Tube 3TF7 Current Regulator 



Here are design data for a 3TF7 replacement. Design was based upon the information found in The Ballast Tube Handbook` by Jacobi and on the descriptive field of the R390x part list: Actually, according to the voltage charts of R390x receivers, the 3TF7 gives approximately 12 Vac at its output, with an input voltage around 26 Vac.
The ballast output is wired to the series detailed below: Giving a look to the voltage distribution along the above paths for both the R390 and the R390A we find different data as resulting from the voltage/resistance tables in the respective technical manuals: The secondary winding nominal voltage, entering into the 3TF7 ballast, is set at 26 Vac for the R390 and at 25. 2 Vac for the R390A. Well, we have at the output of the ballast 11. 3 Vac, or 14. 7 V drop, for the R390 and 12 Vac, or 13. 2 V drop, for the R390A. It was common practice to operate low power tubes at reduced heater voltage for a more stable behavior through their life. From the tables of the R390 we find 5. 3 volts across the heater of V508, the BFO oscillator, and 5. 7 volts across V701, the VFO oscillator, plus a 0. 3 V drop across the RF filtering inductor. In the R390A voltage drops across the heaters of the two tubes are the same, 6 volts each, but we find no drop on the RF filter coil. The inconsistency of different readings for two series connected heaters can derive from tolerances in their resistance values, as we should think of 3TF7 and of the same controlled heaters as temperature depending resistors. Data given for 3TF7, 8. 6 V drop at 310 mA, probably refer to its cold resistance at the start. Since oscillator tubes operate with heater drops around 6 VAC, the current flow must be under the nominal value of 300 mA. Measuring heater...
characters left: