Introduction: Hi Fi 50 Watt Tube Amp..... With BlueTooth.... Why in This Century?.....Really?

About: I am a life long tinkerer. Amateur radio license km4ywt. VP of engineering at S&TGeoTronics llc. Also have been known as a milwright.

A very limited release of this STG ClairAudio BluTube Mono amp is available for purchase in the ColumbusMakesIT Kickstarter October to November 2015 So if you would like to contribute to something much larger than all of us and help insure that no one is left behind, click the above link and follow your heart, thanks.

What is Hi Fi.....?

The experts will argue specs and numbers all day long and far into the night. Ultimately it is an idea or preconception based on perception itself. Often shaped by one's own person physical attributes and then skewed by experience and lastly personal preferences. I can no more tell you what sounds good to you than choose who you will love or your favorite color.

My definition is simple: A true to life reproduction of an audio experience as accurate as possible to the original. Much like a clear picture is much nicer than one that is out of focus.

Why Tubes......Really?

This being said, Why TUBES? Why in the 21st century?

This requires an understanding of audio media (the packaging of our precious music over time). Originally we had vinyl records, then magnetic tape, then digital tape and finally compact disk. At this point, music became a digital representation of an analogue audio experience. Like a digital image is compared to a photograph, it is limited by the size of the information in audio the bits/second like in the digital images: the total number of picture elements.

Some will argue that the new media lacks the noise and negative characteristics of bygone media. This is for the most part true and simply not the argument at hand. Rather the method that the DIGITAL SOURCE is thereby vectored back into music. This is the true argument.... The tube amp is but one simple answer to this decade's long argument. Just as a digital photo can only capture the smallest object that it has a picture element for, the same is true for digital music it can only capture the smallest event that it has a least significant bit for in the prescribed unit of time (samples per second)

It is a well-known fact that tube amps all tend to distort harmonically on the second harmonic. It is a function of the physical design of the tube. Since none of us can avoid the laws of physics, why not exploit them a little. The second harmonic distortion does not replace the missing innuendo that was too insignificant for digital to capture, however it creates a pseudo-event that pleasingly fools the ear and brain into believing that the event was indeed heard. Also, the heavy iron core output transformer is incapable of responding directly to bits of little significance so it then smoothly vectors the content according to its q factor. Lastly the feedback loop causes the tube amp to be able to hear the room it is in through the speaker itself and also through the microphonic of the tubes themselves. This allows the tube amp to modify its sound dynamics to fit the room.

I know this all sounds like a load of BULL!!! Explaining these fundamentals is far beyond the scope of this instructable. It does not matter how these things come to pass, just that they do. Let your own senses decide that somethings are sometimes beyond explanation. Thus I ask that you share in this illusion and feed your amp periodically some classic rock, they thrive on it, should you continue.

Step 1: Tube Selection

The number one factor above all others is tube selection!

For the purposes of simplicity, function, building an amp true to its lineage, and lastly it's an American legacy. I have chosen the historic 6L6GC. This tube is very appropriate, produced by Radio Corporation America in 1936. You may read more on Wikipedia

The 6L6 and its variants have dominated for nearly a century and are still made today in China, Russia, and Canada. I have chosen the SOVTEK 6L6WXT+. This Russian made tube is undoubtedly the pentacle of this tube's perfection to date. The amp may use 6L6, 6L6XXX, 6550, KT66 or KT88 tubes from new old stock or new manufacture with no modification. Just correctly adjust the bias voltage and enjoy. It is also possible to use the mighty KT120 and KT150, however the power and output transformers will have to be up-sized accordingly, also your wallet will be somewhat thinner and the neighbors will call the cops when you test the thing out.

So, not wanting to get some "new old stock" that is less than new, and not wanting to chance the quality of China, the Russians win this time. The test amp sounds nothing short of fantastic even though it is Mono. Solid bass, clear mids and treble that soars far beyond my ear's ability as my oscilloscope indicates.

