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Re: [Amps] AC filament voltage regulator

Subject: Re: [Amps] AC filament voltage regulator
Date: Thu, 14 Jul 2011 12:31:10 EDT
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I became interested in filament voltage control after learning how  
excessive voltage shortens tube life.
Then with the power company incrementally increasing line voltage in most  
areas the concern grew.
Some tube manufacturers recommend using the lowest voltage consistent  with 
good cathode emission. One note suggested reducing filament voltage until  
the emission drops slightly and then increase the voltage about .2 volts.
No mention of observing the voltage or current was made.
What makes sense is to follow that procedure and make note of the resulting 
 filament voltage. Then monitor the voltage until something changes, at 
which  time another calibrate cycle can be done.
The question in my mind is how to measure cathode emission. Can this be  
done observing plate current? Drive on? Drive off? Pulse the bias circuit to  
simulate peak current under full drive? Peak current is normally about 3X  
average cathode current. Does emission effect peak the same as average 
cathode  current?
Perhaps power output is a better indicator of cathode emission?
So many questions; hopefully there are some answers!
Gerald K5GW
In a message dated 7/14/2011 10:18:48 A.M. Central Daylight Time, writes:

If you really want to control the filament, make some optics, lens,  
filters and detectors and observe the
light from the filament and use  that as feedback to the controller. Use 
color temperature or the black  body radiation curve to do it.
Regulating the "average" AC  Voltage rather than the RMS Voltage is just 
as good as long as
the  waveform remains the same. Gee, how did we get by all these years 
without  filament voltage regulators?
Even all those broadcast transmitters with  adjustable (variac) filament 
voltages used average or
peak measuring  meters. And the variacs and human intervention had a much 
slower  response.
Bill wa4lav

At 09:38 AM 7/14/2011 -0400, Ron  Youvan wrote:
>Hsu wrote:
> >    You are  correct. But a problem ,AD's true RMS chip is not a 
> inexpensive  device.
> >      I  have an idea, using a  inexpensive CdS photoresistor-LED or 
> micro-bulb
>  >   photocoupler( like  N110 in  ICOM  PS-35   power supply) because 
> and CdS
> > photoresistor with  very bad frequency response
> >     so the sampling  voltage   will associate  with  true RMS volt  (I  
> guess) ...
> > If  the noise is not a problem and the  tube's cathode is oxide-coated ,
> > the switching regulator is the  best. but monolithic  IC  maximal out 
> put current
>  > only 10A (ST's  L4970) others need external power MOS  FET.
>    Hsu said his idea is to use the CdS  photo-resistor and a micro-bulb 
> create a home  made
>photo-coupler to get a "slow response" version of  a
>"opto-isolator" to measure a tube's heater Voltage, but that will  result 
>more in an
>averaged value.
>     The best idea is to use a true RMS Volt meter such as almost every 
>  Fluke DVM ever
>made to SET the RMS Voltage "at the tube," then use THE  CURRENT Amperage 
>reading to maintain
>that Voltage.  The  advantage is if a connection becomes lose and develops 
>an IR drop, the  current
>will drop and you will turn up the Voltage control for the  current that 
>gets you
>the correct current which is the correct  "at the tube Voltage" until you 
>can no more make that
>current,  then you will look for a bad connection, find and fix it.  (as  
>you tighten connections
>the current will rise)  A lot of  commercial equipment works this way.
>      Ron  KA4INM - Mistakes are often the stepping stones to utter  
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