Dual High-Fidelity System

This amplifier may be used as a conventional audio amplifier, using either of its three input circuits or two output circuits together, singly, or in any desired combinations. Two high-impedance inputs and one universal input make it possible to connect the amplifier to just about any type gear available. The mixing is high-level electronic mixing—it is accomplished after the signals have gone through one audio stage. The advantage of this method is that when the volume is controlled at a high-level point in the circuit there is less tendency for noise to develop. Normal output from either channel is 15 watts. Channels may be paralleled to give an undistorted output of 30 watts. Since the three input circuits may be mixed, an unlimited number of effects is possible.

As a recording amplifier one channel may be used for monitoring and one for recording. This prevents disturbance of the reflected load to the output stage. Output taps are available on the back terminal strip from 4 ohms to 500 ohms. This immediately introduces the possibility of using the amplifier as a cathode modulator, or as a speech amplifier in a medium- or high-power transmitter.

Tube line-up

The amplifier line-up consists of 14 tubes and one neon bulb, arranged as indicated in the block diagram (Fig 54-c). For obvious reasons it is not practical to attempt to place tubes in any specific order, since they are all related to one another and in many cases in identical manner. Tubes used are 3-6SJ7; 3-6J7; 2 -6C5; 4-6A5-G; 1-6H6 and 1-5T4 (6A3's or 6B4-G's can be used instead of 6A5-G's with no circuit changes). The 6SJ7 and 6J7 have only 5 volts on the filament despite ratings of 6.3 volts. The reason for this is reduction of hum and shot effect (hiss due to electrons breaking loose from the cathode) and the minimizing of thermal agitation. The 6C5's are used as voltage amplifiers before the final stage. The 6A5- G's serve as the two separate output stages. The 6H6 is the rectifier for the bias supply, the two diodes being placed in parallel and the neon bulb serving as bias-voltage regulator. The 5T4 is the rectifier for the entire amplifier.

To simplify to some degree construction of the amplifier a full description of the controls is in order. The accompanying diagram may be used as reference chart (Fig. 54-b), Control 1 is the range selector for the volume indicator. This indicator is calibrated in decibels using 6 milliwatts as 0 db. In order to increase the effective range of this scale an attenuator is inserted in series with the meter. Actually the meter is nothing more than a reference indicator, but as such is invaluable for audio work. After determining correct levels with the db meter, it may be continually referred to as a check for correct levels. Its most important use is in recording to prevent overcutting and undercutting, and in connection with this phase of the amplifier's operation is an absolute essential.

 

When the amplifier is used as a speech amplifier in a transmitter, the db meter is useful primarily to supplement the class B plate meter, which is the normal modulation indicator. The db meter may be calibrated against the class »B plate milliammeter or better yet against an oscilloscope. This operation is quite simple. A calibration chart is drawn up showing at what point on the meter 100-percent modulation is shown on the scope. These percentages may be recorded from 0 to 100 in whatever steps the operator desires, although 0, 25, and 100 should be ample.

The meter also will serve as an indicator of popular response at a party, or in amateur contests at various clubs, etc. It is often referred to as an applause meter, and while relatively unimportant the availability of the gadget should not be overlooked.

Control 2, as indicated on the diagram, is the output switch. Position 1 connects the two output stages in parallel to give a maximum audio output of 30 watts into an 8-ohm load. Position 2 switches a monitor speaker to channel 2 and connects the recorder to output channel 1. Position 3 connects the monitor speaker to channel 2, as in the case of Position 2 but switches a second recorder, if used, to channel 1. The purpose of this rather elaborate set-up is to permit continuous recording with dual turn tables. The builder may, of course, select any switch set-up to suit his needs, provided the impedances are available on the transformer.

Control 3 affects only input channel 1. It was found desirable to obtain more flexibility in matching input impedances in at least one channel. Commercial microphones and some high-quality pickups as well as the output of some tuners, are low impedance.  

Its first position connects the first grid to a chassis connector on the rear of the amplifier Position 2 connects the grid to the secondary winding of a hum- shielded line-to- grid transformer. This postion is used to match low-impedance devices to the amplifier. This is in direct contrast to the high impedance available when the switch is on the first position. Position 3 connects this transformer to the grid of channel 1 through a fixed pad. In this position the channel may be used to amplify the output of a telephone line, high-output low-impedance pickup, or any other low- impedance high-level source.

 

Control 4 is the volume control for input channel 1. Controls 5 and 6 are volume controls for input channels 2 and 3, respectively. Control 7 is the power switch. Controls 8 and 9 are volume controls for output channel 1and 2.

The neon bulb is used as a voltage regulator because of its ability to change its resistance, depending upon the voltage drop across it. The circuit is extremely simple as Fig.54-a illustrates. The theory behind it is that the neon bulb draws an initial load.  

A higher voltage across A and B will cause the neon bulb to draw higher current, thus causing a greater drop across C. A lower voltage will increase in voltage drop across C. Regulation is not perfect, but it is better than the regulation available when using a small tube like the 6H6, with a high-resistance filter. At no time is regulation poor enough, or a heavy duty bias supply.

Consider input is used for the rectifier circuit to obtain a voltage high enough to be usable as bias. No condenser is necessary at the output of the bias supply, because of the tendency of the neon bulb to oppose change in voltage.

Transformer details

The audio transformer used in this amplifier are high-fidelity units with shielded cases and electrostatic shielding between windings to reduce hum pickup. The input transformer is designed to match a multiple line to a 50,000-ohm grid circuit. The driver transformers have a 3:1 ratio (pri. to 1/2 sec.). The power transformer supplies 360 volts each side of center tap and has one side tapped at tapped at 150 volts for the bias supply. Two 2.5.volts winding are connected in series to provide 5 volts for the filaments of the 7J7's and 6SJ7's. The transformers (both power and audio) are the limiting factors in the amount and quality of output obtainable from the amplifier.  

They should be carefully selected if high quality and maximum undistorted output is the goal. A cheap power transformer may have poor regulation causing plate voltage to drop when volume level is raised. This of cous will reduce the undistorted output rating of the amplifier. For the same reason, the two filter chokes should have as low d.e. resistance as possible. A cheap output transformer may have poor efficiency, particularly at high and low frequencies. There's no economy in spending time and money on a PA amplifier only to trolled it with a cheap output transformer.

Needless to say a good common ground throughout the entire unit is essential. To obtain low grid-to- ground capacity, crystal microphone cable was used for all grid leads. In order to obtain a somewhat neater job of wiring, spare prongs on tube sockets were used as mounting lugs.