For the pre-amp and driver stage, I have chosen the 6SN7GT, an often overlooked hi mu twin triode. This classic tube was replaced by the smaller 12AU7 which is similar electrically, just in a smaller package and nearly driving 6SN7 out of existence. For some reason, the military community of the world rejected the smaller tube and kept the larger 6SN7 alive. Most of the ones today are “mil spec” which ensures reliability and durability regardless of whose missile they were pointing down range. They are also a phenomenal pre-amp tube with up to 20 dB of gain and little noise. They are also big and bad enough (5 Watts of plate dissipation) to be used as a driver for multiple output tubes. The 6SN7 also makes a dandy head phone amp or can drive a small speaker all by itself. Try that 12XX7! So this design could be scaled to 200 watts or more by simply multiplying the number of output tubes and up-sizing the transformers, power supply diodes and caps. Again the Russians make great 6SN7's. You are welcome to read on about the 6SN7 at

Russian tubes...... Really?

Russian tubes, you say what is American about that? Well I spent many years as an American Soldier stationed in Germany. I find that fitting these Russian tubes, (that at one time may have been pointing a weapon at me) to a truly wonderful use is quite satisfying. After the wall came down (yes I was there) I found the "Bad Russians" are really pretty ordinary folk much like You and I. Nothing as sinister as we were led to believe for sure.

Step 2: The Design

Homage...... Recognition where it is due!

The Williamson.

This amplifier is influenced by two historic units. First, the Williamson: Designed in the late 30's, the historic implication is that “the Willie” although a British concoction, is the first well known true Hi Fi amp. The plans and schematics were published, so it was absolutely the first "DIY Hi Fi"! DIY is what Instructables are all about! Feel free to read about the Willie here: . That is the original publication describing the Willie. Most all modern tube amps in one way or another have been touched by the Williamson. There is much controversy over how good Willies were........ Again "experts touting ambiguity". I am certain that in its time, folks were floored and flabbergasted. Simply a must have for any top ten most significant amps of all time. The influence I am referring to is that the Willie uses a whole twin triode as a driver stage after the phase split. Some argue that this is excessive and unnecessary but it makes for a very robust device void of any chance of clipping in the drive stage. Also, it allows one to expand the output stage and drive several pairs of "power tubes" if necessary. This will allow some hacking room for someone wanting to up-scale and build a monster.

The Dynaco Mark II and III,

In the 50's and 60's, Dynaco offered these in kits and assembled/tested units. They fancied themselves as Willie inspired, however later “Dyno's” drove the outputs directly off of the inverter. The big advance was the Acrosound output transformer with built-in windings acting as grid traps. This greatly improved the linearity of the amp over a greater frequency bandwidth. It also improved the ability to deal with sudden transient changes in input. Also most Dynacos that I have seen, have a bias adjustment for each output tube. Some have a channel balance pot to ensure the same amplitude of signal coming from each side of the phase splitter so that both output tubes are driven evenly. You may read more about Dynaco here : .

Our own Special Sauce..........

Being the crazy individual that I am, I like to put my own controversial twist on everything. So here is the short list:

1: A better power supply; Bigger, badder, filter caps. No tube rectifier, no sagging voltages (diodes work just fine and do not change the dynamics of a tube amp but only to make it much more efficient). Separate resistor and filter cap for each stage, it is just the right thing to do.

2: Separate bias voltage power supply; (keeps the darn thing from running away even when it is cranked all the way up) allows for a separate pot and test point for each tube to verify proper idle current per tube. Best of all, does not rob total plate differential voltage from ground, producing more power to speaker and improved linearity over all. (1 pair 6L6GC 54 mA of plate * 2) = (108mA * 470Volts) = (50.76 Theoretical Watts * 98% efficient output transformer) = 49.7448 Theoretical WATTS to speaker! Pick up 1 Watt off of each grid trap, and Bamm! 51 and a half Watts. Out of a 6L6GC pair, who'd thunk it???

