by Fred Nachbaur, Dogstar Music ©2001



It's been said that an amplifier is just a modulator for a power supply, so we'll have a look at this important circuit component first. The diagram below shows that power supplies don't have to be complicated, especially in low-power designs.

Schematic, Spunky Power supply
Schematic, Spunky's Power Supply

The filaments are powered directly from the 120 volt AC line, minimising the size of the transformer(s) required to drive the amplifier. Note the slightly unusual series-parallel arrangment; each pair of 35C5 output tube filaments is in series with one half of the 12AU7 dual triode. The center-tap of the 12AU7 sums the two branch currents (150 mA each) into the preamp tube, giving 300 mA as required for this valve's heater supply.

While unusual, this geometry is eminently stable and safe. If any tube is removed (or its filament burns out), none of the others will suffer as a result, unlike some series-wired television sets where a single burnout could take out several other tubes at the same time.

Each branch of two 35C5's drops 70 volts. Add in the 6V drop across each half of the 12AU7, and the 9V drop across the 9JW8 input tube, and we have 85 volts. The two dropping resistors R1 and R2 each drop 36 volts (150 mA x 240 ohms = 36 V), for a nominal line input voltage of 121 volts. Because of this dependance on 120 volt line voltage, this design is unfortunately not practical for 240 volt AC supplies.

Incidentally, the two dropping resistors get quite hot during normal operation. I suggest cementing them to the chassis, and placing a small heat-sink on the other side to help dissipate the generated heat. The heatsinks supplied with fans for 486 processors are perfect for this. (... just in case you wondered what the CPU heatsink was doing in the pictures!)

The B+ supply is very conventional, consisting of a small plate transformer (150 volts either side of center-tap), a full-wave rectifier, and typical CRC filtering networks. The idea here is to get approximately 165-200 volts DC at the first filter output (B1) at quiescent current (about 80 mA), with enough headroom for full-signal conditions (about 125 mA). At 165 volts, maximum output power will be on the order of 7 watts; at 200 volts you can expect as high as 9 watts.

A small (12 volts at 100 to 300 mA) separate transformer is used to feed a voltage-doubler circuit, which supplies the four variable bias outputs for the finals. The absence of regulation on either the B+ or bias lines is actually not a problem, since it in fact gives a certain degree of self-adjusting capability. If the power-line voltage sags, causing the B+ to decrease, the bias will also decrease, tending to increase the bias current and keep the plate dissipation relatively constant.


Here is a schematic of the main amplifier portion of our circuit:

Spunky - Main Amplifier

There's nothing really surprising here, either. Each pair of tubes has its plates and screens tied together, but the control grids are brought out separately to allow independant bias adjustment. The output transformer is a Hammond 125C "universal" 8 watt push-pull device, that sounds surprisingly good considering its small size and relatively low cost.

Note that there are 4700 pF capacitors (C35, C36) at each plate junction to ground, i.e. effectively in parallel with the output. This is to limit high-frequency response. In guitar amplifiers, excessive high-frequency is a bane rather than a boon, since typical electric guitar signals will rarely have much content higher than a few kilohertz anyway. Leaving the bandwidth too wide only invites instability (due to cable runs, proximity to other gear, etc.) and contributes to excessive hiss and other high-frequency artifacts. For this reason, you'll find bypass capacitors at every stage in this design, as with other well-thought-out guitar amplifier designs.

The 27 pF capacitors C30-C33 at the output valve grids are really not needed, but again I put them there as added insurance against ringing or outright oscillation. They're cheap, and can do no harm being there.

As in the "Li'l 4x4", the screens are supplied by a simple regulated supply consisting of 110V zener diode D6 and resistor R40. This keeps a relatively constant voltage on the screen grids, yet allows the voltage on them to sag in the event that screen current becomes excessive. C34 is a screen bypass capacitor, and should be a reasonable quality mylar or similar capacitor. Interestingly, unlike the 30A5/HL94, the 35C5 has a proclivity to oscillate if electrolytic screen bypass caps are used.

The small 10-ohm cathode resistors are provided to make it easy to measure cathode current, giving an excellent way of adjusting the control grid bias for each valve. The measured voltage will be ten times the cathode current; for instance, if you read 0.250 volts, the cathode current is the recommended 25 milliamperes (0.025 A). The grids are independantly adjustable for bias, as mentioned above, requiring separate input coupling caps at each valve.

The 35C5 grids are fed from a cathodyne phase-splitter, also called a "concertina" circuit, since the plate and cathode voltage push and pull in opposite directions, much like the little accordions for which the circuit is named. Given that the cathode current and the plate current in this 12AU7 triode section must be the same in class A operation, it follows that the output voltages will be equal but opposite under equal-load conditions. It should be noted that the concertina circuit does best if the plate and cathode load resistors are a good deal lower than the input resistance of the following stage - a requirement that is eminently met in this design (about an 11:1 ratio of effective grid resistance to plate/cathode load).

Finally, the input stage consisting of the other half of the 12AU7 is a pretty generic class A resistance-coupled amplifier. Again, C25 is there to limit high frequency response. The stage is cathode biased by R30, which is unbypassed to give some local current feedback to linearise the stage's response.

At the opposite end, the speaker output is switched via an integral switch in the headphone jack. Note that it is important in valve designs to provide a load at all times, which is why it is necessary to use a double-throw switch and resistor R43 (8 ohms, 10W). R41 and R42 are dropping resistors to the headphones, preventing excessive power from being delivered to the phones. Note that the use of two resistors and a stereo jack allows the use of either stereo or monaural headphones.

The output voltage is also applied to the switchable compressor circuit, described in the next section.


This is the schematic diagram of "Spunky's" pre-amplifier circuitry, completing the project:

Spunky preamplifier circuit

Again, the approach is direct and straightforward in the preamp circuit. Like the Li'l 4x4, a pentode stage is used for input, partly because of its high gain and partly because I just like the sound of small-signal pentodes. Another bonus is that pentodes (even the sharp-cutoff variety) lend themselves quite well to gain control by varying the grid bias -- a property we make use of in the compressor circuitry.

Pentode stages also exhibit a high output resistance, making it almost trivial to include a line in/out jack right at the output of the stage. This is also a good place to put the volume control, which in turn feeds a completely standard triode 2nd-amplifier stage. Again, both stages are cathode biased, with the cathode resistor unbypassed to provide local negative feedback. (It should be noted, however, that the amount of feedback in the pentode stage varies when the compressor is enabled and active.)

Once again, small capacitors are connected at every node, limiting high-frequency response. The result is an extraordinarily stable amplifier, albeit unsuitable for hi-fi applications where the sharp rolloff around 10 kHz would be considered inappropriate.

The compressor is very similar to the compressor option for the Li'L 4x4. The main difference is that component values are fixed, and compressor control reduced to a simple toggle switch. You might want to experiment with the values, though I doubt that your chosen values will be too far off from the ones given here.

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