08-06-2023, 03:34 PM
A 5Y3 or an 80 is NOT a direct substitute for an 84. The 80 and 5Y3 are directly heated cathodes and their power must come from a transformer secondary winding that is isolated from ground. The 84 has an indirectly heated cathode that is insulated from the heater, therefore the heater and the heaters of the other tubes can be grounded at either side or by a center tap. The 84, 6X4 and 6X5 were commonly used in Automotive radios of this vintage.
The only way that you can substitute a 5Y3 for an 84 is by also replacing the power transformer with one that has a separate 5V heater winding that is well isolated from ground, as that winding will be at B+ potential. Most sets of this vintage other than Philco used an 80 or a 5Y3 (the modern (?) octal base replacement for the 80), as the 5Y3 had a much higher available output current. Philco possibly used the 84 in these vintage radios, as their output power was likely similar to a car radio, Philco was a major manufacturer of car radios and used the 84 for them, therefore they could get a volume discount for the tube, and since there was one less winding on the transformer, it was cheaper to manufacture, all a win-win for Philco.
The "Now" drawing of the 5Y3 circuit is incorrect and will not work. A cap can't couple DC.
The "correct" version may be slightly incorrect, though much better. The junction of the - terminal of the 12uF cap and the 146 Ohm resistor goes to the center tap of the HV secondary of the power transformer. this connection is NOT grounded. The - terminal of the 16uF cap and the 15 Ohm resistor is at chassis ground. This series of resistors between chassis ground and the B secondary center tap was a common method to get the bias required for the audio output tube and some others.
The filter circuit as drawn in the actual schematic 41-250 works with either the 84 or the 5Y3.
The only way that you can substitute a 5Y3 for an 84 is by also replacing the power transformer with one that has a separate 5V heater winding that is well isolated from ground, as that winding will be at B+ potential. Most sets of this vintage other than Philco used an 80 or a 5Y3 (the modern (?) octal base replacement for the 80), as the 5Y3 had a much higher available output current. Philco possibly used the 84 in these vintage radios, as their output power was likely similar to a car radio, Philco was a major manufacturer of car radios and used the 84 for them, therefore they could get a volume discount for the tube, and since there was one less winding on the transformer, it was cheaper to manufacture, all a win-win for Philco.
The "Now" drawing of the 5Y3 circuit is incorrect and will not work. A cap can't couple DC.
The "correct" version may be slightly incorrect, though much better. The junction of the - terminal of the 12uF cap and the 146 Ohm resistor goes to the center tap of the HV secondary of the power transformer. this connection is NOT grounded. The - terminal of the 16uF cap and the 15 Ohm resistor is at chassis ground. This series of resistors between chassis ground and the B secondary center tap was a common method to get the bias required for the audio output tube and some others.
The filter circuit as drawn in the actual schematic 41-250 works with either the 84 or the 5Y3.
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Best Regards,
MrFixr55
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