3: A modified feedback loop to account for some lossy losses; Tube amps are usually coupled by resistors and capacitors. Well, resistors resist by nature and that is a good predictable thing, On the other hand, capacitors are supposed to stop DC electricity cold and pass only AC, and they kinda do. Somewhere in my distant past, there was an electronics professor and Uncle Elmer in the shack just a telling me about "X of C". “Boy, you got to count for X of C”. In memory of Eric Erickson (Uncle E to all of us novices) and James F. Cornell, here it is:

Xc = 1/(2*pi*f*c). Xc is the resistance in Ohms, f is frequency in Hz and c is capacitance if Farads. So now, me being the great approximater autistic problem child with now many years of experience, I will roughly guess within 5% that these .47 mFd coupling caps are about like a 5K Ohm resistor at 60 Hz and 2.5K Ohm at 120 Hz, wait for it..........1.125K Ohm at 220Hz. Above 6K Hz, more like a dead short... Usually about 4 or five RC (resistive capacitance) couplings in a tube amp....... Yup, that explains it: a constant 9 dB loss of Bass at 60 Hz. Well, we used a 6SN7 (@ 20 dB gain) tube in lieu of a 12AX7 (10 dB gain), guess we can cut the first stage gain in half and put a .047 uF cap in-line to attenuate...... wait........ 8.3 dB of the base feedback. And a tiny 47 pF cap to short to ground the feedback through a 1 MOhm resistor so that the Treble overcomes the resistive lag of charging screens through a resistor (also a function of charging a cap through a resistor), BAM!!! The thing has a nice flat response, totally linear from 40Hz to far beyond what I can hear these days. The scope says all the way to 18K Hz, all within 1.3 dB totally inaudible unless you are blessed with golden ears that defy the limits of human perception.

Step 3: The Parts List.........Bits and Things


You are welcome to use other transformers. I feel that the Edcor transformers are very conservatively rated. Also that they are a superior product. However they are made to order and usually take six weeks to arrive after ordering. Parts Express has a Hammond version that arrives much faster. Much more expensive and they are not as pretty. The Edcor transformers have a very fine fit and finish and are spot on electrically.

Here are the links to Edcor.

1 each Power transformer:

1 each Output transformer:

1 each 5 Henry choke capable of at least 180 mA:

The Tubes (The brand and lineage of the tubes will play a big part in how your amp sounds)

The Sovtek tubes work well and sound fantastic, and are the most reasonably priced. You are welcome to choose any tube from the 6L6 family to the limit of your pocket book.

2 each 6L6WXT+ (matched pair preferable but not essential):

2 each Sovtek 6SN7GT :

Look at the parts list above as some of the other parts can be ordered at Tube Depot as well.

Filter Caps & Resistors and other bits and pieces:

Most of the list can be purchased from Mouser:

Some other pieces come from Parts Express:

If it can't be found anywhere else like the clarostat get it from MCM:


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You can get it here:

Aluminum Plate

I used the Diamond cut Aluminum plate from Tractor Supply because of its availability and character.

It can be found here:

Step 4: The Build! - Part 1 (The Layout)

Step 1: Cut the Aluminum plate to desired size. I used 12" x 12" and found this to be the ideal size for a Mono Block. It is exactly half of a 12" x 24" sheet (I used one half for the top and the other for the bottom) and exactly a quarter of a (24" x 24" sheet yielding 2 Mono Blocks). I used my trusty old 4" grinder with a cut-off wheel.

Step 2: Top Component layout. Using the above footprints, lay the hole pattern on the cut plate and mark off the hole centers with a punch.

Step 3: Using the appropriate hole saw (1 inch - 25mm), drill all the holes for your 5 Tube Sockets. The 4 Transformers just require a drill bit adequate for the screws you will use. I used a 1/2" bit for the wire holes.

Step 4: Take a round file and carefully de-bur all holes to prevent the wires from being cut and shorting (it might be a good idea to install a rubber grommet in each of the wire holes).

Step 5: Cut 2 pieces of wood 3/4" x 12" x 2" and screw them to the 2 outside edges so that the plate is lifted providing room for your underneath components during construction. I have found that 2 inches is pretty much the ideal height of the Amp.

Step 6: Screw down transformers and tube sockets to the top plate making sure of proper orientation.

Note: I used a Radio Shack Choke Transformer which is no longer available. The appropriate replacement Edcor is in the Bill of Material provided in the previous step. The tiny bias voltage transformer also came from Radio Shack and no longer is available. Replace it with this one at Mouser: (115 VAC to 12.6 VAC @500 mA)

Step 5: The Build! - Part 2 (Power Supply)

Step 1: Mount 2 5-Terminal solder strips in front of the power transformer using 6-32 x 1/2" screws.

