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 Post subject: Re: FM Tube Radio
PostPosted: 10 Dec 2010, 20:47 
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Joined: 14 Feb 2010, 13:13
Posts: 663
Location: Lisbon, Portugal
Some aspects and updates...

Well the receiver is working. I had almost every part needed for this one. It was quite a job, and a real pain to tune-up (without some special equipment).

As said, it is working. I was expecting some better performance. It gets almost every FM station with a single 1 meter wire, but to get rid of background noise it really needs a very strong signal.

The audio is not as good as a cheap pocket radio (this is not a superheterodyne).

Right now i´m having some fun, to expand the complete FM band to work with fully variable capacitor turns (it´s kind of compressed), but i´m inside this subject.

What I really can´t get rid off?

HUM!!!

I hate hum, and there is some hum in the background, though it does not get louder by cranking up the volume.

It does get louder, when the volume potentiometer is near minimum and does not go away when fully CCW (grounded).

There is a little hum on the 12AV6 output (very light).

The hum does not go away even with no sound input on the 30A5 grid (disconected from the 12AV6).

I followed an AA5 schematic for the audio based on a 12AV6 and 30A5 ( I also have a 50C5).

The PSU consists on a 220/110v step-down transformer and a half-wave 1N4007 diode, having the chassis (B-). Heaters are AC in series.

Please help me to get rid of HUM, to keep moving-on into a tuned RF pre-amplifier...

Some pictures attatched!


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 Post subject: Re: FM Tube Radio
PostPosted: 10 Dec 2010, 23:09 
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Joined: 05 Jan 2010, 21:16
Posts: 680
Location: Las Vegas Nevada USA
How about DC heaters? For $10 you can get an SMPS to test out that theory. If it works and you don't like SMPS you can construct a regulated supply with a filament transformer, rectifier and regulator, or whatever DC supply you like the most.

Also, why the half wave rectifier? Wouldn't that be harder to filter than a full wave? Do you have any way of measuring your power supply ripple?

The hum could also be directly induced by the power transformer. Simply moving the transformer a couple of inches or rotating it while listening will tell you if that's happening - that's an easy test if you have a little slack in the wires.

Just some thoughts. Cool project. I've looked at some schematics of good radios and I don't even want to go there. Tuners are hard to build. Transmitters are easy. Tuners are much more difficult than both transmitters or audio amplifiers IMHO. To build a good one would be a very difficult project, and then most people don't have a way to tune the filters and make all the critical adjustments necessary for quality sound.

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 Post subject: Re: FM Tube Radio
PostPosted: 11 Dec 2010, 10:18 
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Joined: 26 May 2009, 18:51
Posts: 130
Location: Lilburn, Georgia USA
Hello, I have purchased on ebay in the last few days 5 different tube type tuners and all are very high quality units and were quite cheap. I am sure some will need some new caps, especially the two Heathkit ones. Also got a Eimac, an old Realistic, two ss tuners one NAD and one Proton. I am set up to tune now!


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 Post subject: Re: FM Tube Radio
PostPosted: 11 Dec 2010, 11:31 
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Location: Las Vegas Nevada USA
rparsh wrote:
Hello, I have purchased on ebay in the last few days 5 different tube type tuners and all are very high quality units and were quite cheap. I am sure some will need some new caps, especially the two Heathkit ones. Also got a Eimac, an old Realistic, two ss tuners one NAD and one Proton. I am set up to tune now!

Wow, yes, I agree, you are set up to tune! You're as crazy as I am with all the tubes I ordered.

I bid on a Scott tuner but I wasn't willing to go high enough and somebody else got it. :bawling:

There are some inexpensive tube tuners available on ebay. The HeathKit should have the original construction manual with full instructions for tuning it up available, probably on ebay.

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 Post subject: Re: FM Tube Radio
PostPosted: 11 Dec 2010, 14:38 
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Joined: 14 Feb 2010, 13:13
Posts: 663
Location: Lisbon, Portugal
I´ve been playing with the PSU designer from Duncan´s.

Modeling the original power supply circuit (some posts ago in the thread), gives a very bad filtering specially for the power amplifier tube. I wonder if someone can help me to design a simple and better PSU for my tube radio? The ripple is huge! So HUM is big for sure...

