Antique radios are beautiful and restoring them is also fun. But if you are crazy about repairing radios, sooner or later, your attic will filled with those classic items. Ordinarily for your family these treasures would be just a dump.(T . T).
So about ten years ago I thought if there was no way to listen to favorite music with my old radios. The first idea was putting music on the air with an AM transmitter. I made a dozen of transmitters in years. It was very fun to hear favorite songs among local broadcastings, but to my regret, the quality of sound was terrible, so I soon got tired them.
Old radios often have an AUX terminals to connect an analogue turntable.CD player or other sound sources can be connected them. Quality is not so bad. But connecting with cables seems to be lack of romanticism.
Now Bluetooth audio receiver can resolve this annoyance. Only connecting Bluetooth receiver to AUX terminal, you can transmit music stored in your smartphone. But it is something inelegant to expose receiver outside the set. Moreover audio amplifiers of most tube AM radio reveivers are pentode single ended with very small output ttansformer, so their sound charactariscs are narrow ranged.
Only remaining way is re-construction.
Nowadays, antique radios are precious cultural legacies, so I cannot stand feeling guilty. But It would be better for radios themselves to use in daily life than to keep them idle. Finally I decided to do it.
Step 1: RCA Victor 9-X-571 Original Schematics
Original circuit is as indicated below.
Power amplifying stage is 50L6 single ended (ultra linear connection). concentric circle shaped front grills have not only visual attraction but also acoustic effects.
Step 2: The New Circuit
I have built 4 bluetooth tube radios since starting this projects. I have been strictly observing these rules below.
1. Aiming building devices with Hi-Fidelity audio level. Use torerable level Speakers,tubes and other parts.
2. The circuits must be constructed of tube distributed at same age as receiver. If using Amplifier IC, you can build them only with small percent of costs and time. However for such "mixing apples and oranges" devices do you feel Romanticism?
3. Aiming qualities for daily use.
The reconstructed circuit is as below. It is very simple and ordinary triode connected push-pull. I love metal tubes, and that time There are a 5693 red metal tube and a 6N7 in my stock. I decided to make all metal tube amplifier. On eBay I bought a pair of 6V6 metal tube.
Step 3: Install Speaker, Deaden the Case
Triode connection class A push-pull amplifier is not so powerful so select a high-sensitivity speaker. Electrovoice 409E (almost same as altec lansing 409E) is matched this purpose. Plastic case is very thin and light-weighted, so to deaden them I employed REAL SCHILD(TM)
Step 4: Remove All Parts and Modify Chassis
After removing all part on the top and inside of chasis, built amplifiere circuit. Electrovoice (same as Altec) 409E was originally designed for built in ceiling. So it only needs very short depth. but this radio's original speaker unit is thinner than it . So it needs some works on chassis to get more clearance.
Step 5: Put Brand-new-old Wine Into New Bottle
After removing all parts from chassis, build a brand new audio amplifier circuit in it. Finally install Bluetooth audio receiver and power source for it.
For this project I decided to install Belkin Bluetooth audio receiver first, because of its stable work.But this authentic receiver needs pairing process every time turning on, it is somewhat annoying.
Recently I found out a satusfactory module on the web market. It reminds connecting information,
And I insttal remote controller for turning on/off main switch.