Step 2: Take 3 1 Amp 1000 Volts Diodes and 4 100uF 100 Volts Caps and build the voltage multiplier circuit making sure to respect polarity of caps and diodes. (Note to self: electrolytic caps hooked up backwards tend to explode.)

Step 3: Connect the circuit to the 12.6 volt output leads of the small transformer and power up the tiny transformer with 115 to 120 v household current and verify that the voltage multiplier is yielding 51 or so volts. Don’t be alarmed if unloaded it is a little higher, when we load it up it will fall in line.

Step 4: Be sure to ground the positive output of this little supply as we will be using the negative output to bias the tubes.

Step 6: The Build! - Part 3 Power Supply High Voltage

Now. This is where "The Rubber Meets the Road"

Up until now it has been fun and games..... Now we are going to build a power supply capable of shocking the crap out of you, causing severe electric burns, or possible death by electrocution! Think about this for a while before continuing........

Step 1: Gather up 2ea 5 terminal solder lugs, 2ea 100 uF 500 V caps, and 2 ea 1K 1A diodes.

Step 2: Solder the two caps into one of the solder lug strips as shown above. This is the best way I have found to mount these pcb type caps into the lug assembly.

Step 7: The Build! - Part 3 Power Supply High Voltage (continued)

Make no mistake, read this again: Up until now it has been fun and games..... Now we are going to build a
power supply capable of shocking the crap out of you, causing severe electric burns, or possible death by electrocution! Think about this for a while before continuing........ At best a shock from this will be excruciatingly painful!

Step 3: Take the second 5 lug strip and temporarily mount it to the first with a screw and add the diodes as shown. Be mindful to observe the polarity of the diodes and caps. The white banded end of the diode indicates the cathode end. This needs to be hooked to the positive lead of both caps (only one cap is shown in photo). Diodes conduct electrons from the cathode end to the anode end thus positively charging the positive pole of the capacitor.

Step 4: Once the caps and diodes are soldered up, carefully remove the screw and mount the assembly on one of the power transformer mounting screws by adding a second nut to the stud. In the photo above, I mounted both but on separate studs. This is OK too but wasted some space and does not look as neat. (This is what I call an after taught). The first one had only one primary filter cap, and you can build that way too, but it benefits greatly from the second primary cap with less voltage sag under a full load.

Step 5: Connect one red lead from the power transformer to each diode anode end lug by soldering.

Step 6: Connect the power transformer black/red wire to the ground lug between the diodes with solder, also connect the ground lug on the caps at the same time.

Step 7: Connect the center three lugs on the cap strip together grounding both cap negative poles.

Step 8: Solder a jumper wire between both cap + lugs.

Step 9: Connect one lead from the inductor to one of the positive lugs.

Step 10: Build up another lug strip with one 100 uF 500 V cap, mount it to another power transformer or inductor mount stud and connect the other lead of the inductor to the positive lug.

Step 11: Connect all center lugs that have not been connected with hookup wire and solder together. This is the ground plane and must be solidly connected to ensure quiet operation of the amp.

Step 12: Now is a good time to find one of the 1 Meg Ohm resistors and solder it across the last installed cap. Making for sure that the negative pole is grounded. This resistor is responsible to bled off the high voltage after the unit is turned off. This is a safety device once the power is cut, it slowly discharges the high voltage.

Step 8: The Build! - Part 4 Output Section

The output tubes:

Step 1: Attach two 5 lug solder strips to the stud part of the tube socket with another nut so that each output tube has one.
Step 2: Connect the blue wire from the output transformer to pin 3 of the left output tube socket.

Step 3: Connect the white/blue wire of the output transformer to pin 4 of the left output tube socket.

Step 4: Connect the brown wire of the output transformer to the right output tube socket pin 3.

Step 5: Connect the white/brown wire of the output transformer to the right output tube socket pin 4.

Step 6: Connect one of the brown wires from the power transformer to the left output tube socket pin 2, also connect the right tube socket pin 2 to the left tube socket pin 2 with a jumper wire.