Cheers in advance,
Miguel


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 Post subject: Re: FM Tube Radio
PostPosted: 11 Dec 2010, 15:53 
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Joined: 06 Apr 2009, 10:08
Posts: 1497
Location: US Pacific Northwest
Which power supply are you using? This one...
Attachment:
ps1.png
or this one...
Attachment:
ps2.png


I hope it's first one and not the second. I am assuming that it's a supply like the first one with a transformer and a full wave rectifier. I will recommend that you add a choke. The filter capability of an LC section is vastly superior to an RC stage.

For an RC stage the ripple reduction factor is approximately 2πfRC+1 where f is your ripple frequency (usually 2x the mains frequency). For an LC section the ripple reduction factor is approximately ((2πf)^2)*LC-1. So lets run a comparison. For 60Hz mains, a 100µf capacitor and either a 150Ω resistor of a 5H choke the ripple reduction factor for the RC stage is 2*π*120*150*100e-6+1≈12.3 but for the LC section it is ((2*π*120)^2)*L*C-1≈283.2. This is an improvement of 23x simply by replacing one resistor with one moderate sized choke.

Also, the choke has the added benefit of attenuating higher frequency noise. At 1kHz that same 5H choke has a series reactance of over 31kΩ. As such there really is nothing to be gained by adding snubber caps across your downstream filter caps so this also simplifies your design.


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 Post subject: Re: FM Tube Radio
PostPosted: 11 Dec 2010, 16:51 
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Joined: 14 Feb 2010, 13:13
Posts: 663
Location: Lisbon, Portugal
Hi Sun!

Thanks for your quick and good reply. I am using the 2nd schematic PSU.

Firstly running one 1N4007 diode as a rectifier, a couple of minutes ago, I decided to swap for a 35W4 tube rectifier (still half-wave) just for fun. I don´t know why, but things got a lot better, but still away from acceptable.

I don´t have much space for a big choke, neither I have one. Although I have a lot a dust cores (ferrite) and enameled wire? I also have some ordinary 12v and 24v transformers laying around, but without having a capable system to measure their inductance.

Cheers,
Miguel


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 Post subject: Re: FM Tube Radio
PostPosted: 12 Dec 2010, 02:25 
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Joined: 05 Jan 2010, 21:16
Posts: 680
Location: Las Vegas Nevada USA
HT Performance wrote:
Hi Sun!

I also have some ordinary 12v and 24v transformers laying around, but without having a capable system to measure their inductance.

Cheers,
Miguel

Yes you do if you have a multimeter and and any sort of sine wave generator. I think this is really cool - it's too easy:

http://technologyinterface.nmsu.edu/fal ... nduct.html

Also, VA rating of a transformer will tell you the current rating on each winding, and if the DC resistance is acceptable then you really don't have to know the inductance - just use it. It's better than a resistor of the same value in any case.

BUT, now you have a way to measure inductance with only an AC voltmeter and a sinewave function generator, and you can use any 120 volt primary transformer with a low voltage secondary for your "function generator" so really all you need is an AC voltmeter. A scope would be better, but even some cheap meters read RMS voltage correctly for sine waves, but sine waves only.

Now why the heck are you using a half wave rectifier? A full wave will recharge the cap twice as often and the ripple will go way down just from that one change. Then you can address the remaining hum. That's what I would do anyway.

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 Post subject: Re: FM Tube Radio
PostPosted: 12 Dec 2010, 07:34 
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Joined: 14 Feb 2010, 13:13
Posts: 663
Location: Lisbon, Portugal
That´s a nice way to measure inductance! My homemade capacitance/inductance meter has excellent precision, but it was built for Ham use, and so only up to 1H readings.

The reason for half-wave rectifier, is only because I have several AA5 tubes laying around and wanted 100% tube tuner. Maybe I should get an EZ80? What do you recommend?

Thanks so much for helping! :)

Cheers,
Miguel


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 Post subject: Re: FM Tube Radio
PostPosted: 12 Dec 2010, 19:01 
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Joined: 14 Feb 2010, 13:13
Posts: 663
Location: Lisbon, Portugal
Hurray!!!

I managed to send the badly HUM away! It was all about AC heaters! It´s impressive as my Oddwatt is running AC heaters and there is absolutely no HUM at all! So from now on, DC heaters will always be present in further RF projects.

But after all that, the project review is bad.

Reception is noisy with some distortion, I always knew from the beginning that this wasn´t a Hi-Fi project, so i´ll give a 9 from a 0-20 scale.

I´ll keep trying to improve it as I still want a 100% homemade tube FM tuner. And this means that I may go on for a real superheterodyne receiver next year.

Thanks again to everyone! I´ll keep posting if some good improvements appear! :)

Cheers,
Miguel


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