Tube Type PC Speaker Project

Using a single 70L7 to provide 1.8 watts of Class A output per channel in a compact speaker enclosure.

We have been told that this is a silly project, and asked why we'd do it when the power is only 2 watts. Well, the answer waits in repose for all of those technical-minded folks who use computers as well as vacuum tubes. Simply, the answer is this: "We did it because we can, and because it is cool". Stick that in your plastic pipe and smoke it!!

The 70L7 heater is supplied from the 120VAC line through a 369 ohm resistance. The current through the heater is 150mA, so therefore the value is enough should the nominal line voltage rise to 125.35VAC. The heater is placed so that it is near the B- end of the circuit, so that the heater-to-cathode voltage rating of the pentode section is not exceeded. The resistors in the heater circuit will dissipate 8.3 watts, so it is important to mount them away from sensitive components.

The repetitive peak current rating for the 70L7 rectifier section is 420mA, so values of resistance and capacitance must be chosen for the B+ filter that will prevent the charging peaks from exceeding this level. The values chosen also should cause the DC output to be close to the 110VDC operating plate voltage rating of the pentode section. Such calculations are tedious to do by pencil and paper, so we used Duncan Munroe's PSUD II program.

The values chosen provide about 40mV ripple at 114VDC for the plate supply.

Screen grid decoupling is provided by a 1000 ohm resistor and a 47uF capacitor, the combination of which has a time constant of 47mS.

Cathode bias is provided by a 175 ohm resistor, bypassed by a 470uF capacitor, for a time constant of 80mS. A 7 or 7.5 volt Zener diode could be substituted for the cathode bias resistor, and such a diode may be bypassed with a 10uF capacitor in parallel with an 0.01 uF capacitor to eliminate noise.

Safety measures are provided by the use of a grounding line cord, and a decoupling network between the neutral side of the line and the ground. Additionally, the B- connection is the neutral side of the line, and the use of a grounded cord forces the plug to be inserted with the correct polarity into the outlet. A fuse protects the circuitry in case of a short. A 1 amp fuse is shown, but a 0.5 amp fuse could probably be used, as cold start inrush at 125V is only 370mA. The input source is isolated from the "hot chassis" by T1 and T2, which have a high voltage isolation rating.

The speaker enclosure will need a few small holes in it in the bottom (or bottom sides) and the sides near the top. Ten 1/4" holes should be enough at the top, and again at the bottom, to pass sufficient air. The enclosure should be of die cast aluminum, as a kind of ruggedized speaker sold by Radio Shack is encased in, or of ABS or other high temperature plastic. The cabinet should be selected carefully due to the high temperatures present with tubes and the heater dropping resistor. A usual PC speaker cabinet is generally too cheaply made, although a large enough one might do well and not melt or deform.

T1 is a small audio isolation transformer of the kind used in telephone service. It is rated for 250VDC isolation, and has a 1:1 ratio at 600 ohms. Radio shack sells it as part # 273-1374. The frequency range is specified as 300Hz-5KHz, but it will do a bit better with no loading on the secondary except the grid of the 70L7.

The speaker output of most audio cards is easily capable of an 8V p-p signal (2.8V RMS), and so a 1:1 ratio is fine for the input transformer.

T2 is a commonly found "filament" transformer, with a 110VAC primary and a 6.3V secondary. A 1 amp transformer is shown and is the best size for this tube in this circuit. This kind of transformer also has good insulation and will provide isolation needed for safety. A 120V-to-12.6Vct transformer can also be used, by using just half the secondary winding.

The impedance match is provided by the voltage ratio of T2. The voltage ratio is 110 to 6.3, or 17.46:1. the impedance ratio is the square of the voltage ratio, and is 305:1. The tube needs a 2000 ohm load, and so a proper match is obtained with the filament transformer when a 6.6 ohm load is connected. An 8 ohm load placed here reflects an acceptable 2440 ohm load at the plate. A 4 ohm load reflects a 1220 ohm load and is much too low of a load impedance. A 4 ohm impedance would be well-matched to a 2000 ohm plate load impedance by a 5V (4.9V) secondary. Reasonably good performance will be achieved if the transformer has a power rating of 6-8 watts. Smaller transformers will begin to suffer from core magnetization of the DC plate current, and larger transformers will consume too much power from the audio signal. These effects are slight near the 5-8 watt range, but become more noticeable in transformers rated below 4 watts and above 10 watts.

Now is the point where objections are raised concerning the transformer selections. The objections would have more weight if the amplifier was driving a high fidelity speaker system. In fact, it is driving a 4" wide range speaker and a 1/2" dome tweeter in an 0.16 cu. ft. PC speaker enclosure. Argument closed.

Since the amp is transformer coupled on the input, Class A2 operation is possible. The grid dissipation figure is not given in the manual, but the output of the majority of PC sound cards is not enough to harm the tube. The amp can be made to provide quite a loud volume, if an efficient speaker is used. The kind of speaker design such as the square frame 4" type with a small magnet used in intercoms and small radios is very good and efficient, and will make the most of this amp. Look for an efficiency in the mid 90's -that is such as "94db SPL 1W/1M". The "midrange driver" type of hi-fi speakers with a 350-5000 Hz frequency response can be used as well. They will often perform well outside the range specified, especially in a good enclosure.