[Boatanchors] (no subject)

[Barrie Smith]

Because, anymore, I've got more time than money!

73, Barrie, W7ALW

----- Original Message ----- 
From: "Bob Macklin" <macklinbob at msn.com>
To: "Drew P." <drewrailleur807 at yahoo.com>; <boatanchors at mailman.qth.net>
Sent: Sunday, April 18, 2010 7:51 PM
Subject: Re: [Boatanchors] (no subject)


>I have not seen anywhere that says how much the voltage is off.
>
> Why not just buy a good 5V filament transformer. Allied and Mouser have
> them.
>
> Bob Macklin
> K5MYJ
> Seattle, Wa.
> "Real Radios Glow In The Dark"
> ----- Original Message ----- 
> From: "Drew P." <drewrailleur807 at yahoo.com>
> To: <boatanchors at mailman.qth.net>
> Sent: Sunday, April 18, 2010 7:42 PM
> Subject: [Boatanchors] (no subject)
>
>
>
> Barrie Smith wrote:
>
>> I've been juggling 5 volt transformers all day, trying to
>> find one that would actually produce close to 5 volts for a
>> pair of tubes.
>>
>> They were all either too high in voltage, or too low.
>> One did give me almost exactly 5 volts, but it ran way too
>> hot.
>>
>> So I've selected one with plenty of current capacity and am
>> playing with resistance in the primary to get the secondary
>> voltage down to 5 volts.
>>
>> Using a 25 ohm, 150 watt rheostat, I find that 11 ohms gets
>> me real close. The rheostat runs quite warm, which
>> leads me to my question.
>>
>> The voltage drop across the rheostat is a measured 13
>> volts. Knowing that and the resistance, Ohm's law
>> tells me that I have 1.18 amps running through, which equals
>> 15 watts.
>>
>> I don't want to install the rheostat in the transmitter
>> because it is quite large, so I thought I'd substitute a
>> fixed resistor.
>>
>> I tried a 15 ohm, 30 watt fixed resistor. It got
>> very, very hot real quick!
>>
>> I must be making a mistake. I thought I should use
>> the "dropped voltage" in Ohm's law. Perhaps I must use
>> the full 115 volts, instead.
>
> Another method of reducing the secondary voltage would be to remove turns
> from the secondary winding, assuming that the transformer construction is
> amenable to this type of modification.
>
> I've done this to low voltage transformers on several occasions with good
> success.
>
> Drew
>
>
>
>
>
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