Text preview for : RCA BA-6A Manual.pdf part of RCA BA-6A Manual with balancing instructions, trouble shootingh, repair and tube replacement issues. Link two for stereo.
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RCA BA-6A LIMITERS Operation: Apply power to the limiters and allow them to warm up during your setup. The meters will light up. If not, check that the power switch on the chassis of each limiter is on. A balance check will not normally be needed. See balance instructions. Turn the input and output controls to the marks, (unity gain). Turn the BAL-LIM switch to Limit - Single. Turn the METER switch to GR, (gain reduction). Switch, stereo, fast, and slow switches down, (off), on the panel between the limiters. The units are now ready to be used. Mono: Apply a 600 ohm balanced line source from a pre-amp, an EQ, or console buss out, to the input of the limiter. Connect the output of the limiter/s to the Line In of the recording device, the A to D converters, or the console. Observing the limiter meters, adjust the controls of the source to obtain the desired amount of limiting. Monitor and adjust the console or recording device on the output of the limiter for normal levels. Normally it should not be necessary to move the limiters input and output controls off the marks. Typically 5 db of limiting is desired, though very dynamic instruments or singers may cause up to 20 db of limiting. If in that circumstance the Pre or EQ feeding the limiter starts to distort, raise the input control 6 db. Three clicks at 2 db per click, and lower the pre-amp or source gain. Stereo: Does not apply to unmodified unit. Start with the stereo switch off. Connect the limiter to left and right inputs and outputs as above. Adjust the pre-amp gain or console output until limiting begins. When the meters are showing equal limiting on both sides, left and right, switch to stereo (up). See notes. Fast slow switches: Does not apply to unmodified unit. Start with these off, (down). Monitor and study the effect of these switches as you try them. Raising the fast or slow switches will quicken or slow the release time respectively. Using one switch in stereo mode will give half the effect. Using both slow and fast at the same time will slow the attack time, possibly improving the attack of drums. Limit single/double: Turning the BAL/LIM switch to "double", will introduce an element of compression as well as limiting, since the gain reduction will not completely release for several seconds. It's intended to adapt to a section of program that is louder than normal without excessive pumping of the limiter. Notes: This is about the modification. The 1/4" phone plugs from the panel between the limiters must be plugged in to each limiter for the panel switches to work. When they are all down it is electronically the same as not having them plugged in. This is the factory normal. In the stereo mode, left and right program material will both contribute to total limiting. The limiting is kept equal to prevent drift of any center channel information. This is needed when limiting a mix with center channel elements. This might be a finished project or a stereo mix of drums. The snare or vocal will stay in center as the left side or right side tom tom is struck. This is true stereo limiting, and is not always desirable when left and right sides should simply stay equal, particularly in the absence of any center channel image or stereo effects.
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RCA BA 6A BALANCE ADJUST Meters and balancing: The balance must be done before using the equipment for the first time, when tubes have been replaced or have aged unequally. Every two hundred hours of operation or every few months would be advisable. The symptom of poor balance would be a popping or thumping noise upon the attack of program material. The adjustments are made on the chassis using the labeled flat blade screwdriver type controls on the top of the chassis. Balance Adjustment: The balancing procedure is the same for any application of the BA-6A. When checking the balance skip steps 4 to 8, and make any adjustments in FINE BAL. The position of the meter needle when the unit is off, (at rest on the left), is the minimum meter reading mentioned in this procedure. 1. Open the front panel by pulling on the monogrammed knobs, and turn the ON-OFF power switch to ON. Allow the limiter to warm up for at least 30 minutes 2. Turn the METER switch to the FIL position and adjust the heater voltage control until the meter pointer is on the HEATER mark. 3. Turn the INPUT control to the OFF position. 4. Turn the METER switch to the COARSE BAL position. 5. Turn the BAL-LIM switch to position A. 6. Adjust the BAL A control on the chassis for minimum meter reading. 7. Turn the BAL-LIM switch to position B. 8. Adjust the BAL B control on the chassis for minimum meter reading. 9. Turn the METER switch to FINE BAL position. 10. Turn the BAL-LIM switch to position A. 11. Adjust the BAL A control on the chassis for minimum meter reading. 12. Turn the BAL-LIM switch to position B. 13. Adjust the BAL B control on the chassis for minimum meter reading. Note: If meter deflections are hardly noticeable, no adjustment is necessary. Meter Adjustment for Zero Gain Reduction: Turn the BAL-LIM switch to OFF. Turn the METER switch to GR. Adjust the ZEROADJ. Control on the top of the chassis until the meter reads db. Semi pro application: In a semi-pro environment, a "bump box", would be needed at the input. The inputs of a stereo Bump box have RCA jacks and/or 1/4" phone jacks. These inputs will accept the 10 db unbalanced audio from the semi-pro gear. The outputs are XLR, +4 db 600 ohm for the XLR inputs of the limiters. One box would handle both limiters. The outputs can simply be reduced 14 db with the output controls and wired to unbalanced by tying the low side, black wire, to shield, when the signal must return to the semi-pro connections. Some bump boxes include the adaptation back to the semi-pro standards. With a "bump box" on hand, the limiters could be used in any application. 1. Determine if the audio source to be limited is unbalanced or balanced. 2. Use the "bump box" if the source is unbalanced RCA or 1/4" phone jacks. 3. If the box has gain control/s, play audio at normal levels from the source. 4. With the limiter input controls on the marks, adjust the gain until limiting begins. 5. Connect the outputs using adapters. Lower the output controls 14 clicks (14 db).
