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This is a Norton Chimes unit currently under restoration. The machine is composed of a turntable, mixer/preamp/driver, power amplifier, and power supply. The system is mounted in a 6FT BUD rack. This is what a 150-200 watt tube-type audio amplifier looked like in the 1940's.
The turntable visible in the top section is a Garrard RC60. A collector friend has already spoken for it, and as of today it has gone. In its place will go an RCA BA-6A limiting amp and a Symmetra-Peak phase rotator.
The next lower section is the control section. It contains a mixer and a small push-pull 6B4G amplifier of about 10 watts. A switch selects whether the 10 watt amplifier drives the church PA system or drives the large power amplifier for the church bells (Chimes is the industry standard term).
The next section is a 200 watt (approximately) power amplifier using four 838's in push-pull parallel. The chassis contains from right to left, the output transformer (500 ohms), driver transformer, and filament transformer. Each 838 requires 10 Volts at 3.25 Amperes for the directly heated cathode (what we called a "filament" in the olden days). The chassis also contains two regulator tubes for the grid bias circuit. The 838 uses zero bias and operates class B. The schematic is here. Note the interface for the drive. It comes from the plates of the 6B4's in the control section. That odd extra wiring in the control section makes more sense now, eh? The VR tubes in the power amp serve to cut off the 838's during idle times.
Note that I have stated 200 watts for the presumed output. This is based on the size of the output transformer over the voice range and the type of the power supply rectifiers, which are rated for 500mA maximum. Four 838's can consume 640mA. This equipment is a type that required utmost reliability and was to be operated by non-technical personnel, i.e. the clergy or custodian. It is my opinion that the rectifiers therefore would be loaded no more than 350mA for reliability reasons. Although two 838's would fully consume the available power, four were used so that they could age for many years with no trouble, a MTBF of 5-10 years for the system. If one failed, there would be no need to stop using the machine. What a pity today's all-too-cheap electronics gear is so poorly made by comparison. Well my fellow Americans, that's what you get when you want new oats for the price they're worth -after- they've been through the horse!
The lower section is the power supply. The high voltage for the 838 stage is rectified by a pair of 866 mercury vapor rectifiers and the low voltage for the mixer/driver is provided by a 5U4. The circular device next to the 866's is a timer. The mercury rectifier requires a warmup time of 30 seconds minimum in order to vaporize the mercury before high voltage is applied. More detail will be given on this section later. It will be very interesting to know the HV output voltage, because that will determine the amplifier's power output.
Its ultimate destination will be as a high fidelity driver for the modulator of the Tucker transmitter. The transmitter does not "require" anywhere near 200 watts to drive the grids of the 3-500Z modulators, but the specifications call for 25 watts and it is well known by those competent in the art that a driver for a class B stage should have "several" times the required capacity in order to provide good regulation of the drive voltage under varying load conditions found in a class AB2 or class B amplifier's grid circuit.
I'm sure a few people who don't like AM on the ham bands will wail and gnash their teeth at the bare thought of all this high fidelity and headroom, but if they do, it is because they have little understanding. The facts are that the signal must be preproduced as perfectly as possible so that unwanted harmonics are not added to the modulation. If I set my audio passband to 3000 or 3500 cycles in the low level sections of the audio chain, which is a correct technical practice, it is imperative that later stages do not distort the signal or else unwanted frequencies (harmonics) outside my desired passband could be produced. Nobody wants that.