Step 7: Now would be a good time to also jumper both pins 2 to their respective ground lug on the terminal strips. This will help ensure Quiet operation.

Step 8: Connect the other brown wire of the power transformer to both output tube sockets on pin 7. In the same way. Do not ground the pin 7 side of this circuit. This will provide the 6.3 volts for the tube heaters.

Step 9: Grab 2 5 Ohm 3 watt resistors and connect them between pin 8 and ground on both output tube sockets.

Pictured are two 10 Ohm resistors in parallel it yields the same value as a 5 Ohm resistor just looks ugly.

Step 10: Install the two 10 K clarostats near the output tube sockets, also install the two female banana plug jacks near the two output tube sockets.

Step 11: Connect each banana jack to pin 8 of the output tube it is nearest. This will be a test point to set that tubes bias with the 10 K pot to set that tube's idle current very important!

Now things get interesting.

Step 12: Remembering the 50 something V power supply we made in part 2 lets go and get the negative feed and attach it to one end of both clarostats. With jumper wire of course.

Step 13: Now we will ground the other end of the clarostats with a 22K 1/4 watt resistor, to the center lug of each respective 5 lug solder strip.

Step 14: now would be a good time to mount an "orange drop" .47 uF 630 V film capacitor to each solder strip across two unused terminals.

Step 15: Choose one end of the newly installed caps to connect to pin five of the output tube with a 1 k ohm resistor. The other end of the cap will be the input for that respective tube.

Almost done with this little bit.

Step 16: Connect with jumper wire the wiper of the clarostat to the junction of the .47 cap and the 1K resistor.

Now is a good time to review everything to this point. Look at the instruction and the schematics and closely compare the actual circuit that you have built to them.

Step 9: Testing! - Part 1 First Basic Testing, Smoke? No Smoke, Good...

Now it is time........ time for some initial testing... just to verify a thing or two or three or four.

First things first. We are now going to start down a new path, we are going to apply very real household voltage to a homemade circuit for the first time. It is a reasonable consideration to prepare yourself a power cord that has an old fashioned 25 watt lamp (the one with a filament like Thomas E is so famous for) in series with the line supply side of the plug. The Edison lamp will act as a current limiting device. It is also a consideration to use an isolation transformer to separate yourself from the electric co. These may be found on eBay with regularity. Unfortunately, electric safety is somewhat beyond the scope of this instructable, on the other hand there is such a thing as due diligence. The lamp will limit the total current at 120 volts to 250mA. According to nfpa70e standards, 250mA is dangerous but unlikely to cause sudden instantaneous DEATH, just an extremely bone rattling electric shock that should smart.

It would also be a wise thing to be sure that there is a gfi interrupter in the circuit, be it the panel braker, the receptacle or one in a dropcord form available at Lowes or Home Depot and other fine retailers. This limits the power lost to ground (perhaps through you) to 5 mA or less (just a tingle).

Next things next.

Let's power up the primary winding of that tiny 12v transformer by attaching temporarily the primary leads to our special power cord and plugging it in...... wait for it no smoke, good, now let's check the voltage from the two 6L6 sockets on pin5 to ground for around 30 to 50 volts. This is "the C supply" that will bias our 6L6 tubes so that they will not run away and burn up. Adjust this voltage with the two clarostat pots to the max, should be > 40v. The pin should be negative opposed to ground (held below ground). It is a good thing to turn the pots for the max voltage.

This will keep things turned down during the rest of the initial tests. Eventually we will adjust this voltage for each tube to match the tube's individual needs.

If you got a small puff of smoke out of the transformer it is a sure sign that something went wrong. Get another transformer, replace it, and check the previous steps. No smoke but the lamp is very bright, not good either, re trace the previous steps and compare circuit to schematic and clear the fault.

If negative voltage appears on pin 5 for both 6L6 sockets, the lamp is dim or better yet unlit and you can adjust the bias voltage with the respective pots. Take a step back, breath deeply and take a bow, you are in good shape so far.

More next things next......

Let us disconnect the small transformer and connect our Edcor power transformer.


When we plug this baby in, it is expected that the lamp will light and then dim as the capacitors charge.

Now we will test both sockets of the 6L6 between pins 2 and 7 for 6.3V AC, may be as high as 6.8 with no load.

