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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 1st Dec 2021, 9:43 pm   #1
Ed_Dinning
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Default Saving sets from butchery

Hi Folks, for some time now there have been a number of "antique" dealers and Steam Punk fans who would take an old valve radio, rip the guts out and fit a module or Bluetooth device to allow music to be played over the speaker in the set.
Direct connection, even when PU sockets are fitted will often not work as there is insufficient signal to drive the amp to sufficient volume.
A little design work has come up with a modern version of the "intervalve" transformer shown below. This is wound for a 4:1 ratio and gives sufficient volume for most sets and allows the set to be both used as intended and as a means of playing music from your phone.

Note that while it provides isolation it is NOT intended to be used on AC/DC sets with live chassis such as a DAC90/a; a totally different design would be needed for that application.

In use the signal from the phone is fed into the primary of the transformer, the secondary is connected across the volume control pot. This small transformer can then easily be mounted in the rear of the set.

Enjoy your digital music !

Ed
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Old 1st Dec 2021, 11:41 pm   #2
turretslug
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Default Re: Saving sets from butchery

That's a really nice idea Ed, a straightforward-to-use passive step-up that suits both modern source device and high-impedance classic circuitry, and has "vintage approach" appeal. I reckon it should be pretty popular here

Colin
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Old 2nd Dec 2021, 12:53 am   #3
dave walsh
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Default Re: Saving sets from butchery

That's a very clever/simple notion that Ed has come up with there. I can see why he doesn't recommend it should be used with a live chassis but it does remind me of recording from an AC/DC Television [they all were] when I was about 14 in the sixties! It was a bit of a dream to be able to get a direct feed, rather than holding a microphone in front of the speaker [very crude]. I think it was Practical Television that recommended using an inverted radio output TR [secondary side across the across the TV spkr] to provide a step down match to the Tape Recorder and isolation from the live chassis as well. [I always had a light bulb clipped across earth to the chassis anyway to ensure it was negative].

The TV was rented and I had to remove the audio output valve to connect up. Eventually I stood on it one day and then had to shovel the bits out of the carpet and into the back of the set. The Service engineer was very suspicious of this "fault" [more related to pouring cold water on a red hot valve]. I told him my mother had just said that there had been a bit of a bang and played the innocent. I think I gave the game away by my interest in his work though! Eventually he gave me an "old fashioned" look, replaced the valve and left. It was all a very big deal in those far off days

Dave W

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Old 2nd Dec 2021, 6:22 am   #4
Gabe001
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Default Re: Saving sets from butchery

Ed, can I quickly clarify

If this goes between the pickup/phone input and the grid of the first valve in the output stage in a radio or amplifier, will it increase the input sensitivity fourfold without any negative effects?

How big is it?

Gabriel
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Old 2nd Dec 2021, 9:15 am   #5
Ed_Dinning
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Default Re: Saving sets from butchery

Hi Folks, It is wound on an EI 48 core so just under 2" long.

For use on a live chassis set it should be designed to give the correct creepage and clearance, so would be on a double section bobbin. It should also be certified as meeting CE standards. This would slightly reduce the coupling between the windings.

In theory it would boost the signal bt a factor of 4, but due to loading effects it could be a bit less. I have used it with a switch to prevent it loading the radio signal when in use.
I've also added a 0.1 uF cap as a DC blocker to prevent it altering the bias on the OP valve which it may do in some circuits.


Ed
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Old 2nd Dec 2021, 10:36 am   #6
bobhowe
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Default Re: Saving sets from butchery

Hi Ed you have done a great job hopefully this will cutdown the bluetooth speaker hipster muppets in ruining sets .
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