Jesse French Export Co. U-1 Schematic

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Manufacturer:Jesse French Export Co.

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THE U - 1 SUPERHETERODYNE CIRCUIT The U - 1 Chassis uses seven tubes as follows: one 551 variable Mu tube for the first tuned R.F. stage, one 224 screen grid tube for first tuned detector, with a 227 oscillator tube signal beating into the frst detector stage. One 551 Variable Mu tube for the intermediate R.F. stage and a 224 for power detector. This second detector or Power Detector is resistance coupled to the power tube which is a PZ Pentode type tube. One 280 tube is used as a rectifier.
The grid bias of the Pentode is obtained by the center tap of the Rectifier Plate passing through the 1620 ohm field coil to ground instead of leading direct to ground for negative potential. The power grid is tapped into the field coil at 1320 ohms or 300 ohms from ground, making a positive flow to ground. The resistances are so arranged in the grid circuit of this power tube, that it gives excellent tone quality because it presents a constant positive few to ground of circuit.

A 385 ohm filter choke connects the source of the plate or 280 filament with the plate filter by passes which are of the 8 mfd wet electrolytic type condensers and the remainder of the circuit being by-passed by paper and mica condensers.

The first electrolytic condenser by passes the plate positive source to the center tap of the rectifier plate winding or negative potential which will have a negative voltage of approximately 83 volts before it passes through the field coil to ground. The body or negative of the electrolytic case being insulated from the chassis permits this by passing arrangement.

It is quite a question in the Loftin - White direct coupled amplifier where detect ion actually takes place, but for the time being, we will call the type 224 tube the detector, and the type 245 tube the audio frequency amplifier. The detector can be considered of the high bias type. A 100,000 ohm resistor in the cathode circuit of the 224 tube connects the cathode approximately 15 volts positive with respect to ground. This is too high a bias for the 224 to operate as a detector. Therefore the grid retura is brought back to a position on the network about 12 volts position with respect to ground. This leaves a three volt bias on the grid of the detector which is the pro per value for detecting weak signals. When a strong signal is delivered to the grid of the detector, the detector plate current increases. This changes the cathode volt age from 15 volts approximately 20. At the same time, the plate current in the network decreases making the grid returns approximately 8 volts positive with respect to ground. The effective bias on the grid of the detector tube is therefore about 12 volts which is the proper value for detecting the strong signals. In measuring the bias on the detector, the readings will be affected a great deal by the type of voltmeter used. It is best for the service man to take these reading on a set which is known to be good with his own voltmeter. In the future these readings can be taken as standard and questionable sets compared to them.

The peculiar part of measurements on this audio system is the high voltage from the 245 tube plates to ground, the high voltage from the filament to ground and their possibility to read the grid voltage with a meter. The best indication of the Loftin. White detector amplifier condition is the plate current of the type 245 tube. This should be approximately 38 milliamperes. This reading will vary quite a bit with different tubes and with the line voltage.

A special dynamic speaker with a 4700 ohm field coil is used as part of the Loftin White resistance net. work.
The rectifier tube is used as a full wave rectifier and supplies the total plate current of the set which is approximately 38 milliamperes at 400 volts.

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