[Amps] 4pr1000 heater voltage?

[Dr. David Kirkby]

gd0tep wrote:
> 
> Hi,
> 
> My specs say that the 4pr1000 tube needs 7.5v for the heaters, I've got a
> transformer from Dhal, but on testing the voltage with the tube in the deck
> I see I'm getting about 8.3v
> 
> Is this to high?
> 
> Andy
> GD0TEP

I think the tolerance is normally +/- 5% (that should be on the data
sheet), so the answer is yes. 

You can easily add an external resistor in series with the heater. I
don't know what the heater current is, but if you need to drop 0.8V,
R=0.8/I, where I is the current in amps, with a power rating of 0.8*I*I,
where I is the current in Amps. 

Assume I is about 8 Amps (guessing off the top of my head), you will
need something around 0.1 Ohms at 7 Watts, but I would do the maths and
work it out. 

It might be more convenient to add the resistor in series with the
transformer's primary, where the resistance needed will be higher in
value, but will produce a bit (not much) more heat. The current in the
primary will be about 1/30th of the secondary current (since you are
stepping down the voltage by about 230/8.3 ~ 30), so the value of
resistor will be about 30x larger -i.e. something around 3 Ohms. 

Hence a 2.7 Ohm resistor in the primary or 0.1 Ohms in the secondary
will get you somewhere near what its required. But do the maths, as I
don't know the heater current and have not bothered using a calculator.
There are only guestimates. 

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Dr. David Kirkby PhD,
Senior Research Fellow,
Department of Medical Physics,
University College London,
11-20 Capper St, London, WC1E 6JA.
Tel: 020 7679 6408 Fax: 020 7679 6269
Internal telephone: ext 46408
e-mail davek at medphys.ucl.ac.uk  
Web page: http://www.medphys.ucl.ac.uk/~davek