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1- "MERCENARY FIELD AMPLIFIER" | by Andrew Pilling
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Output: 30 Watts per channel into 8 ohms / 40 Watts 16 ohms

THD: .4% (most likely better)



I have constructed this Audio Contraption to nestle right into my upstate N.Y. cabin retreat. This amplifier is aptly Nicknamed the "Mercenary Field Amplifier" loosely referencing old Military field equipment (not a cross field (radar) amplifier or Carver's marketing gimmick called the "magnetic field coil power amplifier"). This Amp is an integrated (OTL) amplifier in design which essentially is an independent Stereo Preamp plus (2) Mono Power Amplifiers all wrapped up and merged into one unified package. The Amp cabinet was constructed from recycled knotty pine tongue & groove paneling, aged copper; fashioned into edge trim. Rusty screws & some old luggage hardware.


Popular (OTL) Amplifiers such as Atma-phere's Music System's S-30, which has a patented Circlotronic, auto-biased, dual-mono, Balanced Differential, Direct Coupled amplifier Design, or Transcendent sounds product line of acclaimed (OTL) Amps both have successful track records (sales, quality & favorability). Their designs are with merit however they are not necessarily unique. Prominent elements of these designs can be traced back to the early 1950's, having evolved, cross pollinated and have morphed into many of the acoustically eclectic (OTL) flavors that we see today.


About My Mercenary field Amplifier:

There's an Ei-core trannie concealed inside of an old gutted-out Emerson electric motor casing; mounted on top of the amp cabinet that supplies the Hi current Mu follower preamp (6CG7's driving 12AU7 output tubes). Each Mono amp has (2) vigorous toroidals supplying the filament & B+ demands, in total; Juicing (4) 6SN7's that drive the (8) 6AS7G's. The remaining electronic components pretty much obey the law, however some may be arbitrary.


The Preamp features a Master Volume Knob, L-R Trim knobs, input select knob, Headphone output. An upward movement of a hand lever on the left side of the Mercenary Field Amplifier activates an industrial on / off button switch lighting up a jeweled lamp indicator for the Preamp. The Amplifier also has on / off - standby switches with jeweled lamp indicators and a quick check and adjustment via a built-in switchable (L-B-R) null meter with precision trim Pots helps to keep me within the Zero Zone. Testing and evaluation is still in progress, which is very promising considering that I'm only monitoring with 8 ohm speakers and that the amplifier circuit test results seem to be within an acceptable range.


I have incorporated (8) Brass cap & chain covered fuses to guard the 6AS7G output tubes, limiting current and to protect against tube failure and possible speaker blow-out. Speaker demise can happen from an intense sustained motor force of the voice coil hammering away on the speaker cone that will cause a mechanical blowout and lets not forget that a thermal short will most certainly leave you mono-fied; Your speaker mortified (Dead); Thus terminating one of Your Beloved twins.


Speakers for Listening to the Mercenary Field Amplifier:

While browsing around during a timely thrift shop trip, I "stumbled" across a package with an old UPS shipping label addressed: (William's Furniture Co. Grand Rapids MI) that contained a boxed pair of (RCA 16 ohm 15" Duo-Cone "HIGH-FIDELITY" Speakers). These Speakers were manufactured in one of the many (defunct) buildings at RCA's Dog factory in Camden N.J. This "FIND" was indeed "A Dream Come True" and an unexpected blessing. Subsequently I will construct a pair of cabinets for the RCA duo cone speakers that will match the amplifier cabinet using the old recycled pine paneling and copper trim that I have. The finished Speaker cabinets will sit on a pair of antique folding Tex-Mex hammock stools. Think & design outside the box.....


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Taken on January 25, 2012