Objective: A low wattage amp which still has enough muscle to get vintage speakers working to where they "bloom", and produce enough sound pressure to induce feedback; an amp in which the modes of non-linearity (preamp and phase inverter overdrive, power tube overdrive, cathode bias squish, power supply sag, output transformer saturation, etc.) are more or less activated simultaneously (onset-balanced), creating a highly complex envelope; an amplifier that morphs its character in response to its player's touch.
The input circuit is based around the EF86 pentode, and is very similar to the input circuit of the legendary four-input Vox AC30. After initial voltage amplification and the VOLUME control, the next stage is an ECC83 triode; encompassing this stage is a switch called FOCUS, which first rolls off low guitar frequencies (whatever can be played on the big E string), and then boosts everything above that same frequency range using a shelving filter. With the switch engaged, the component values are similar to what's found in a 100 Watt Marshall Superlead. With it disengaged, the low frequency range extends and flattens, and the overdrive takes on a less aggressive character.
After a treble-rolloff-style TONE control to tame and smooth the harmonics generated in the previous stage, the signal then sees the other triode of the ECC83 arranged as a cathodyne phase inverter, very similar to what can be found in a 5E3 Tweed Deluxe. The values are toned down by a hair, because the signal coming from the previous two stages has more voltage gain than a 5E3. The components on each side of this phase inverter are balanced (same resistance and capacitance values) but asymmetrical in their composition (carbon film vs carbon comp, and polyester film vs paper in oil), enhancing the generation of harmonics this stage is known for.
The mirrored signals then encounter a switchable voltage divider (OUTPUT RANGE "Master" switch), which allows the player to select the sensitivity of the output stage to the preamp signal (full or half). The output stage is a pair of Mullard EL33 pentodes (or GEC KT61s) in cathode-biased push-pull operation, for about 6-7 Watts RMS at low distortion, and 10 to maybe 12 Watts on peaks. These are extremely sensitive valves - the EL33 requires 6V on the control grid to be driven to full output, and the KT61 only requires 4.4V! For comparison, the EL84 requires 7.3V and the 6V6 wants 12.5V.
The power supply employs a GZ30 or 5Y3GT rectifier, and a robust pre-filter comprised of a huge military C-D 4uF paper in oil capacitor followed by a low value Dale 10 Watt wirewound resistor. This network gives a soft knee to the ramp-up of high voltage in the entire amp, which is kind on all the tubes and components, promoting long life; it offers some gross smoothing of the rectified pulsating DC by taking the brunt of the swing (so the electrolytics see a cleaner, attenuated wave); and it gives the amp a juicier, more organic response as the power supply becomes more reactive to the input signal.
The output tubes interface with the speaker load through a NOS 1950's Thordarson paper-interleaved output transformer, part number 26S59, 10000 Ohms P-P to 4-8-16 Ohms. Its nominal rating for power is 10 Watts - suitably sized to provide full transfer of clean signals, but introduce sweetening musical compression, harmonic generation, and frequency bandpassing on signals large enough to cause the output stage to become overdriven. A NOS 10 Amp Grayhill rotary switch is used to select output impedance.
Back panel removed
The wiring is milspec teflon jacketed silver plated copper in the power supply and heater circuits. Throughout the signal section I used rubber & cloth jacketed wire salvaged from two mid-20th century donors, a Sylvania oscilloscope and an RCA 16mm projection amplifier. Shielded wire (both 1- and 2-conductor) in the preamp is again silver-teflon and came from a Royal Canadian Navy/FAA oscillator. The jacks are all Switchcraft with long bushings and shoulder washers. The toggle switches are JBT USA in the signal section, and 1HP (yes, horsepower) Cutler-Hammer in the power section. Potentiometers are Canadian-made PEC RV4 2-Watt. The power transformer is a new Hammond for best reliability (they still make great trannies) and ideal choice of parameters.All components received teflon sleeving on exposed leads. The preamp is laid out using two parallel sets of heavy-duty terminal strips (milspec/Marantz style), and the larger components in the power section are mounted using silver-plated terminal posts with ceramic standoffs. All tube sockets are ceramic (Amphenol octals and ELCO nine-pin).
Chassis shot 1
I used a mixture of capacitors... in the B+ rail, the aforementioned Cornell-Dubilier PIO 4uF/600V is first in line in the power supply, then a 20uF/500V Sprague Atom to feed the plates through the OT, then a 2x16uF F&T can cap for the screens and preamp stages. Sprague Atoms bypass the cathodes of the EF86 and EL33s. A Wima metallized polyester is part of the FOCUS control adding some grind to the midrange, arranged as a switchable bypass on the first ECC83 gain stage. For signal caps, a mixture of Sprague Vitamin Q hermetically-sealed paper in oil, and Philips "Mustard" polyester film/foil are used. The treble rolloff cap is a Good All paper/mylar foil type, as seen in classic Marantz tube gear.
Chassis shot 2
Resistors: super-tough Dale wirewounds as surge limiters before the rectifier and throughout the B+ rail, and Sprague Koolohms on the power tube cathodes and screens (the screen Koolohms are non-inductive). NOS maroon-bodied carbon film resistors (as seen in the earliest Marshalls and many German tube radios of the early-mid 60's) are used in the preamp on the plates and the grid leak positions, as they are quiet and stable yet still very warm and detailed. All other positions in the preamp received Allen-Bradley RCR carbon comps (the established high-reliability version with 5th yellow band), with the exception of the input resistor, a Tektronix/Welwyn carbon film. The primary output jack incorporates a switching circuit with a tubular Dale high-wattage wirewound, which protects the tubes and OT should the speaker cable inadvertently be unplugged.
The chassis is 11-gauge Aluminum (.091")... I brought the raw sheet to Jack at Star Sheet Metals where he still uses a hand brake to bend it (I think he's in his eighties, he opened in 1953!). I used two sizes of lettering stamps to label it. Every stainless machine screw is mounted through a tapped (threaded) hole in the chassis, then secured on the other side using a stainless/nylon pozi-lok nut.
Chassis from above
The cabinet is constructed from pine, employing a hand-finished variation of the dovetail joint. The underside is vented with a perforated steel port to facilitate air flow (the chassis also has ventilation holes in key locations), and the bolts which secure the chassis from the underside are mounted through steel bars for added strength. Super-sized feet and edge-style chrome corners. German-made Marshall RI handle. Military knobs. Dialco lampholder with translucent blue jewel. The aluminum badge was drilled, filed, sanded and polished by hand.
Sound sample ...Junior and X into a pair of 1972 25W greenbacks in a closed-back pine cabinet... recorded with a B&O BM5 stereo ribbon mic, Ampex MX10 mixer, and Ampex 960.
Here's an instrumental recording I made with the X back in 2006 when I finished building it. All 1960's era mics, preamps, and tape machine.
More recently, I used the X on a cover version of Gordon Lightfoot's The Way I Feel
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