02-02-2024, 08:21 AM
Hi Gang,
Mancave, if the radio plays loud and proud, this proves out all of the amplifier section except the phono preamp. Try injecting your Bluetooth audio right at the phono-radio switch by either using jack with internal disconnect or adding another switch at the junction between the plate coupling capacitor of the phono preamp and the radio/phono switch. IMHO, this is the correct junction to add a "line level" or "aux" input anyway.
The reason for the extra stage of amplification for the phono is because this unit uses a magnetic pickup, whose output is much less than that of a ceramic pickup. The output of a magnetic pickup is somewhere between 5 and 20 mV except for some of the really early Victor horseshoe heads. The output of a ceramic or crystal cartridge is between 500mV and 3V. (Many portable phonos of the 40s and 50s employed a crystal cartridge with an output between 1 and 3V and only 1 tube, usually a 117L7 output tube / rectifier or a 25L6 output tube and a selenium rectifier. If your Philco used a crystal or ceramic pickup, the extra stage of amplification and the transformer would not exist. The advantage of a magnetic or the Philco "Beam of Light (BOL) pickup is much greater fidelity and lower distortion.
The output of a Bluetooth, MP3, etc. is usually over 1 volt, and this will "rail" a phono preamp designed for an input between 10 and 100mV. In addition, phono preamp stages usually provide a level of equalization that is not appropriate for another input such as TV, MP3, Bluetooth, CD Player, Computer, etc.
Hope all of this helps!
Mancave, if the radio plays loud and proud, this proves out all of the amplifier section except the phono preamp. Try injecting your Bluetooth audio right at the phono-radio switch by either using jack with internal disconnect or adding another switch at the junction between the plate coupling capacitor of the phono preamp and the radio/phono switch. IMHO, this is the correct junction to add a "line level" or "aux" input anyway.
The reason for the extra stage of amplification for the phono is because this unit uses a magnetic pickup, whose output is much less than that of a ceramic pickup. The output of a magnetic pickup is somewhere between 5 and 20 mV except for some of the really early Victor horseshoe heads. The output of a ceramic or crystal cartridge is between 500mV and 3V. (Many portable phonos of the 40s and 50s employed a crystal cartridge with an output between 1 and 3V and only 1 tube, usually a 117L7 output tube / rectifier or a 25L6 output tube and a selenium rectifier. If your Philco used a crystal or ceramic pickup, the extra stage of amplification and the transformer would not exist. The advantage of a magnetic or the Philco "Beam of Light (BOL) pickup is much greater fidelity and lower distortion.
The output of a Bluetooth, MP3, etc. is usually over 1 volt, and this will "rail" a phono preamp designed for an input between 10 and 100mV. In addition, phono preamp stages usually provide a level of equalization that is not appropriate for another input such as TV, MP3, Bluetooth, CD Player, Computer, etc.
Hope all of this helps!
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
