The Li'l 4x4 Logo

Li'l 4x4 in cabinet
THE "LI'L 4x4"
by Fred Nachbaur, Dogstar Music ©2001


This article documents my quest for a high-quality general-purpose amplifier, using inexpensive parts. My need was for a clean, accurate instrument (guitar) amplifier, and the resulting unit suits this need to a tee. I dubbed it "Li'l 4x4", since it is a four-stage amplifier with an output of a solid 4 watts before clipping.

Before we get into the nuts and bolts of this new design, let's take a little walk down memory lane and discuss the history behind the tubes and other parts used in the circuit.


Prior to the 1940's, home electronics was a luxury only affordable by the relatively well-to-do. Radio sets were all constructed completely by hand, and even those who could afford them spent careful hours researching. The arrival of a radio set in the household was easily on a par with purchasing a new car nowadays; perhaps even more significant in some ways. Those who did purchase a radio or phonograph often found themselves suddenly quite popular in their neighborhoods, with friends and neighbors stopping by to socialise and listen.

After WWII, demand for radios and phonographs spurred manufacturers into finding ways of providing them at costs affordable by the average household. Tubes and other parts were mass-produced, and engineers found ways of cutting corners -- and thereby costs -- to produce units that were relatively compact and affordable.

One of most expensive parts was the power transformer. It was also heavy and bulky, making it difficult to produce electronics that appealed to a growing public interest in miniaturisation and efficiency. With this in mind, designers found ways of manufacturing tubes that would operate well directly using the relatively low (by tube standards) home power line voltage - typically 110-120 volts. Another factor was that there were still some regions powered by the otherwise obsolete DC power grid system developed by Edison; these new radio designs would operate from either type of supply, hence "AC/DC".

Manufacturers started churning out table-top radios using this new tube technology. Filaments were wired in series, powered directly from the line, sometimes with additional dropping resistors (or even line cord with a third resistance wire) as required. Plate and screen supplies were similarly derived directly from the line voltage. Before long, a great many models hit the market with essentially the same basic circuit, which came to be known as "The All-American Five" because of the five octal tubes it utilised. These were:

12SA7 (a pentagrid converter, or heptode, combining oscillator and mixer functions)
12SK7 (pentode, IF amplifier)
12SQ7 (triode and dual diode, used for detection and first audio amplifier)
50L6 (beam pentode, power output)
35Z5 (half-wave rectifier for B+ supply)

The filament voltages of these tubes, when added up, came to 121 volts, which meant that no additional dropping resistor was required. Because of the directly line-operated power supplies, the ground return for the power supply was directly connected to one side of the power line. Since there is a 50/50 chance of having the chassis "hot" (this was before the days of polarised power plugs), the chassis was isolated from user contact by being placed in a non-conductive cabinet, with non-conductive knobs. Even so, many models appeared with "phono" inputs, allowing the connection of a turntable outfitted with a crystal or ceramic cartridge. I'm perhaps betraying my age by reporting that there were many occasions when I'd get a "tickle" from the phonograph, necessitating reversing the power plug.

By the late fifties, the new miniature 7- and 9-pin tubes appeared, allowing radios to be made even smaller and cheaper. The typical lineup in the miniature version of the "All-American Five" was as follows:

12BE6 (pentagrid converter)
12BA6 (pentode, IF amplifier)
12AV6 (triode and dual diode, used for detection and first audio amplifier)
50C5 (beam pentode, power output)
35W4 (half-wave rectifier for B+ supply)
Here is a typical application of this idea (from the RCA Receiving Tube Manual):

The All-American Five
The "All-American Five" circuit using miniature tubes

The output tube, 50C5, was capable of a maximum power output of 2.3 watts, though this maximum was rarely achieved in real life. Since, as a single-ended circuit, it had to run in Class A operation, it typically dissipated about 5 watts on the plate, 1 watt on the screen, and 7.5 watts on the heater, for a total dissipation of over 13 watts in a miniature tube. These things got hot! Still, they held up very well. There's a machine shop in a town near where I live, that has a little table radio that's been running continuously since the 60's, and is, to my knowledge, still on its original set of tubes!

