My first Bluetooth tube radio has been completed. Because of its neat appearance and satisfiable sound quality, it seems good enough for daily use.
That is all metal tube pushpull amplifier. I love metal tube because of its unpretentiousness. But most people don't know there are such lightless vacuum tubes. Every time I boasedt to my friend about it, they always said "Is this really tube amplifier?". ...It somehow got on my nerves, So I decided to build with "shiny" tube next time.
And though I had been a single-minded plastic radio lover for a long time, I wanted to try something different.
Step 1: Finding Out the Treasure
I like 1950s plastic radios. Almost all of my radio sets are made in this era.
One day last year I fond an old and shabby wooden radio on an auction site in Japan. That is pre-war era, tomb stone regenerative radio.
At that time, Japan had very few broadcast station because there were no commercial broadcast, so selectivity is not so important for radio sets. Moreover Japan of the time was very poor, superheterodyne was not populer for the ordinary Japanese. So most receivers of that generation are regenerative type.
This radio was not so good condition without knobs , windowglass and backpanel, and had many scratches.
Step 2: Select Circuit Type and Parts
This radio was made on 1930s,original circuit is 12F-12A-26B-27 regenerative 0V2. 12A was a popular power valve in Japan, and at that time ordinary built-in speaker for Japanese radio is magnetic type. It is sensitive enough to be driven easily by such small-power tube but very narrow-ranged and far from hi-fidelity. So this time I should employ a more recently made speaker and more powerful amplifier
And, the appearance is as important as its performance.It would be so cold if miniature tube or GT tube is used in such radio. So all tube in this radio MUST be ST type.
Finally I selected 71A pushpull. 71A is a direct heated triode made around same years as 12A. For driving stage I selected 27 that was used as a detector in original receiver. And according to the manner of that generation, I employed driver transformer.
71A pushpull is certainly far more powerful than 12A but its power is still small for the modernistic speakers. I employed 1970s' high efficient model made in Japan - Coral Flat 8.
Step 3: Remove All Parts on Chassis
We are not going to re-form this receiver but re-construct. There was no parts can be reused on chassis, so I decided to remove all of them.
There were a lot of suspicious parts. Some of them might contain suspicious subject ,that would be harmful for life. So I treated them carefully.
Step 4: Power Supply
Direct heating tube needs individual power supply for its filament. But this receiver has so few space on top that the second transformer could not place.
27 tube is one of the earliest heater cathode type tube, whose heater is terrible power eater. So it requires a leeway transformer.
Though there were small space inside chassis, hight was not enough to put heater transformer in it. so I made a tough choice - employ switching power supply.
But this decision would bring down a future trouble....
Step 5: Parts Arrangement and Wiring
Now, build amplifier according to schematics above.
This circuit has no such impressive feature, very classical non-feedback pushpull with phase inverter transformer.
Coupling condenser and inter-stage (phase inverter) transformer is placed on flat lug plate.
Step 6: Make Dissapeared Knobs and Tuning Window
When arriving my house, the glass of tuning window were broken out the grill cloth was worn out,and all of three nobs had been dissapeared.
I have another radio that was made around same age, so casted its tuning knob and make new nobs with Plarepair(TM).
The brand new tuning window was deformed of plastic plate with heating.
The instruction to reproduce knobs is here.
Step 7: Brus Up Wooden Case.
Body of case had many scratches,Some of them are deep.
Such oldness sometimes lets people feel nostalgia as the flavor of age. But... Only a fine line exists between shabbiness and such "flavor". It seemed that this reciever had already been beyond the line. So I decided to repaint it.
After repainting, set up new grill cloth.
Step 8: Setting Up
Put all the components together, and attach Bluetooth audio receiver inside case.
Just completed. It is not wide-ranged but sweet and very smooth to listen.
Step 9: A Problem Has Occurred
But happy events tend to be accompanied by problems. About 30 minutes after starting to listen, suddenly sound was distorted.
After a spell of searching ,the cause of it was finally found out. Because of lack of air flows, switching power supply was overheated.
So I removed power supply from chassis, put another USB power supply inside case.
Then switched on, oops! very large hum noise was heard. It was caused by new power supply . Its leakage magnetic flux was so strong that it excited the voice coil of speaker. So I made magnetic shield of stainless plate.