[Amps] Filament transformer needed

[Roger (sub1)]

On 5/25/2011 10:18 AM, Glen Zook wrote:
> The main reason for wanting some "head room" is to prevent excessive transformer heat.  Being able to control the heater voltage (the GS-31b has a separate cathode) is just the "icing on the cake".  In the pair of GI-7bT amplifier that I built for 160-meters only a few weeks back the filament transformer that I used is rated at 5 amps and the tubes draw just over 4 amps.  Although not hot enough to "burn out", the transformer is a "bit" warmer than I like.

Modern transformers and materials may be rated at between 80 and to over 
100C temp rise at the rated power so that transformer *may* have gotten 
hot enough to burn you and still been within ratings at 5A.
Of course under the normal load a transformer with lots of head room may 
not draw the voltage down enough to stay within the tube's filament 
ratings.  Depending on the core material some of them may get hot just 
at idle.

73

Roger (K8RI)
> Also, a 10 amp transformer is pretty common and should be much more available than most others.
>
> Here is the 160-meter amplifier
>
> http://k9sth.com/uploads/160-meter_amplifier_article-1.pdf
>
> and here is the start of the GS-31b amplifier
>
> http://k9sth.com/uploads/All_Band_Linear_Amplifier-1.pdf
>
> Glen, K9STH
>
> Website:  http://k9sth.com
>
>
> --- On Wed, 5/25/11, Roger (sub1)<sub1 at rogerhalstead.com>  wrote:
>
> You may want to rethink that headroom.  The transformer will tend to have a bit higher voltage if running at less than rated load. And higher than optimal fil voltages equate to shorter tube life.  In addition, there is less inrush trama to the filament if you have a matched transformer.  I think the latter is subject to some debate however.
>
> In any event, you would want to trim the actual fil voltage with a series resistor for max life.
>
>
> I look at it as you need the extra headroom allowing for trimming the voltage to one that is ideal. Of course with the line variations we run into today, ideal might be right at the bottom of the permissible range to keep the top within the permissible range.
>