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RCA BA-6A TECHNICAL HISTORY The Rebuilding: In 1997 both of these units were rewired. The electrolytic capacitors were replaced and all the tubes were replaced. The tubes were operated 48 hours, and the balance was done repeatedly until the tubes had, "cooked in", and the balance became stable after warm up. Buying Tubes: The first 6SK7 tubes purchased for these units were counterfeit. The printed marking on the tube was 6SK7, but the stamping on the base of the tube was different. The BA-6A will not limit at all with substitute tubes in place of the 6SK7's. Tubes can be obtained online at reasonable prices. Sites like, vacuumtubes.net, and tubedepot.com are examples. Prices are 2004. There are nine tubes altogether. $80.00 Two - 6SK7 $16.00 for a matched pair One - 6H6 $ 2.00 Two - 6J7 $16.00 for a matched pair One - OD3 $ 4.00 Two - 6V6 $30.00 for a matched pair One - 5R4GY $12.00 Input and Output Pads: These limiters were designed to limit the level fed into AM radio transmitters to that required by the FCC. They received program material from a Class A Line provided by the phone company, connecting the radio station studio, to the distant transmitter and its broadcasting tower. Ample gain was provided to make up for any losses and to drive a 50,000-watt transmitter. They had an unmanageable amount of gain both at the input and the output. To adapt the gain structure to the +4 db recording studio environment, an 18-db pad was added to each limiter at the input, and a 12-db pad was added at the output. Marks on the input and output controls show where they would be when toned out for unity gain in a studio. Raising the input controls a click or two may help to get the GR desired. At the mark on the output control the limiter will deliver +4 db at it's output when it is limiting. Repair and troubleshooting: The big advantages here are that there are two units to compare, and that each limiter has a built in tube tester for all the audio tubes! Remove the 1/4" stereo linking phone plugs to isolate the two units for testing purposes. 1. Turn the meter switch to each test position, (V 1 - V 6) and watch the meter. This is the tube test feature. Compare readings to the other limiter. 2. Replace suspect tubes and repeat the test. If OK rebalance the limiter. All 13 steps. 3. If not, consider replacing all the tubes or switching them with the ones in the other unit. This will prove that it is a tube problem and not a chassis component. It will also mean that both units will have to be rebalanced, doing all steps. 4. If the unit is not passing signal, substitute the other limiter to be certain. Switch the XLR cables at the backs of the limiters. If limiting action is seen on the meter, then the input is OK. Look for an output connection problem. 5. If the unit does not power up at all check the fuse. These fuses are often found at auto supply stores. They are 2 amp fuses. If it blows again, change the 5R4GY, or consider replacing all the tubes. The tubes removed would all have to be tested. 6. Beyond this, or if smoke came out of the unit, the limiter must be put on a bench and checked out by an electronic technician. Give him both units for comparison. What follows is a detailed description of circuit operation to aid in trouble shooting.