A few manufacturers came up with push-pull outputs, using the 50C5's "cousin", the 35C5. This tube really should have been given a new number, since it is not identical with the 50C5 in other ways than filament voltage. Its filament current is 150 mA, just like the rest of the tubes used for series heater operation, so with the lower design voltage it dissipated only 5.25 watts. Maximum plate dissipation is similarly somewhat lower (5.2 watts instead of 7) and maximum output power is given by the manufacturer as 1.5 watts (as compared to 2.3). As for the 50C5, the maximum output power rating is a bit optimistic, and 2 watts was all one could typically expect from a push-pull pair.

The fly in the ointment, of course, is that two 35C5's plus the other four tubes come up with a filament voltage of over 140 volts. One solution was to use an early solid-state rectifier called a "selenium rectifier" instead of the 35W4, and take up the 15 volts excess using a 100 ohm dropping resistor. Even later, the much cooler-running (and smaller) "top-hat" silicon rectifiers made their appearance, and were called into service for power supply rectification. Similarly, the push-pull design required an additional amplifier stage for the phase inverter, so the dual diode built into the 12AV6 was lost. For radios, a germanium diode was used instead.

Push-pull 35C5's (or even 50C5's) were also commonly found in phonographs amplifiers. These were essentially stripped-down radio circuits, sans oscillator/mixer and IF strip. I don't have a picture of a radio to show you, but I do have a couple shots of a phono amplifier that uses two 35C5's, a 35W4 rectifier, and a 12AX7 dual triode. I'm thankful to Col. Larry Daniel for providing this, as it makes an excellent "benchmark" against which to compare my new design.

The Lafayette KT-92

The Lafayette KT-92 phono/tuner amplifier kit, ca. 1959

This was provided by Lafayette Radio as a kit, and appears in their 1959 catalog. The unit I acquired still works after all these years, requiring only replacement of the filter capacitors and a couple off-value resistors to bring it back to life, managing to (barely) squeeze out 2 watts of power on the test bench. It was intended primarily as a phono amplifier, though the input jack says "Radio/Phono", so it's quite conceivable that Lafayette also offered a radio tuner kit that could be plugged into this amplifier.

The KT-92 Underside

The Lafayette KT-92, wiring side

Here's the underside of the copper-plated steel chassis, showing the typical wiring technique of the day, employing tube socket pins for connections whenever possible, and terminal strips for everything else. It sports "tone controls", which by today's standards are almost a source of amusement; the "Treble" control is merely a high-pass filter at the input, and the "Bass" control (more properly this would be termed a "high-cut" control) is a simple high-pass filter in the global feedback network.

This unit actually had a design error*; the on/off switch was placed in the ground return rail of the AC line. This meant that if you set the power plug to have the ground return "cold" (i.e. connected to neutral) when the switch was on, the ground would ride high (hot) when shutting it off, due to the continuity of the tube filaments. This error has been corrected in the schematic diagram below, also repeated later in the article for easy reference. [* Aug. 2001: see footnote below].

If you do decide to build a clone of this 1959 Lafayette circuit, either for hobby historical purposes or as a benchmark for your own designs, please use an isolation transformer. At the very least, use a polarised power plug (the wider prong is the ground return). Still, you'll have the devil's own time when interfacing with other, power-ground-referenced gear because of hum. This is due to the fact that there will usually be a slight AC voltage drop on your house's neutral line, caused by currents flowing to lights and applicances in the house. This voltage is effectively added in series with your input signal, making hum avoidance next to impossible. So, use an isolation transformer and save yourself a lot of grief, not to mention injury or death.

*FOOTNOTE: Dave Brooks offers this explanation for the apparent wrong polarity on the AC line: "On the page describing the Lafayette push-pull amplifier kit, you describe as an "error" the placement of the on/off switch in the chassis AC line. Back in the 1960's, I recall reading an article about the All American 5 design, which (in that instance) did the same thing. The author explained that this was no error. Given the AC line enters by the rear of the chassis, and the switch (on the volume control) is at the front, there's necessarily a length of AC line traversing the width of the chassis. By putting the switch in the chassis line, this length of AC wire is at ground potential when the set is on, so can't radiate hum. The hot AC wire is very short, going direct to the HT rectifier and the start of the heater string."

The Lafayette KT-92 Schematic

Schematic of the KT92 phono amplifier.
Note that this is an inverting amplifier, phase at speaker is reversed from input phase.

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