[HBR] Filament Voltage Dropping Scheme

[Hopperdhh at aol.com]

Hi Jim,

Thanks for your thoughtful reply.  Let me address your 4 drawbacks.  See my 
replys below.

Dan K9WEK




In a message dated 12/2/2006 6:09:42 PM Eastern Standard Time, N2EY at aol.com 
writes:
In a message dated 12/2/06 8:45:42 AM Eastern Standard Time, 
Hopperdhh at aol.com writes:


> how I reduced the line voltage to 62.6 volts for 
> the filaments.
> 
> I used capacitive reactance to do it.  There is no heat dissipated in the 
> capacitor like there would be if the 120 volts were dropped with a 
resistor. 
>  
> 

3.89uF
> 
> 
>  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.
> 
> The nice thing is that no power is dissipated so the capacitor runs stone 
> cold as it does the job.  I don't think the power company will mind the 
very 
> 
> slight power factor increase.
> 

Actually, the capacitive reactance may help! Most loads are inductive, and 
the utilities often have static capacitor banks on the poles to correct it. 
I've 
long been a fan of synchronous condensers but that's another topic...

> This should work just fine in line operated receiver circuits as well.  Of 
> course all the usual precautions apply for all line operated equipment. 

Agreed.

Today, 
> 
> with houses wired for 3 wire outlets, line operated equipment is much safer 
> than it was back in the 50s and 60s.
> 

Only if the house is wired to code. Many aren't.

Some years back, we were thinking of moving to the point that we had a home 
inspection done on a house we were looking at. The inspector went around with 
an outlet tester - the three-light kind.

Besides finding numerous 2 prong outlets, he found almost every possible 
problem on the various 3 prong outlets:

- No ground
- Hot and neutral swapped
- Ground and neutral swapped.
- Completely dead outlet

About the only thing he didn't find was hot on the ground pin. 

We didn't buy the house!

> OK guys, what do you think?

It's certainly a useful technique, but it has some drawbacks:

First, the needed value will need to be found experimentally in almost all 
cases. Not really a problem if you are only making one or two units and have 
a 
handful of capacitors.
As you say not really a problem.  I have always only built one rig at a time. 
 We're not in production here!


Second, the caps have to be rated to take the voltage and current. IMHO, 250 
volts is kinda close, because the peak value of a 125 volt RMS sine wave is 
177 volts, and if a surge comes along....pop? If a cap shorts, your precious 
bottles have full line voltage on the string. 


This is, to me, the only real concern.  This one does need some additional 
validation.  Good point Jim.

It would be hard to fuse the circuit to protect against this.  However, the 
capacitor does not have the entire line voltage across it, typically.  In my 
case above

VC=current * Xc = .15 * (1/(377*C)) = 102 volts

OK that's pretty high, but capacitors typically are rated with a 2 to 1 
safety margin.  A 250 working volt capacitor will likely fail at 500 volts, not 
250.  I know that this could cause some eyebrows to raise!

Third, the setup will only work right on one frequency. It is probably 
waveform-sensitive, too.

The only frequency I use is 60 Hz.  I have noticed that the waveform is 
pretty much sinusoidal.  I don't think this is a problem really.


Fourth, when you turn on the set, there will be surge as the caps charge for 
the first time. The charging current is only limited by the cold resistance 
of 
the heaters.


I just measured the cold resistance of the 12SK7 plus the 50L6 to be 59 ohms. 
 The time constant is then R times C or 229 microseconds.  Hardly enough time 
to cause any thermal shock.  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.




Just be careful!

73 de Jim, N2EY





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