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Old 10th Jul 2017, 11:48 am   #1
bobskie
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Default First Radio Build - 12V HT system

Hello everyone, I'm very new to radio technology and don't fully understand how it all works yet, what better way to understand than building my own radio? I have decided to use 12V for this project so it's non lethal incase I make a mistake.
I found this circuit which uses a 12v solid state amplifier
chip, that seems like cheating to me so I was wondering if I could use this circuit to amplify the signal. I received a load of TV valves so this is perfect. Another problem I have is that I only have 1 ECC 82 valve, could I use an alternative in any of the two places it's used? I will have a look through the box for any that would work. I will no doubt need more help as the project proceeds but I'm hoping my understanding of radios will increase. Thanks,

Cameron
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Old 10th Jul 2017, 11:53 am   #2
peter_sol
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Default Re: First Radio Build - 12V HT system

The ECC82 is 2 valves in one so you only need one.
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Old 10th Jul 2017, 11:56 am   #3
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Default Re: First Radio Build - 12V HT system

Quote:
Originally Posted by peter_sol View Post
A 12AU7 is an ECC82
I know I only have one ecc82 valve though, I have just reread the amplifier instructions however and it says an ecc81 can be used instead which I do have so that should be fine.
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Old 10th Jul 2017, 12:21 pm   #4
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Default Re: First Radio Build - 12V HT system

Use of a solid state amplifier is, IMHO, a bit of a cheat though still an interesting introduction to valve technology.

HT voltages such as 90 volts from batteries are low risk, it is no doubt possible to kill yourself with a 90 volt battery but I have never heard of it happening !

Mains voltage is potentially more dangerous and can not be recommended for the complete novice without help or guidance from someone more experienced.
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Old 10th Jul 2017, 12:48 pm   #5
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Default Re: First Radio Build - 12V HT system

The problem is finding a 90v battery or building one, it seems like a lot of work when an alternative could be used. I think I would probably be ok with mains I'm not a complete novice when it comes to electricity but still I would rather have a less deadly voltage for my first build.
I may just build the solid state version first and see if it actually works then add on the amplifier later if it's even possible.
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Old 10th Jul 2017, 5:02 pm   #6
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Default Re: First Radio Build - 12V HT system

I think it is probably a good idea to use a solid state amplifier, although I would go with something like an old germanium design with transformers (similar to early transistor sets) if parts or a module are available to keep a vintage feel.

The trouble with valves at low voltages is not so much that the gain is down, but that they will not supply much in the way of power as they are running at minute currents. This makes driving a speaker problematic, whereas transistors handle the lower voltages well.

Working with that handicap can turn a 'toe in the water' into a bit of a survival exercise.

For a more traditional approach I would agree with 90V (which can be reduced to maybe two thirds or less of that if you don't need too much volume) and use a valve which has been designed for that voltage in a fairly traditional setup (say a DL96). Note that this would complicate the heater supply though with the lower voltage heater.

Personally, as a first or early project I would stick with the given solid state but design in the option to replace it later when the circuit has been tested as you suggest. I use a small audio amp (although early germanium) in a separate case for test purposes. You may wish to do similar by separating the amp out from the unit for use on other projects.
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Old 10th Jul 2017, 7:39 pm   #7
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Default Re: First Radio Build - 12V HT system

Yes I think I'll give the original design a go first, I have plenty valves to move onto more complex designs later. It will give me a bit of practice on winding coils etc.
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Old 10th Jul 2017, 8:09 pm   #8
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Default Re: First Radio Build - 12V HT system

You could try some 12volt space charge valves for the output if you can come across them for reasonable money, 12K5 etc.

Lawrence.
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Old 10th Jul 2017, 8:39 pm   #9
broadgage
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Default Re: First Radio Build - 12V HT system

Building a 90 volt battery that looks like an original is fairly time consuming, but does look the part in a vintage set.

Simply connecting a number of 9 volt transistor batteries in series is very quick and easy, you will of course need ten for the full voltage but as has been posted by others, a lower voltage will suffice if full audio volume is not required.
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Old 10th Jul 2017, 9:55 pm   #10
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Default Re: First Radio Build - 12V HT system

bobskie- before venturing into the world of valves, ( and this is said without criticism) etc)might I suggest you look out for a valve manual/s(Brabinski used to publish these) ,where the name of the valve ( in this case ECC82) , gave a good insight into the data of the valve. I'm a bit rusty on this, but I believe E =6.2V HEATER, C=TRIODE (CC MEANT DOUBLE TRIODE) , and 82 referred to other parameters. Again from memory, I recall that the ECC82 needed a HT of circa 200-250 Volt.
For 90v HT- you will ( I think ) need to look at the D range. Must be plenty of old circuits out there- PYE used to produce a multitude of designs, but the aerial coils might prove a problem.
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Old 10th Jul 2017, 11:27 pm   #11
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Default Re: First Radio Build - 12V HT system

Many later valve car radios used "selected" current valves, e.g. ECH81 became ECH83
with a 12 volt HT rail and could perform well. A solid state step up inverter module for
60-120 volts output is a much cheaper alternative to multiple batteries.
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Old 11th Jul 2017, 11:14 am   #12
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Default Re: First Radio Build - 12V HT system

