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Old 16th Apr 2006, 11:50 am   #1
DoctorWho
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Default Graves Vulcan 2 - How to change the tuning range

I have a Graves Vulcan 2 battery set which works very well, I can pick up a variety of stations on it, the strongest being Radio 5 Live, but I cannot get my own local BBC Radio devon on 375 metres as this seems to be just outside of the range of the set.

Can anyone suggest how I could adjust the range slightly to allow me to receive BBC Radio Devon without having to make any substantial changes to the set please?

Many thanks,

Peter.
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Old 16th Apr 2006, 12:05 pm   #2
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Default Re: Graves Vulcan 2 - How to change the tuning range

try adding a small variable capacitor in line with the aerial. Sometimes tuning the aerial helps.

Not sure I would want to do anything to that radio, Its too good.
Does it have a plug in coil? perhaps you could wind one specially
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Old 16th Apr 2006, 12:53 pm   #3
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Default Re: Graves Vulcan 2 - How to change the tuning range

I don't know anything about this set. Is it a superhet or TRF?

Is 375 metres (800 Kc/s) above or below the present tuning range? If the lowest frequency tunable is above 800 Kc/s you'll need to add a capacitor in parallel with the tuned circuit(s). On the other hand if the highest frequency tunable is below 800 Kc/s you'll need to remove some capacitance or inductance.
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Old 16th Apr 2006, 1:26 pm   #4
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Default Re: Graves Vulcan 2 - How to change the tuning range

Thanks for the input. I've attached a few pictures. The set doesn't have a plug in coil, and thankfully it hasn't been modified at all since it was made, it all looks totally original and works extremely well. I don't know what frequency Radio 5 Live is on, but in this area BBC Radio Devon is on 375 letres, I suspect that the set needs to tune down lower than it currently does.

The set is a 2-valve TRF from the mid 1920's and uses a PM1HF and a PM2 valve. It's housed in an oak cabinet with an Ebonite panel at the front.

It has "Short" and "Long" wave, short wave is of course medium wave. What sort of value variable ondenser would you consider using with the aerial? I certainly don't intend to modify the set at all, but I don't mind connecting something externally, or even across the tuning condenser so long as it can be removed.

I was amazed actually at the qualitry of the sound from such an early set. It would just be nice to be able to pick up the local BBC radio Devon transmissions. The signal is pretty strong on 375 metres, so if I can just get the set within that range it should work well.

Peter.
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Old 16th Apr 2006, 1:36 pm   #5
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Default Re: Graves Vulcan 2 - How to change the tuning range

BBC Radio Five Live is on 909 & 693 Khz Medium Wave, or in old money 330 metres and 433 metres.
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Old 16th Apr 2006, 1:42 pm   #6
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Default Re: Graves Vulcan 2 - How to change the tuning range

That's interesting. It seems that the set was originally designed to receive 2LO and 5GB, I'm not sure what frequencies they operated on in 1926-8.

The original tuning dial is calibrated in degrees and goes from 0 to 180, I am picking up BBC Radio Five Live at 80 degrees, so would you imagine I'm receiving the 330 metres or the 443 metres? I'd guess at 433 metres, but that is only a guess.

What sort of value condenser would you advise using to try and lower the range, and where would you try fitting it?
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Old 16th Apr 2006, 1:52 pm   #7
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Default Re: Graves Vulcan 2 - How to change the tuning range

Quote:
What sort of value condenser would you advise using to try and lower the range, and where would you try fitting it?
It's difficult to say without seing the circuit, knowing what the value of the tuning capacitor is and knowing the present coverage.

To establish the present coverage you need to identify stations at the extreme ends of the scale and know their frequencies. It's nice to talk in metres, but using Khz makes calculations so much easier.

A suck it and see approach would be to connect a 50 pF capacitor (or whatever similar value you have) across the tuning capacitor and see how this changes the coverage. You can then increase or decrease the capacitor value as required.
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Old 16th Apr 2006, 1:59 pm   #8
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Default Re: Graves Vulcan 2 - How to change the tuning range

Quote:
Originally Posted by DoctorWho
That's interesting. It seems that the set was originally designed to receive 2LO and 5GB, I'm not sure what frequencies they operated on in 1926-8.
2LO was on 361 metres in 1922 and 5GB on 491.8 in 1927.

See Radio Radio 3rd edition Page 228.
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Old 16th Apr 2006, 5:36 pm   #9
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Default Re: Graves Vulcan 2 - How to change the tuning range

Well I think the tuning condenser is 0.0003, that was what it looked to be stamped with.

Interesting about the frequencies of 2LO and 5GB, theoretically it should then tune in to 375 metres surely?

I'll experiement with some low value condensers and see what happens.

Thanks for the advice.

Peter.
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Old 16th Apr 2006, 6:02 pm   #10
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Default Re: Graves Vulcan 2 - How to change the tuning range

Just a thought, Is your aerial horizontal ?

If so you could be at 90 deg to the signal in which case you will be in the null and reception will be weak try a trinmmer cap in series with the aerial any thing around 30pF will be enough
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Old 16th Apr 2006, 6:23 pm   #11
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Default Re: Graves Vulcan 2 - How to change the tuning range

Thanks for the suggestion, yes the aerial is horizontal. When yoy suggest putting in a trimmer condenser in series with the aerial, do you literally mean between the aerial and the receiver, or between the aerial and earth?

Peter.
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Old 16th Apr 2006, 6:33 pm   #12
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Default Re: Graves Vulcan 2 - How to change the tuning range

Hi Peter

I notice from the circuit for this set that there are 2 places you can connect the aerial.

One aerial socket goes to the top end of the medium (short) wave coil and one to a tapping on that same coil.

If you connect to the latter socket the capacitance presented to the set by the aerial will have a smaller effect on the tuned circuit and allow you to tune slightly further up the band.

Just how much difference this will make in practise is outside my experience, but it's worth a try.
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Old 16th Apr 2006, 8:45 pm   #13
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Default Re: Graves Vulcan 2 - How to change the tuning range

Hello Colin. Well I have tried this, in the Aerial 1 socket the set is pretty sensative, but in the aerial 2 socket it is not very sensative and only picks up a couple of stations. Thanks for the suggestion though.
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Old 18th Apr 2006, 11:17 am   #14
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Default Re: Graves Vulcan 2 - How to change the tuning range

Hi Peter -

As I'm sure you are aware, the tuning range of sets like these (where a single tuning coil is connected directly to the aerial) is greatly affected by the type of aerial and earth you are using.

The natural aerial / earth capacity of the aerial is effectively added to that of the variable tuning capacitor. This extra capacity is affected by the height, length and proximity of the aerial to buildings etc.

I have Vulcan V2 like yours, and I checked the tuning range using my signal generator. The results were:

No aerial connected
===============

Short wave: 1500 kHz - 670 kHz (about 200 - 450 metres)
Long wave: 230 kHz - 140 kHz (about 1300 - 2140 metres)


With my aerial and earth connected to top of tuning coil
==========================================

Short wave: 1000 kHz - 550 kHz (about 300 - 545 metres)

With this arrangement the selectivity on Long wave was so poor I couldn't get a meaningful reading of range!

If you want to tune to higher frequencies, you can use a shorter aerial or put a trimmer (say 50 - 500 pF) between the aerial and your set. Both these ideas will also increase selectivity, but decrease the volume. If you want to tune to lower frequencies, put a capacitor or trimmer (say 50 - 200 pF) across the aerial and earth connections of your set.

Hope this helps

Robert
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