BAUER 707

GENERAL DESCRIPTION

THE Bauer 707 - Model 707 AM Transmitter 1000/500/250 watt

DESIGN FEATURES OF A BROADCAST TRANSMITTER KIT

A new development in the broadcast equipment field is the availability of kits. The new 1000/250 watt transmitter kit offered by Bauer Electronics Corp. is described in this article.

THE "do-it-yourself" trend has reached the broadcast equipment field with the introduction of the Bauer Model 707 AM transmitter. The design was based on an existing one-kilowatt Bauer model but has been simplified mechanically to meet the modern concept of "kit form" electronics.

Not only did this simplification make the transmitter easier to build, it also provided a layout that permits complete component accessibility. Note in Figures 2 and 3 the case with which every component can be reached. All wiring is accomplished with just one harness which is supplied with the kit, properly laced, and with each wire number coded. All small components are mounted on well marked component boards (such as Fig. 4). Eleven of these insulated boards are used in various sections of the transmitter. An illustrated assembly instruction book shows the correct placement of each part and outlines each step of the wiring.

The average assembly time is 100 hours. When a Bauer kit transmitter is completed the builder sends a notice to the manufacturer, who then sends a representative to the station to run a proof-of-performance on the completed transmitter. When the representative is satisfied that the transmitter meets factory specifications he installs the Bauer nameplate and it is ready for use.

Bauer 707

The RF Section
Looking from the rear (Fig. 2) the RF section is on the left side of the transmitter. Figure 3 shows a closeup of the oscillator-buffer section with the cover removed. This section is assembled on a separate shielded chassis, factory wired and checked, to insure stability. There are provisions for two vacuum crystals and either one can be selected by a relay which is controlled from a switch on the front panel or remotely. The vacuum crystal supplied with the Model 707 is capable of controlling the carrier frequency with an accuracy of ± 5 cps without the use of heaters, thermostats or ovens.

The oscillator is a Type 6AG7 connected in an electron-coupled circuit and is followed by another Type 6AG7 functioning as a Class A buffer. The driver is a Type 6CA7/EL-34 operating as a Class C stage. The driver excites two Type 4-400A tetrodes operating in parallel as a plate modulated power amplifier. A motor-driven rheostat in the cathode circuit of the final stage controls output power so as to compensate for variations in line voltage. The final tank circuit is unique in that tuning is accomplished through the use of a variable vacuum capacitor, a top quality method not usually found in one-kilowatt transmitters.

The transmitter is designed to match a 50-ohm unbalanced load. The RF output circuit provides the required impedance transformation and adequate harmonic suppression through the use of a "Pi" network followed by a "Tee" network. Additional suppression of second harmonic output is provided by connecting the load to the "Pi" network coil at a point where the impedances of the coil and a fixed capacitor are series resonant at the second harmonic frequency. A simple adjustment of this circuit provides harmonic suppression well beyond that required by the new FCC rules made effective last Jan. 1.

The Bauer Model 707 was one of the first transmitters type accepted by the FCC under these new rules. In looking at Figure 2 you will notice the dummy antenna switch. The built-in dummy load is made up of four "ohmspun" grids mounted in the outgoing air stream. Since all the dummy antennas used in the broadcast band are reactive, a means is provided in the Model 707 to automatically cancel out this reactance at any frequency and provide a pure resistive load.

Bauer 707

The AF Section
Looking from the rear (Fig. 2) the AF section is on the right side of the transmitter. Four tubes are used in this section. A pair of push-pull 6SJ7's drive a pair of push-pull 4-400A tetrodes operating as Class AB-1 modulators. 8DB of inverse feedback is provided over the two audio stages. One interesting feature of the design is that the modulator plate current when fully modulated does not vary more than 10 per cent over a 30 to 12,000 cycle range. The over-all response of the transmitter is flat within ± 1.5 db over a 30 to 12,000 cycle range. Distortion is below 2 per cent and noise is down -64 db.

Bauer 707

The Power Supplies
Vacuum tubes have been eliminated in all high voltage, low voltage and bias rectifier circuits in the Bauer Model 707 in favor of semi-conductor units. Type 1N2071 silicon diodes are used exclusively, 16 in the low voltage supply, 4 in the bias supply and 56 in the high voltage supply. The low voltage supply is located on the left side of the transmitter (Fig. 2-lower left). The bias supply is located on the lower right and the high voltage rectifier (two plug-in sections) is located on the right above the modulation transformer.

