (Tours over - please visit our gift shop on your way out)


Point to point wiring.


The traditional way of constructing a tube circuit is to use point to point wiring on a tube socket.  By all means, it's a good time tested method of construction. For a stereo pre-amp your going to need three 12AX7 tubes. Each of them can be mounted horizontally, side by side.
Tube socket brackets 
Neatly mounted.
 To mount those tubes, I had to fabricate my own tube socket brackets. Made out of 1/8 inch thick aluminum sheet. One thing the Behringer engineers did do well is the back connector interface board. Well done!..........but I can't use it. So I ripped it out and reinstalled all the connectors to the back panel.

  Oh yes baby, we're at the home stretch. Now all needs to be done is connect the back connectors to the tubes. Install all three 12AX7's and plug in the A.C. cable for the initial "Smoke Test". Once again, do I have to explain everything computer people......hum. A smoke test is the first time a home built piece of equipment is powered up. If you find no burnt fuses or burnt components after the power switch is turned on - then it's a successful build. Oh, also if anytime during the test you DON'T see any smoke leaving the unit.....Thumbs up dude!!    

            

Un-soldering the connectors
Removing the back interface board.


Useless board removed. 


 As an added touch, I improved upon Behringer lackadaisical attempt to justify a working tube. It's back lighting with a solid state LED's was ridiculous. So I put in an old school incandescent  light bulb. Ah yes, a nice warm light to remind you of the warm sounds that comes out this pre-amp.
Kids, you might want to ask your parents what a incandescent light bulb is.
Real tubes light up.












  Let talk about tubes before I close out. Yes I know, I discussed tube theory up the wazoo earlier but I want to cover what type and where you can find tubes. Remember, you can't go wrong with N.O.S. (new old stock) tubes. They might be dirty, covered in dust or god knows what, but they work almost like the day they were born. I've had great success with these new fangled Russian made 12AX7's tubes. Their easily found on Ebay for as little as $15 each. Another thing to consider when purchasing tubes is when your project requires several of them, always buy "Matched Pairs". These are tubes that have been dynamically measured so that each matched pair conduct evenly between each other.
Well, hope you learned a thing or two about analog electronics. Hey it's fun and you get knocked on your @$$ with high voltage......Just kidding folk's. Be safe out there in the real "analog" world. And if I offended any computer people. I'M SORRY! Your just to easy in the analog world.
                 
Digital? Any idiot can count to "1"
                                              Bob Widlar



I like electron tubes!

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