An 8 -Watt PA System

Here is a complete PA system that can be built at comparatively low cost. It is ideal for PA work at dances, for orchestras, and singing, and all general ballyhoo work.

The system was designed as a low-cost amplifier with an 8- to 10-watt output, and that would give excellent fidelity.

The amplifier was designed to be very simple to build. Resistance coupling is used throughout. This enables one to obtain excellent fidelity and gain, without the use and expense of interstage transformers.

A 6J7 high-gain pentode stage is used as a microphone pre-amplifier. This has a high-impedance input and will take almost any level of high-impedance microphone. Because of the high gain of this stage, hum may be picked up in both the plate and grid circuits; for this reason both circuits should be well shielded. A bias cell is used instead of cathode bias; this design insures hum-free performance.

The microphone stage mixes into a 6SF5 along with the high-impedance phonograph channel. The tone control, phono and mike input jacks, and associated gain controls are mounted conveniently on the amplifier chassis There is also another input circuit, mixing into the second grid of the 6SF5; this comes from the phonograph input.

These three channels can be mixed simultaneously with absolutely no interaction. This is due to the 500,000-ohm resistors 'in series with the volume controls that always keep the grid above ground.

These Mixing stages of  resistance-coupled to the final power output stage. A 6L6-G beam power tube is used in this stage because of its high power sensitmty high power output for relatively small inputs, and high efficiency. It has very low third and negligible higher harmonic distortion.

A very simple method of inverse feedback is employed. A portion of he Plate voltage (determined by the value of feedback resistor R14), is tapped off the 6L6-G plate and fed back to the grid. This feedback voltage is out of phase with the input voltage and tends to cancel out distortion generated m the plate circuit of the 6L6-G. This makes the distortion in he output circuit of the 6L6-G comparatively low. The output is slightly lowered by this arrangement but may be brought up again by applying more driving voltages

The output transformer has secondary taps at 2, 4, 8 and 500 ohms lhese taps are brought out to two speaker plugs in parallel on the back of the amplifier, so that by selecting the proper terminals, the proper impedance values can be chosen for the speakers.

The 500-ohm line is also brought out to two banana plugs in the rear. This facilitates measurements on the amplifier under actual operating conditions. The 8-ohm tap is brought out to a terminal strip marked OUTPUT. This is a convenience terminal for an auxiliary speaker.

Two 12-inch PM speakers are used. The speakers have voice-coil impedances of 6 to 8 ohms and will handle 10 to 15 watts. Both speakers are wired with 50 feet of rubber-covered lamp cord. Thus they may be put in convenient places to get maximum coverage of a given location and also to prevent feedback to the microphone. The speakers are con¬nected in parallel across the 4-ohm tap on the output transformer. The loss in running the voice-coil wires through that length of speaker cable is less than 1 db.

The speaker cabinets were built from one-half inch plywood. This design gives better bass response than that of a closed cabinet. The cabinet is cut diagonally through the center and the speakers set in so that it is easy to carry them about. The cabinets are fastened together with suitcase clasps and carried by the handle that is shown on top of one of them. The speaker inclosures are shown partially open so that the reader can see the construction to better advantage. The speakers are first mounted on a piece of sound-absorbing material such as celotex, in which a hole is cut to fit the size of the speakers. The speaker, on the celotex, is then mounted on the sounding board. The sounding board i, placed in the cabinet and secured by screwing to corner blocks put there for that purpose. The celotex baffles close up the leaks around the corners of the speaker openings, thereby making the chamber more efficient.

The cabinets are nailed and glued together, stained and shellacked. After rubbing down with steel wool, a final coat of varnish is put on them to give a hard and lasting finish. Rubber feet help to keep the cabinets from becoming marred. All necessary dimensions are shown in the accompanying sketch. The final touch is to cover the speaker and release openings with a piece of grille cloth. Wire screening may also be used for this purpose but will result in a loss of high frequency response. With a little care in designing and building the speaker cabinets, not only will surprisingly excellent response result, but the cabinet will be a nice looking piece of furniture.

In the power supply an 83-V tube was used because of its excellent voltage regulation characteristics. This is due to the close spacing between the cathode and the plate.

Common procedures should be followed in building the amplifier. Filaments should be wired with leads twisted, to cut down the electromagnetic fields around them, thereby reducing chances of hum pickup. All large parts, transformers, condensers, volume controls, etc., should be mounted first.  Small parts such as paper condensers, resistors, etc., should be mounted where it is convenient and where they will cause the least interaction between circuits. As indicated in the diagram, certain circuits should be shielded. Resistors Rl, R7, R8 and R9 should be of the insulated type. The backs of all potentiometers should be shielded as well. These procedures if carefully followed will result in hum-free operation.

Make sure that tubes and speakers are installed before turning on the amplifier. The input circuits can then be used in safety. A burned-out output transformer may be the result of turning up the gain of any channel without speakers connected to load the output transformer.

List of Parts

XI—Output transformer, single 6L6 plate (3,500 ohms) to line and voice-coll secondary (2, 4, 8, S00 ohms)
T2—Power transformer, 800 v c.t. at 110 ma, 6.3 v at 3 amp, 5 v at 3 amp
L1—Filter choke, 12 h at ISO ma
1—6J7 tube
L—6SF5 tube
1—6XS-G tube
1—83-V tube
1—Chassis 8i12i9 inches
2—Terminal strips (2-terminal)
1—S.p.s.t. toggle switch
2—FM speakers, 12-inch diameter, 10-15 w.
6-8-ohm impedance

1—High-impedance microphone

3—Octal sockets

1—4-prong socket Condensers and resistors as per schematic