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RCA BA-6A Bench work: There is a schematic, and the transformer power supply is easy to trouble shoot. Both limiters could be placed on the bench and circuit voltages could be compared. High DC voltages require caution. Stand on a dry or rubber mat and use a one handed approach when taking voltage readings. Signal flow: At the upper left one can see the signal come into the unit through the input transformer and input gain control, such that the signal is oppositely phased at the grids of the 6SK7's. At the top center, there is a transformer between the first and second stage. In the following stages, the 6J7's are coupled by capacitors to the grids of the third stage 6V6 output tubes by caps C-9 and C10. A very high (and dangerous) plate voltage is applied to the center tap of the output transformer, at top right. Out of phase signals pass through each stage as audio in this push-pull configuration. In the first stage the changing voltage of the gain reduction is fed to the grids of the 6SK7's, in phase and is cancelled out when the limiter is properly balanced. In an out of balance condition, a thumping or popping noise will appear at the output as the gain reduction occurs, and GR voltage rises and falls. The circuit accomplishes limiting by exploiting the non-linear gain of the 6SK7 as grid bias voltage changes. Originally designed for the radio receivers of that vintage, the 6SK7 limited the gain of the radio when strong signals were received. Known as AGC. The output signals from the plates of the 6V6's are applied to a dual rectifier tube in the lower right of the schematic. After rectification to DC, this varying voltage goes negative as the output increases. In today's terms it is called a volume envelope. When this is fed back to the grids of the 6SK7's it reduces the gain in the first stage. This causes the limiting effect. Capacitor C1 is charged with the GR voltage and it is discharged by R1, which determines the release time. When not limiting, there is no voltage at the GR point, see the center tap of the input transformer's output winding, which is also the junction of C1 and R1. In the OFF position of the BAL-LIM switch, the junction of C1 and R1 is grounded by terminal 3 of switch S1-B seen just below the input transformer in the schematic. This disables all limiting even as the unit is passing signal. In double mode, an additional resistor and capacitor in series, is added on to the basic circuit from the GR voltage to ground, creating a kind of compression, along with the limiting. Power supply: The power supply transformer at the lower left on the schematic has windings for the several AC and DC voltages used in the limiter. The 5R4-GY rectifier tube provides the plate voltage. A portion of this is regulated by the gas tube OD3/VR150 which lights up with a purple glow, (normal). This provides a regulated screen voltage for the 6SK7's. Their filament supply is DC to reduce hum. See points "Y" "Y" at the lower left near the heater voltage adjustment. Two other 6.3 volt windings seen near the hum adjustment go to the other filaments "X" "X" and meter lights and to the output connector. A separate 5volt winding is for the 5R4. Beware of the high voltage DC on terminal 10, originally intended to power an outboard pre-amp, right next to the output terminals, 11 and 12.
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RCA BA-6A Balancing and metering circuits: At the lower right center of the schematic, you can see the two light bulbs and the meter with it's plus and minus terminals. Follow the minus terminal of the meter through an 82-ohm resistor near the input terminals at the far upper left. It comes back to the wiper of switch S2-A. The positive terminal goes to the wiper of S2-B near the center of the schematic. The dotted lines show that the sections of S2, the "METER "switch, move together. As they move through the various settings voltages and configurations needed for balancing and tube testing are supplied to the limiter circuits. When the BAL-LIM switch is in position A, a sixty cycle voltage is applied to the grids of the 6SK7 tubes in parallel and the amplifier output signal is disconnected from the out going line and switched to the meter. The output shown on the meter is due to an imbalance of the 6SK7 tubes. In position B, the sixty-cycle signal is increased and the cathode bias voltage is increased. The BAL B control, balances the screen voltages of the 6SK7's. The bridge rectifier near the meter is only to drive the meter during the balancing procedure. When the unit is way out of balance, as when tubes were changed, you must start with coarse balance, BAL, on the meter switch. This will protect the meter from damage. The tube test feature measures the current passing through each tube. These are positions V1 through V6 on the meter switch. In the case of the 6SK7's, the meter reads a shunt resistor of 16 ohms in the cathode circuit. When testing the 6J7's and 6V6's one terminal of the meter is grounded and the other terminal switches through resistors to the cathode circuit of each tube. For example, at S2-B, the V5 testing position, terminal 7 leads to R-49, a 56,000 ohms resistor, and to "G". Point G is found between the two 6V6's, at the cathode of V5. The Modification: The fact that the GR voltage starts at zero goes up with limiting, and is referenced to chassis ground, makes it practical to link the limiters for stereo. 1/4" phone jacks have been mounted on the chassis to allow access to the GR voltage and lead to each side of the stereo switch on the panel between the limiters. Taking a cue from RCA's double position components R37 and C18 at terminal 5 of switch S1B, resistors and capacitors are switched in by the fast and slow switches shown on the linking panel diagram. Adding a path to ground, the "fast" resistors discharge the GR voltage quickening the release time. The slow capacitors store the voltage and slow the release time. Effects on attack time have not been analyzed, but seem minimal. Removing the phone plugs from the jacks, or switching all the switches down (off), returns the circuit to its original form. The GR voltage is low and safe at about 12 volts or so during limiting. Remove the phone plug from the unit during bench testing.