Tube radio and modern sources

rustman

rustman

Member
I'd like to use my beautiful old tube radio with my iphone, cd-player etc. There are gramophone connections behind the radio for simple banana jack mono connectors (mono). Any idea what do I need to do to get my source connected in mono? Do I need to worry about impedances or such? Do I need a preamp in between the source and radio?

Cheers, Matti
 
rustman said:
I'd like to use my beautiful old tube radio with my iphone, cd-player etc. There are gramophone connections behind the radio for simple banana jack mono connectors (mono). Any idea what do I need to do to get my source connected in mono? Do I need to worry about impedances or such? Do I need a preamp in between the source and radio?

Cheers, Matti
Click to expand...

This is a bigger can of worms that you might think!
First off, is that "lovely old radio" earthed? I am not familiar with the mains supply in Finland. If it is NOT earthed, go no further! If it is, I would suggest a PAT test but so long as you have a good mains earth fine.

Now "Gram" connectors normally disconnects the radio section from the audio amp, just the opposite of what you want! The solution, if you are able is to connect to the outer tags of the volume control which will give you a "pre-fade" feed. Another snag however!

That V pot will be 1meg Ohm and so you will need a very high impedance buffer. You could use a single JFET source follower but easier is an op amp such as the TL071. You could put the buffer in a tin and battery power it (the lead from the radio must not be more than about 1/2mtr) or, again if you are 'tronics cute, power it from the HT supply in the radio with a resistor chain and a zener diode.

There is another way to skin this cat. A resistive load to replace the speaker and then an attenuated, transformer isolated feed from that.

I can do you a couple of scrawls if you like?

Dave.
 
Hello ecc83!
Thanks for your reply. I have used old tube radios as guitar and harp amps for almost forty years just plugging into the banana gram sockets. These radios are generally not grounded, but I read from somewhere that there are two kinds of tube radios. The older ones you wouldn't want to use as an amp (risk of electric shock) and the newer ones which work fine, yet are quite low in volume without preamp (like boost or distortion pedal). I do have Teac PA-4 phono amp unit (with ground pin) and TCC TC-400 phono pre-amp lying around without use. Would these work anyhow in my case? How about lowZ to hiZ transformer? I've been using one to use SM57 with guitar amp with harp. Any scrawls would be great :-)

Edit: There is an earth line in the mains supply in Finland.
 
I doubt anything you have suggested will work all well. The detector stage of a radio really does need to see a very high impedance.

I will draw you something up over the weekend.

Dave.
 
Morning Arc'
Yes those "AC-DC circuits were common over here 40 or so years ago but are illegal now.
As you say, it was possible to get a live chassis which at 115V was not fun but at 230V is often not survivable!

You COULD build in a FET follower driving a proper isolation transformer such as one rated for telephone lines but I do not like the idea one bit! Better to just use a 230:230 isolation traff . But then AC-DC radios, in UK at least, were the really cheap end of the market. Can't recall an FM one for instance so I cannot see getting the audio out would of much use?
Squiggles soon. Promise!

Dave.
 
Ah! Now! Look at the grid circuit of that 12SQ7. Very high resistances and the stage is an example of "leaky grid biasing" . Did away with the need for a diode detector and cathode bias components. The audio would be taken from the junction of the 50k and 3m3 resistors and an input Z for the buffer would best be some 5meg ....Geeez! Been a long, long time since I did any of this stuff!


Dave.
 
Hi guys!
Looks like you know your stuff. If you can find a way to do this I have couple of friends who are interested. I have five tube radios and the one I've been working on lately is Grundig 3055 3D Klang. I don't have a schematic for it, but I know a guy who restores tube radios as a hobby and he can build me anything you come up with if it's too complicated for me.
The current here in Finland is 220 - 225V.

Cheers! Matti
 
Hi again,
I hope my attached scribblings are clear.

Top it where you pick off the radio signal to feed the buffer. Note you don't need to cut any wires in the radio, just "tack" onto the pot hot and ground.

Two buffer circuits. The simple FET follower uses any old audio JFET, the ubiquitous 2N3819 would be fine. Hopefully Vsce will come out at about 1/2 18V but a volt or two either way should not matter.

The Zener supply was calculated for 5mA and 300V in the radio. Tap into the HT at a point downstream of the op transformer for minimum hum. If the HT is lower than 300V the 56k can be lower. Not likely to be higher.

A much more linear buffer is made with an op amp TL071. Naturally things also get more complex.

Any questions??? Of course! Fire away.

Going to have a look for that map.

Dave.
 

Attachments

  • radio buffer.pdf
    25.7 KB · Views: 19
Can't get the schematic without downloading another blessed toolbar..No Thanks!

Have discovered however that it is a very top of the range radio (in its day) and should give good quality sound from the FM part. N.B! I read that it uses a Selenium rectifier. Fine as long as it works but you will know when it fails because it will stink the place out!
Replace with Silicon diodes or a bridge but since theses are much more efficient than Se, your tech must limit the input AC to ensure the safety of the electrolytic caps (or change those as well, not a bad idea to be honest).

Dave.
 
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