Pins 3 and 4 should read between 300 and 480 volts depending on the cord you prepared, and loss across the lamp. The dimmer the lamp, the more voltage.

If everything checks out, you have now passed the preliminary testing. Congrats!! If not, compare your circuit to the schematic and clear the fault.

Step 10: The Build Part 5, the Driver

Most common push-pull tube amps, drive the 2 output tubes directly from the phase splitter. In order to reduce the amount of noise generated by the amp, we are going to create our driver stage post phase split.

This will ensure that any noise generated by the driver stage will for the most part cancel itself out. Also, it makes the design scalable as it could easily drive 4, 6 or 8 output tubes instead of two, thus doubling, tripling or quadrupling output power. Of course, going that route, you will need a respectively larger power supply and output transformer.

That being said, onward,

Step 1: acquire from your parts R1 (5KOhm 5 Watt Resistor) and C4 (100 uF 500V Capacitor) and assemble them on a Terminal Lug (TB) strip as you did with C2. R1 goes between the positive post of C2 and C4.This will give us a newly filtered power supply for our driver stage. Mount it near the C3 assembly.

Step 2: Grab yourself another 5 lug strip and mount it on one of the screws holding the 6SN7 driver socket to the plate.

Step 3: Now, connect the positive terminal of C4 to one of the lugs that is not grounded on that strip, and on the same lug connect one leg of R13 and one leg of R14 (47KOhms 2 Watts each).

Step 4: The other end of R13 goes to Pin 5 of the driver tube socket.

Step 5: The other end of R14 goes to Pin 2 of the driver tube socket.

Step 6: Take 2 pieces of hook-up wire (6 inches) and twist them together nicely to make you a twisted pair. (twisted pairs tend to reject EMI noise).

Step 7: Connect one end of the twisted pair to the unconnected ends of C20 and C21. The other end of the twisted pair will go to the driver tube sockets Pin 2 and 5 with the 2 previously installed resistors.

Step 8: Now we need to get R10 a 470 Ohm Resistor 1/4 Watt and connect one leg of it to the ground lug of the TB strip and the other end to Pin 3 of the driver tube.

Step 9: Connect a jumper between Pin 3 and Pin 6 on the driver tube socket.

Step 10: Connect R11 (470 KOhm Resistor 1/4 Watt) between Pins 4 and 6 of the Driver Tube.

Step 11: Connect R12 (470 KOhm Resistor 1/4 Watt) between Pins 1 and 3 of the Driver Tube.

Step 12: Connect C9 (.47uF 600V Film Capacitor) between Pin 1 of Tube socket and unused lug of TB strip.

Step 13: Connect C10 (.47uF 600V Film Capacitor) between Pin 4 of Tube socket and unused lug of TB strip.

Step 14: Verify your work against the schematic above.

You should now have successfully constructed the Driver Circuit.

Step 11: The Build Part 6, the Phase Splitter

Step 1: Once again, it's time to build another power supply. Grab yourself another TB strip and mount C5 to it with the negative pin on the ground lug and the positive pin on another lug (C5 100uF 500V).

Step 2: Attach R2 (5KoHm 5Watt) to Between positive pins of C4 and C5. This will supply our phase splitter with freshly filtered cleaned DC.

Step 3: Select R15 and R16 (22KOhms 2 Watt Resistors) and give them a check with a good multimeter to ensure that they are within 1% of being the same value.

Modern resistors should be very close of each other, but in years past, it was necessary to match these 2 resistors.

Step 4: R15 is connected between Pin 2 of the phase-split pre-amp tube and the positive terminal of C5.

Step 5: R16 is connected between Pin 3 of the phase-split pre-amp tube and the Ground Lug.

Step 6: The capacitor C9 (described in the previous part), is connected via a twisted pair to Pin 2 of the Phase Split tube.

Step 7: The capacitor C10 (described in the previous part), is connected via a twisted pair to Pin 3 of the Phase Split tube.

Step 8: Connect C11 (.47uF 600V) between Pin 1 amd Pin 5 of the Phase Split Tube.

Step 9: Verify your work against the schematic above.
You should now have successfully constructed the Phase Splitter Circuit.

