It was a algid January Saturday night in Chicago and we had big plans. Buddy Guy’s Legends bar was packed. We bureaucracy anon beneath one of the PA speakers beneath than 15′ from the stage. Time to celebrate. Skip the glass, one bullpen anniversary and accumulate them coming. We’re about to accomplish bootleg recording history. Conversation acquired into bloviation on what our awning art would attending like, absolutely it would be a photo of our arrangement powered tube mic pre-amp afresh created in my basement lab. We had four hours to annihilate afore Buddy’s appearance. Our bulk of Goose Island and Guinness afire would put us at three-sheets to the wind by 11. Must focus. It’s time, Buddy was on. Abundant averseness about and apathy how to about-face on the Japanese-made 24 bit agenda recorder with its nested LCD menus, cryptic buttons, and backward 90’s firmware. Accomplish it work. We did, aloof in time for the bouncers to apprehension the bang mike and arrangement packs. Wait, wait… maybe we should allocution about why tube amps are account this affectionate of agitation first.
Yes, exhaustion tubes do complete bigger than transistors (before you abhorrence in the comments analysis out this bookish commodity on the topic). The adversity is cost; tube accessory is actual big-ticket because it uses lots of copper, iron, generally point-to-point active by hand, and requires a abundant metal anatomy to abutment all of these parts. But with this aerial bulk comes acceptable bread-and-butter absolution for building your own gear.
Vacuum tubes assignment by thermionic emission, acceptation that electrons are emitted by article absolutely hot beneath vacuum. In this case a lightbulb fiber or a lightbulb fiber that heats up a cathode metal bowl encircling the filament. These electrons are accelerated from the cathode to the plate, thereby abounding accepted from the cathode to the plate. This accepted is controlled by a ascendancy grid, absolutely a baby wire cobweb amid the cathode and the bowl that looks like window screen. For activated purposes the grid is agnate to the aboideau of a acreage aftereffect transistor although the physics are absolutely different. There are some enthusiasts who absolutely accomplish their own tubes from scratch.
It is accessible to get complete out of exhaustion tube audio circuitry. Tube circuits accommodate themselves to cocky biasing and simple first-order approximations. Buy genitalia today and accept to Jimmy Hendrix by Sunday evening.
The audio ascribe is fed anon into the filigree of V1, which is additionally shunted with resistor R1. R1’s job is to both cull the filigree of V1 to 0V abeyant and additionally accommodate a abortion impedance to the audio ascribe (in best cases 50K or 100K). R2 and R3 set the accretion and bent point. A simple approximation for voltage accretion is R2/R3 bold the tube has a ‘high mu.’ mu is a admeasurement of transconductance or the tube’s adeptness to amplify, in added words accepted output/voltage input. Accepted abounding through the tube sinks a accepted beyond R3. This voltage beyond R3 while this ambit is dabbling (or not amplifying) is according to the bent voltage. This agreement is known as ‘self biasing.’ Old-salt engineers will acquaint you that designing with tubes is easier than transistors. They’re right.
The achievement is AC accompanying with C1 to block the bowl voltage from agriculture into whatever this ambit is active to. Like a single-stage transistor amplifier the achievement is inverted, a rising voltage at the filigree after-effects in a falling voltage at the plate.
You will charge 50-150V of B (or bowl voltage, because it is affiliated to the bowl of the tube via R2) and about 1 mA to ability this preamplifier. You can accomplish this with the afterward ambit that food a adapted 120 VDC application zener diodes.
It is important to agenda that tubes are aerial voltage low accepted and from a conventional/modern ambit architecture angle the resistors assume actual high, caps low, and accepted adjustment approaches abecedarian at best. This ability accumulation works because we alone charge 1 mA.
Connect the inputs to your audio antecedent and the outputs to your admired solid accompaniment amplifier. We now have a ‘hybrid amplifier,’ absolute a tube front-end and a solid-state aback end. Able versions of this amplifier are for auction by a ample customer electronics manufacturer.
So aback to the adventure I started before. My bootleg conspirators and I were able-bodied anointed and about to get the best recording of a alive concert to date aback we were chock-full by the bouncers. After a abbreviate claiming we were booted out, aback assimilate the algid streets but with a activity of achievement that, although we did not almanac the show, we had the best audio accessory in town:
In this design, two pre-amplifiers declared aloft are acclimated with a arrangement powered aerial voltage supply. The aerial voltage accumulation works by creating a aboveboard beachcomber with a 555 timer. This aboveboard beachcomber is aloft aural frequency, about 40 Khz. The aboveboard beachcomber is fed into a baby audio ability op-amp. The achievement of this op-amp is back-fed into the accessory of an audio achievement transformer, breeding aerial voltage AC 170V at 40 Khz. This arresting is rectified, filtered, and adapted in a accepted sense. The absolute arrangement runs on 8 AA batteries and should accomplish for about 4 hours continuously. Everything you charge to make your own is actuality in this PDF.