Another low voltage valve is the ECC86, it's 6.3 volt heaters only though:

http://www.r-type.org/pdfs/ecc86.pdf

Lawrence.
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Old 14th Jul 2017, 8:53 am   #13
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Default Re: First Radio Build - 12V HT system

Thanks all I'll give it a go with the solid state amplifier first and see how that goes. I'll need to check if I have any ecc86's I spent a whole day cataloguing them all!
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Old 15th Jul 2017, 5:14 pm   #14
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Default Re: First Radio Build - 12V HT system

Ok so I have just ordered all the parts for this but there are a few things I don't understand in the circuit diagram. I cannot understand where the negative connects on the circuit diagram. Also I am unsure about what the symbol on the right near the speaker is, is it supposed to be a switch? I have attached a picture of the diagram, thanks for the help. EDIT - the picture seems to go strange when opened, try double clicking to open in a new tab which works or if you are on mobile hold down and click open in new tab.
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Old 15th Jul 2017, 5:40 pm   #15
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Default Re: First Radio Build - 12V HT system

The -ve of the supply should connect to the bottom rail, the one that the earth symbol is connected to.

That symbol near the speaker is a switched jack socket for an external speaker, when an external speaker's jack plug is plugged in to that socket, the speaker shown on the schematic is disconnected and the audio output from the receiver is fed to the external speaker.

Lawrence.
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Old 15th Jul 2017, 6:34 pm   #16
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Default Re: First Radio Build - 12V HT system

Thanks for the help, I had a suspicion that's where the negative connected but I wasn't sure. I'll need to order up a switched headphone jack it would be handy having the option for a better speaker.
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Old 15th Jul 2017, 7:16 pm   #17
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Default Re: First Radio Build - 12V HT system

I have one more question(for now). Can I salvage a ferrite antenna from another radio and use this instead of making my own? Would I just connect up the mw portion of the antenna and that would work? I have an old broken clock radio I can get one from, probably not the best source but I'll be using an external antenna mostly I think.
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Old 15th Jul 2017, 8:37 pm   #18
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Default Re: First Radio Build - 12V HT system

The regen detector as designed in the schematic uses a cathode tap on the coil for the regen feedback, if you want to use a ferrite rod with it's MW coil then you will have to provide a tap on the coil for it to work in that circuit configuration, with an air cored coil the tap would be typically 25 to 30% up from the earthy/ground/chassis end, with a ferrite core (the ferrite rod) you might not need the tap as far up, you could try 10 or 15%.

If you can't manage that then another way would be to wind another coil on the ferrite rod adjacent to one end of the existing MW coil, wind it in the same direction as the existing, connect the two in series and use the series connection as the tap, the other end of the added coil will be the ground/chassis connection, you might have to remove a few turns from the other end of the original coil in order for the MW band to be tuned in full.

Just to note that according to a rough calculation the coil shown in the schematic that was posted will tune from approx 3.7MHz to 12 Mhz if the antenna is coupled lightly, it doesn't cover the MW band.

Lawrence.
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Old 15th Jul 2017, 9:02 pm   #19
bobskie
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Default Re: First Radio Build - 12V HT system

Ok I think I have a better understanding of this now, in the circuit is one wire connected 1/4 of the way up the coil in order to provide a tap? If that's correct then I could adjust the coil to fall within the MW range? I will probably not bother with the ferrite rod if that's the case. Sorry I'm very new to all this but I'm sure I'll pick it up eventually.
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Old 15th Jul 2017, 10:04 pm   #20
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Default Re: First Radio Build - 12V HT system

""Ok I think I have a better understanding of this now, in the circuit is one wire connected 1/4 of the way up the coil in order to provide a tap? If that's correct then I could adjust the coil to fall within the MW range? I will probably not bother with the ferrite rod if that's the case. Sorry I'm very new to all this but I'm sure I'll pick it up eventually.""

Yes the cathode is connected 25% up from the ground end of the coil in the schematic.

The coil in the schematic will not cover the MW band, here's why, the coils inductance is given as 5uH (5 microhenries) the tuning capacitor has a maximum value of 365pF (365 picofarads) the minimum value isn't given but it will be around 30pF or so when all's done, plug those values into a resonant frequency calculator like what's in the link below, the two different resonant frequency results for a capacitance of 365pF and a capacitance of 30pF will be the approx tuning range limits of the tuned circuit using the values given above, eg: approx 3.7MHz to 12.9MHz:

http://www.pronine.ca/lcf.htm

For MW you will need a larger inductance, to get an idea of what value you can use the same calculator, you already know the approx minimum and maximum capacitance values.

For a single layer air cored coil you can use the calculator in the link below, for instance 131 turns of 26 SWG enamelled wire close wound on a 35mm diameter former will give an inductance of approx 273uH which would put you in the ball park for MW coverage using the 365pF variable capacitor:

http://www.66pacific.com/calculators...alculator.aspx

Lawrence.

Last edited by ms660; 15th Jul 2017 at 10:25 pm.
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