Step 12: The Build Part 7, the Pre-Amp

Step 1: Time to construct yet another power supply. So, grab up C6 (100uF 500V), and R3 (5KOhm 5 Watt) and attach them to a new TB Strip. You should be getting good at this by now! (or really really frustrated ...Sorry!)

Step 2: Ensure that R3 is connected between Positive terminals of C5 and C6.

Step 3: Select from the remaining parts R18 (47KOhm 1 Watt) and connect it between an unused terminal lug and Pin 5 of the Pre-Amp socket

Step 4: Connect R17 (470Ohm 1/4Watt) between Pin 6 and Ground.

Step 5: Get R19 (1MOhm 1/4Watt) soldered between Tube Pin 4 and Ground.

Step 6: Solder C12 (.47 uF 600V) between Pin 4 and an unused Lug.

Step 7: Verify your work against the schematic above.
You should now have successfully constructed the Pre-Amp Circuit.

Step 13: Testing! - Part 2 Mounting and Decisions.....

Getting very close to hearing some audio...

Now is a good time to build the front and back of the amplifier. We used a piece of 18 ga sheet metal and we cut it 3 inches x 12 inches and folded it along the long edge so it would have a 1/2 inch lip on top and bottom. We then attached it with sheet metal screws to the top and bottom of the amp. We made an identical piece for the back.

The back piece will need some holes in it: A rectangular hole for the power cord receptacle (1.1 inch X .85 inch) with a screw hole on either side (5/32 inch) to accept some #6 mounting screws.

One hole near the power cord socket should be for the fuse holder. I used a 7/16 inch bit and wallowed the hole a bit to accomodate the threads of the fuse holder.

On the other end of the back plate you will need to install some binding posts for the speakers. Two will work, however four will allow you to use all of the impedence taps of the transformer, it's up to you.

The front panel wil need two holes. One to accept the power switch and one to accept the volume control pot. The volume control is optional, should you choose to simply add an RCA input jack to the back for an input and use it as a simple amp.

Wire up the plug adaptor. The E terminal to ground, the L terminal to the fuse, the fuse to the power switch, the power switch to one side of both transformer primaries and the other side of the primaries to the N terminal of the plug adaptor. Yes, the plug adaptor terminals are marked. Yes, it makes a safety difference and may also affect the amount of noise generated. Yes, use a three prong cord. Yes, use a grounded outlet. Be Safe!

We are now ready, almost, to add power. Let us proceed and add the tubes, go ahead and plug them in to their respective sockets. The first time may be a little stiff, a 6L6GC in each output tube socket, a 6SN7GT for the driver and another for the phase split and pre amp tube.

Right now the input impedence is controlled by R19 at 1MOhm. This is a little high if you are intending to use this as a simple line amp. If this is the case, replace it with one that is closer to the actual impedence of the device you will be using to provide signal to the amp. For the initial test and bias setup it is best to hold pin 5 of the pre amp tube at ground with an alligator clip equipped jumper wire as we will want to set the bias at a zero signal.

Make sure that the speaker binding posts are connected to the secondary taps of the output transformer and connect a speaker of the appropriate impedence to them.

Now it is time to say a little prayer that all is right (Deity of choice), if not available, a good stiff drink will do...

I have seen enough blood shed over the gods, I prefer to recognize my fallen brothers in arms (true heros).

At this point we must plug it in. Use of an isolation transformer is advisable and maybe a current limiter, though at this point we do have the 3 amp fuse in the fuse holder, I hope.

Throw the switch............ Wait for it..............No smoke, tubes glowing nicely........Good! (Bon - for the French...)

Step 14: Testing - Part 3 Setting the BIAS

By now, your amp should be taking form Quite nicely.

We will now set the bias...... But first a word about bias....

The bias is very important. It is the setting of the idle current of the output tubes. This impacts the quality and quantity of power available at the output terminals of the amp, also it will have a direct bearing on the life expectancy of the tubes.

