[HBR] Re: Filament Voltage Dropping Scheme

[Hopperdhh at aol.com]

Hi Walt,

I did a copy and paste from my original post just below.  I believe that the 
equation I derived is exact.  I'll try to write it out more clearly and hope 
that it doesn't get corrupted when sent:

C = filament_current / (2pi*line_freq*sqrt(line_voltage^2-filament_voltage^2))

This equation was derived using vectors where the line voltage was the 
hypotenuse of a right triangle and filament voltage and capacitor voltage were the 
other two sides.  Of course a sine waveform is assumed.  As you stated, one 
needs to verify the filament voltage with a meter to be sure theory and practice 
agree.

Also, I didn't make it clear in my original post that the capacitors that I 
used were taken from equipment that had them directly across the power line.  
See "computer circuit boards" below.  I should have said "switching power 
supply circuit boards."  These seem to be showing up at ham fests, and are a great 
source for these capacitors as well as electrolytics that can be used in 
doubler and tripler circuits.  But, there I go confusing the issue again!

I don't think that the initial surge will last long enough to fail many 
capacitors, unless you try to carry this idea to extremes -- like supplying an amp 
or 2 to a 6.3 volt filament.  I didn't mean to imply that.  The small 150 ma 
current like in this case is not much compared to the several amps that RF 
capacitors could see.  Not to say that it couldn't be done, but in the case of 
really high current, all the concerns would become a lot more serious!

This is a great forum.  I really appreciate the feedback.  If this scheme 
were used in a more elaborate project, it would be a good idea to add a filament 
position to the meter switch in order to be sure the filament reactance keeps 
doing its job.

Going off-topic for just a short paragraph: My little transmitter is an 
updated version of the "22 Watt Monster" that was in December 1963 CQ magazine.  
The 12SK7 oscillator really made a great improvement.  I don't have a meter, 
just a pilot lamp in series with the B+ to the final.  Its totally uncrowded on 
just a 5 by 7 inch chassis including the filament capacitor and half wave 
doubler power supply.  The 50L6 really doesn't seem to mind working at 300 volts 
and 73 milliamps!  Maybe sometime I'll build a compact version!

Regards,
Dan Hopper
K9WEK
 

>From original post:

To calculate the capacitor required for this job, here is the equation:

C=0.15/(377*sqr(120**2-62.6**2))=3.89uF

where 0.15 is the filament current and 120**2 means 120 squared, etc., and 
377 is 2 pi times 60 Hz.

You would put your values in for the current and voltage for your 
application, of course.

Obviously we can't use polarized electrolytic capacitors for this job.  I 
used mylar capacitors rated at 250 volts to come up with the right value.  
Computer circuit boards as well as solid state TV boards found at ham fests often 
have these types of capacitors.  Its easy to experimentally arrive at the right 
value after finding the target value from the equation.



In a message dated 12/3/2006 12:44:13 AM Eastern Standard Time, 
waltah at ntelos.net writes:

> In fact the capacitive reactance seems to cause the filaments to come
> up to temperature more slowly than with a resistor.  I need to do an
> actual experiment to verify this, though.

When you do the calculation for the capacitance, remember that the 
voltage across the combination will lag the current by just about 45 
degrees due to roughly equal voltages across the cap (90 degree lagging) 
and filaments (in phase).  So it's not just a matter of getting a 
reactance equal to the correct series resistance.  The easy way is to 
calculate that number and then measure the voltage across the tubes (not 
the cap!) and adjust.  The tubes are resistive; if they have the right 
voltage they'll have the right current because it will be in phase.  I 
think you'll wind up with rather more than the expected capacitance -- 
consistant with your observation that the tubes come up more slowly than 
with a resistance.

Good idea -- very uncommon in vacuum tube practice because when vacuum
was king, we didn't have caps that could do such things -- but still a
good idea.

Walt Hutchens
KJ4KV
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