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 Post subject: First tube radio [WIP]
PostPosted: Jun Mon 03, 2019 10:17 pm 
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Joined: Nov Mon 16, 2015 1:44 am
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Location: UK, England
After many years of repairing vintage radios I've decided to have a go at making my own tube radio.
The design is a simple one tube regen using a 12AU7 I had laying around, after looking at a bunch of other peoples designs I came up with this:

Image

It could use some adjustment, I have no RF input coil because I forgot to add it and there is no more room left on my coil former, I'll probably wind a new one soon.
I haven't really thought too much about how I'm getting the audio out either, I have a high impedance ear piece for crystal radios that might do the job or I could make use of the other half of the 12AU7.
The regen control pot might also need to be increased, or the throttle capacitor adjusted, I can't quite stop the oscillation at the moment, at least I've verfied it covers the entire MW broadcast range 497 kHz to 1900 kHz.
Here's what it currently looks like, note that this is just for testing, I intend to make something much nicer for the final version.

https://i.imgur.com/qp9Bu9Q.jpg (linked due to size)

Any ideas or suggestions are very welcome.
(Also before you ask, I am using a variac and isolation transformer)


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 Post subject: Re: First tube radio [WIP]
PostPosted: Jun Mon 03, 2019 10:25 pm 
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Chryseus wrote:
"I haven't really thought too much about how I'm getting the audio out either,"


So .... how do you know if it works or not?
.... kinda like a radio controlled submarine... right? Who knows what it's doing 20 ft under water?

You could keep your home-brew radio like this and sell it to deaf people maybe?

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 Post subject: Re: First tube radio [WIP]
PostPosted: Jun Mon 03, 2019 10:43 pm 
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Pbpix wrote:
Chryseus wrote:
"I haven't really thought too much about how I'm getting the audio out either,"


So .... how do you know if it works or not?
.... kinda like a radio controlled submarine... right? Who knows what it's doing 20 ft under water?

You could keep your home-brew radio like this and sell it to deaf people maybe?


Well it doesn't quite work at the moment, I need to be able to fully stop the oscillation before there is any hope of getting an audio signal out, I may have placed the tickler coil slightly too close to the grid coil which would explain why it oscillates so easily.

Update

I've managed to get some sound out, bypassing the plate side L3 to ground with a piezo speaker I was able to clearly hear an AM modulated signal coupled in from a nearby wire, I also reduced the throttle capacitor to 470pF, although only about 10% of the pot is usable before it reaches oscillation, maybe this is normal I've never used a regen radio before.
Now that it actually works I'm going to concentrate on the audio output circuit, I think I will make use of the other half of the 12AU7.

Update 2

After connecting an antenna (VHF whip so not very suitable) I was able to receive the two strongest AM stations in my area, the audio was very hard to hear over the high pitched noise of the oscillation.
I'm going to wind a new set of coils so I don't have to directly couple the antenna to the grid, hopefully once I get that and the audio amp done the performance will be a lot better, probably should get a better antenna while I'm at it.
Here's the current layout:

Image


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 Post subject: Re: First tube radio [WIP]
PostPosted: Jun Sat 08, 2019 12:34 am 
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Time for a progress update, I made a new coil on a 55mm cardboard tube, not much difference in performance but I now have an antenna winding.
The audio amplifier stage has been added which sort of works, I don't think the audio transformer I have is suitable, it came out of an old battery set and I measured the turns ratio as 185 which unless I'm doing the math wrong that works out around 100k input impedance with a 3 ohm speaker (this also came from the same set), so while it works I can barely hear it, the gain of the audio stage may also be insufficient.
The biggest problem though is the antenna, I clearly need something better, best reception I've got is by holding on to the antenna connection :lol: , I'd prefer something compact but I guess a long wire is preferable ?

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 Post subject: Re: First tube radio [WIP]
PostPosted: Jun Sun 09, 2019 12:59 am 
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Location: Lexington, KY USA
The schematics don't really show a tickler coil, although L1 might serve as one if it is near L2.

Even without a tickler, it may be possible to get usable regeneration by making the plate load high enough Z at RF. There were radios that made a BFO out of the IF stage by putting a pot in series with the screen grid bypass capacitor. This scheme benefited from a fixed operating frequency. I suspect that large regineratioin adjustments might be required as the radio is tuned.

You will get much more audio out of the detector by providing a higher audio load impedance at the plate of the tube. Try increasing R5 (now 1k) to 100k or even 220k. Increasing the grid resistor on the audio amplifier (R6, now 220k) will then help also. 1 Meg might be OK for R6.

The 12AU7 will drive a speaker, but you need to get the load impedance right for a useful amount of sound. I can't imagine any radio audio output stage using a 100k output transformer. Surely there is a bad transformer, or some sort of measurement error. What sort of audio output tube and B+ voltage were used in the donor?

Ted


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 Post subject: Re: First tube radio [WIP]
PostPosted: Jun Sun 09, 2019 3:47 pm 
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Usually Lurking wrote:
The schematics don't really show a tickler coil, although L1 might serve as one if it is near L2.

Even without a tickler, it may be possible to get usable regeneration by making the plate load high enough Z at RF. There were radios that made a BFO out of the IF stage by putting a pot in series with the screen grid bypass capacitor. This scheme benefited from a fixed operating frequency. I suspect that large regineratioin adjustments might be required as the radio is tuned.

You will get much more audio out of the detector by providing a higher audio load impedance at the plate of the tube. Try increasing R5 (now 1k) to 100k or even 220k. Increasing the grid resistor on the audio amplifier (R6, now 220k) will then help also. 1 Meg might be OK for R6.

The 12AU7 will drive a speaker, but you need to get the load impedance right for a useful amount of sound. I can't imagine any radio audio output stage using a 100k output transformer. Surely there is a bad transformer, or some sort of measurement error. What sort of audio output tube and B+ voltage were used in the donor?

Ted


L1 is the tickler coil, it's about 1/4" from the grid coil.
I will try increase R5 and R6 and see how it performs.
The output tube used in the donor radio is the DL96 which barely qualifies as an output tube with 0.2W max output :o , B+ I can't exactly remember but I'd assume 90V, the datasheet gives a plate resistance of about 20k.
I went and repeated the measurements and got different results so I must have screwed something up, it's 17.6k at 4 ohm, so I'm not exactly sure why they chose a 3 ohm speaker.
I think the main cause of the trouble is the antenna, it sounds a lot louder with a strong signal so I need to get something suitable built.


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 Post subject: Re: First tube radio [WIP]
PostPosted: Aug Wed 07, 2019 8:47 pm 
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Location: West Point, PA 19486
Chryseus wrote:
The regen control pot might also need to be increased, or the throttle capacitor adjusted, I can't quite stop the oscillation at the moment


Attachment:
RegenRadioEdit.png
RegenRadioEdit.png [ 34.16 KiB | Viewed 476 times ]

Try the regen control this way. It worked for me.


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