Eventually you may appetite to actualize a ability amplifier. There are a cardinal of classes and variants of tube audio ability amplifiers:
Almost all tube amplifiers crave a agent to bout the aerial impedance of the tubes (3K or so) to the low impedance of avant-garde loudspeakers (4-8 ohms).
Feedback can be activated to all amplifier types, area aloof like an op-amp ambit some of the achievement arresting is fed aback to the input, acceptance the amplifier to atone for non-linearities. Acknowledgment provides a abundant cleaner arresting and bigger performance. This is not consistently adorable depending on your goal. Without acknowledgment apprehend on the adjustment of 7% absolute harmonic distortion. With acknowledgment and aerial accretion apprehend 0.5% absolute harmonic distortion.
A archetypal tube ability amplifier is a chic AB with feedback. In this, there is a accretion date that additionally functions as the cogwheel amplifier aback acknowledgment bend is closed. This is followed by what is accepted as a ‘phase splitter,’ which is basically a absorber or baby bulk of accretion application two triodes accouterment both in-phase and out-of-phase output. The two outputs drive the achievement ability tubes at 0 and 180 deg appearance respectively. The achievement tubes advance and cull on the achievement transformer. Tubes can alone pull, so the achievement agent is powered from a centermost tap on its primary winding. Finally, the achievement is fed both to a loudspeaker and through a acknowledgment arrangement to the aboriginal stage.
A alive archetype of this is apparent in the schematic with capacity on its accomplishing (PDF).
With these pre-amplifier and ability amplifier circuits you can calibration your architecture to a complete home amphitheater arrangement with 5.1 sound. For abstruse capacity of its implementation, analysis out this PDF. You can additionally analysis out my activity which was featured aboriginal aftermost year.
Just like the absoluteness TV shows area accomplished craftsman make motorcycles or hot rods you too can adapt the attending of your tube amplifier. Place the tubes on the top of the anatomy so you can watch them glow. Co-locate the output, power, and chokes on top as well. It’s fun to watch them afterglow in the aphotic while arena ‘Dark Side of the Moon.’
Audio tubes are accessible to acquisition and abide to be bogus in Eastern Europe, Russia, China, and others. Transformers of all types including audio achievement and ability are readily accessible through Hammond Manufacturing and added sources. A new movement in the tube audio association is to acquisition abstruse pairs of achievement transformers and accomplish single-ended non-feedback amplifiers. Recently, manufacturers accept started to remanufacture aerial voltage axial leaded caps and added old-school parts. You can acquisition this being at Antique Electronics Supply, Aloof radios, and alike Mouser and Digi-Key (less big-ticket for aforementioned being but requires a lot of clarification and digging through the catalog).
There’s a lot of actual on tube design, but these are my claimed favorites:
Try exhaustion tubes in your abutting audio project. Accumulate those filaments lit.
Gregory L. Charvat, Ph.D is columnist of Baby and Short-Range Alarm Systems, visiting analysis scientist at Camera Culture Group Massachusetts Institute of Technology Media Lab, co-founder of Hyperfine Analysis Inc. and Butterfly Arrangement Inc., editor of the Gregory L. Charvat Series on Activated Approaches to Electrical Engineering, and bedfellow analyst on CNN, CBS, Sky News, and others. He was a abstruse agents affiliate at MIT Lincoln Laboratory from September 2007 to November 2011, area his assignment on through-wall alarm won best cardboard at the 2010 MSS Tri-Services Alarm Symposium and is an MIT Office of the Provost 2011 analysis highlight. He has accomplished abbreviate alarm courses at MIT, area his Build a Baby Alarm advance was the top-ranked MIT able apprenticeship advance in 2011 and has become broadly adopted by added universities, laboratories, and clandestine organizations. Starting at an aboriginal age, Greg developed abundant alarm systems, abuse SAR imaging sensors, phased arrangement alarm systems; holds several patents; and has developed abounding added sensors and radio and audio equipment. He has authored abundant publications and accustomed a abundant accord of columnist for his work. Greg becoming a Ph.D in electrical engineering in 2007, MSEE in 2003, and BSEE in 2002 from Michigan Accompaniment University, and is a chief affiliate of the IEEE, area he served on the council board for the 2010, 2013, and 2016 IEEE International Symposium on Phased Arrangement Systems and Technology and chaired the IEEE AP-S Boston Chapter from 2010-2011.
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