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Old radio -> tube amplifier
Old radio -> tube amplifier
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Old 15th July 2012, 11:48 AM   #1
remick is offline remick  Norway
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Default Old radio -> tube amplifier

Hi guys,

This is my first post on this forum so to be nice I'd like to say hello to everyone.

At the begining I want to point out that lamps are total mystery for me. Generally, analog electronics is something I do not feel. Programming microcontrollers, writing programs from the PC and recently for Android is what I like the most. Therefore I hope you'll forgive me if some of my questions will be too naive or even stupid

It happened that my woman pulled from the parents basement, old German radio. Of course, the receiver does not work. I have no ambition to restore it to life, but I was thinking about something else. Marriage of digital electronics with the vacuum tubes. If I could run amplifier stage of this radio and connect to it to Internet radio - I'd be most happy person on the Earth (well for few days I guess).

Here is schematic of the radio.

Click the image to open in full size.

I measured voltage at the points marked in the figure.

Click the image to open in full size.

A-GND: -0.48V
B-GND: 422V
F-GND: 6.7V AC

After removing diode(?) AZ11, I measured the voltage at tube socket.

A-C: 5.8V
E-D: 8.26V
E-GND: 4.13V
D-GND: 4.13V

Now another thing. What is this "transformer" marked with a green rectangle in the schematic? What is it used for? It seems to me that it is mounted directly on the speaker, as one pair of wires going from the speaker directly to the points A and B. The second pair of speaker wires goes to the transformer. After desoldering these wires and measured the resistance it is >10Mohm (Should not there be a smaller resistance if it is a "transformer"?)

Fundamental question - what can still be measured? How to approach the problem?

Another question - does it make sense to attempt to revive the antiquity, or should I rather pull lamps off, and try to do a simple amplifier using these?

And the last question. Radio power cable. Original one has 2 wires. Is it OK to use 3 wire cable and connect PE wire to radios ground?

So, can someone guide me a little bit?
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Old 15th July 2012, 12:18 PM   #2
jane is offline jane  Norway
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Location: N-6600 Norway
Originally Posted by remick View Post
Now another thing. What is this "transformer" marked with a green rectangle in the schematic? What is it used for?
Field coil for the speaker?
Life is hard - Then you die.
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Old 15th July 2012, 03:13 PM   #3
mvd is offline mvd  Germany
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Location: Schaumburg Lippe
Yes, this is the Field-Coil of the Speaker. In the old days it was used as a choke for the power supply as well to save money.

Remick, look at the speaker. There is no magnet like nowadays. The magnetic field is created from the field-coil and inside tis field-coil is the normal coil, feed by the output-transformer.

I would suggest that you leave the old radio alone, maybe find a guy who would love to restore it back to live.
What would you do with a monaural Amplifier?

best regards
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Old 15th July 2012, 08:35 PM   #4
remick is offline remick  Norway
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Join Date: Jul 2012
Hi guys,

Thank you very much for interest in the subject.

OK - now it is clear for me. Looks like coil/choke is damaged.

Then I have to find new speaker. Of course I guess it will be not possible to buy such old type without magnet, so I need to buy modern one and use choke for power supply.

At the moment radio is used as a stand for photo frame and table lamp, so of course I could give it away to someone who would like to fix it, but unfortunately that is out of question. My wife like it very much as a piece of interior decoration, so I decide to "mod" it.
I said what I'd like to do with monaural amplifier. I'd like to connect internet radio to it, to listen morning program before we'd go to work. I don't need there stereo.
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