Standard bridge rectifiers are used throughout and transformer center taps play an important part in the low and high voltage supplies. The tap on the low voltage supply provides the 400 volts necessary for the low level audio stages and the oscillator-buffer section. In the high voltage section the center tap provides the 1500 volts necessary for power reduction thus providing a transmitter that draws no more power during the cutback operation than any of the many 250-watt transmitters now in use.

An interesting feature of the power cutback circuit is that the reduced final plate voltage has an additional filter allowing excellent noise specifications (-64DB) at 250 watts. When reducing power a reduction in drive to the final and a 6DB reduction in the audio input are automatic. Power cutback to 500 or 250 watts is standard equipment on the Bauer 707. Cutback is essential for the Class IV station with a lower nighttime power and is a bonus for the kit builder who can perform initial tune-up at low power.

The Control Circuits
With the use of semi-conductor power supplies the Bauer control circuits were greatly simplified and automatic starting was provided. Actually, only one master start-stop switch is necessary since the silicon power supplies require no warmup time. High voltage comes on automatically as soon as there is sufficient grid drive to the final tubes to close an underdrive relay. The master start-stop switch is of the new illuminated bar type (Fig. 1-middle), three inches long and easy for even the newest third-class operator to find. An interesting feature of the control circuit is a "second chance" device that automatically resets the overload relays in the event of an outage. This circuit is adjustable so that single short overloads will not take the transmitter off the air although continued overloads will. The relay protective system can be easily reset by remote control. The modulator and final RF stages, as well as the high-voltage transformer, are well protected by reliable delay-type overload relays that eliminate nuisance outages due to momentary overloads. Low voltage and control circuits are fused by the new indicator type fuse holders.

An additional feature fonnd in the Bauer 707 is automatic voltage control. A Sola constant voltage transformer of the new low harmonic type (Fig. 2-left side) maintains all filament and low voltage supplies within one per cent. Filament rheostats that require manual adjustment are thereby eliminated and tube life is extended.

Cooling of tubes and components is controlled through the use of a pressurized cabinet. Filtered air is drawn in by a high quality blower on the rear door, circulated throughout the cabinet, and then forced through the 4-400A tube sockets for maximum cooling. All switching and control functions are pre-wired to the main terminal board making remote control a simple matter. In addition the plate voltage and plate current kits are built in-a standard part of the 707 circuitry. Note in Figure 1 the number of meters, nine in all, providing continuous metering of all circuits.

Summary
The engineer who builds the 707 kit can gain valuable experience during the construction period. Also he achieves a familiarity with the transmitter that will prove very helpful over the years that he will service it. Since professional tools are supplied with every kit he will be able to turn out a first class transmitter and capture the personal satisfaction that goes with a job well done.

Bauer 707 diagram

TECHNICAL SPECIFICATIONS
Type Of Emission	A3
Rated Power Output	1000/500/250 watts
Power Output Capability	1100 watts
R.F. Output Impedance	50 ohms, unbalanced
Frequency Range	540-1600 Kc
Frequency Stability	±5 cps
Audio Input Level (100% mod.)	10 dbm
Frequency Response (0-95% mod.)	1000/500/250 watts 50-10,000 cps : ±0.5 db 30-12,000 cps : ±1.5 db
Distortion (0-95% mod.)	1000/500/250 watts 50-10,000 cps : 2.0% max
Carrier Shift	1000/500/250 watts less than 3%
Noise Level (below 100% mod.)	1000 and 500 watts : -60 db 250 watts : -57 db
Power Consumption	(For one kilowatt carrier power) Average modulation : 3300 watts 100% modulation : 3950 watts
Power Requirements	208-240 volts 50/60 cycles Single phase
Dimensions	Height : 75" Width : 30" Depth : 25"
Net Weight	800 pounds (approx.)
Specifications may change without notice

TUBE COMPLEMENT
RF stages		AF stages and modulator
Number	Type	Number	Type
2	4-400A	2	4-400A
3	6AG7	2	6SJ7

THIS TYPE OF TRANSMITTER IS INSTALLED IN THE FOLLOWING COUNTRIES
	ITU	Country	ITU	Country
	CAN	CANADA	COD	CONGO DEMOCRATIC REPUBLIC
	VEN	VENEZUELA