In this configuration, the 6L6GC tube will accept a maximum bias of 52 mA to the plate. I find that I bias a little on the conservative side and shoot for 40 mA. The tubes seem to work as high as 60 mA but at this level, their life will be considerably truncated. In a nut shell, 50 or so mA bias will render several thousand hours of run and brilliant performance, 60 mA will yield a grittier sound and a substantial boost in output. You might luck up and get a thousand hours but keep a spare pair of tubes handy. ~40 mA gives good performance, 38 watts or so to the speaker and tens of thousands of hours of musical bliss.

Under the cathode of each output tube, we have a 10 ohm resistor. The cathode is also connected to a respective test point. We will be measuring the voltage across the 10 ohm resistor between the test point and case ground. So go ahead and hook your meeter up to read DC Volts and test from tp (testpoint) to case ground...... Adjust the respective trim pot to get .42 volts or .5 for each tube. I would start out at .42. As the tubes burn in, the conductivity usually increases slightly over the first couple of hundred hours of operation. Get them as close to equal as humanly possible.

Now remove the power, unhook the R19 jumper and hook a signal source such as an MP3 player across R19 and start everything up and enjoy some music for a few.

After several hours of inital run, it will be necessary to adjust the bias again as the conductivity of the tubes may change over the first 500 hrs or so of operation. As the amp accumulates more run time, the tubes will settle down for the long haul. Toward the end of tube life, again it will become necessary to do more frequent bias adjustment to compensate for excessive conductivity or the lack of conductivity. When a reasonable bias can no longer be maintained, the respective tube's life is over and it must be replaced.

While it is desirable to have matched tubes, because of independent biasing, it is not nessesary. In fact, tubes of different ages or manufacturers and even different types may be used together so long as they bias the same. This practice will however affect the sound quality unpredictably, none the less is posible in a pinch.

Step 15: The Build - Part 8 Wrapping Things Up!

Again we find ourselves soldering.

1: Grab up the volume control pot (10K to 100K linear taper 1/4 watt). Connect a couple of leads to it. I prefer to use coax (old stereo wires work well for this). A twisted pair will also do, but at this point will be subject to some E.M.I. noise.

2: Mount the pot in the front panel

3: Install R20 4.7KOhm between C12 and the wiper coax of the newly installed pot, connecting the shield to ground.

4: Connect the other coax to the "BluTube" socket: shield to pin 1. Conductor to pins 4 and 6 (pins 4 and 6 are left and right outputs in the stereo application, here we are mono)

5: If desired, you can also connect another coax to a RCA connector or 1/4 inch jack on the back panel.

6: If you so desire, you may also install a selector switch in this circuit of your choice (beyond the current scope).

7: Build out the little 5V DC supply as shown in the schematic. You may attach the bridge rectifier to the top plate with a screw or build it on a tb strip, the choice is yours.

8: Connect the new power supply to the BluTube socket, positive to pin 2 and ground to pin 1.

9: Ground pin 5 of the BluTube socket.

10: For better bass and treble response and a slight noise reduction, install C14 R21 C13 and R20 as shown in schematic. This is a feedback loop, it will slightly boost the bass and treble as to make up for a 5 dB loss at 60 Hz and below, due to the capacitive couplings.

11: You may go to your local trophy shop and get them to design a front panel for your amp with the volume label and tick marks, also, don't forget on and off for the power switch. I prefer just the look of a little thin mahogany or sapele trim. We also may try some in cherry.

12 : Attach the bottom plate with several short machine screws and enjoy for years to come.

Step 16: The Future!!!!

A very limited release of this STG ClairAudio BluTube Mono amp will be available for purchase in the ColumbusMakesIT Kickstarter October to November 2015

So if you would like to contribute to something much larger than all of us and help insure that no one is left behind, click the above link and follow your heart, thanks.

Keep an eye on

Your source for ready made ClairAudio BluTubes , PCB for this amp, and kits to build this amp. All coming in the very near future.

Always keep in touch with Instructables as eventually we may also release the stereo version pictured above as an instructable.

You may find other STG projects on Instructables such as

And be sure to keep looking to kickstarter for the release of the STG ClairAudio BluTube Stereo

with aux input, 1/8 inch jack, BluTube, and phono inputs.

Thanks STG Crew Marc, James, The ColumbusMakesIT Gang

Make IT Love IT Live IT

If you want something better...... MAKE SOMETHING BETTER!