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Re: [Amps] AC filament voltage regulator

To: "" <>, "" <>
Subject: Re: [Amps] AC filament voltage regulator
From: "Fuqua, Bill L" <>
Date: Thu, 14 Jul 2011 20:46:31 -0400
List-post: <">>
The emission will increase as long as you increase the temperature. Has to do 
with the thermal work funciton of the cathode material. 
Mind you that material changes over a lifetime of use. 
So maximum emission can't be the goal because the cathode would just increased 
the voltage until you got no more emission.
Lots of things determine the cahtodes life. Temperature, time, material, 
contamination, etc. 
Somewhere you have a balance between emission or cathode life time. 
The manufacturer draws a line in its specification. 
  You can draw your own line but with out lots of experience and many samples 
to experiment with that is not
practical. The brightest candles burn out the quickest.
  If you want to be precise control the cathode temperature. The method you use 
is up to you.
You never know how long it will really last. Just that some day you will notice 
the drop in output and that will be that.

Bill wa4lav

From: []
Sent: Thursday, July 14, 2011 12:31 PM
To: Fuqua, Bill L;
Subject: Re: [Amps] AC filament voltage regulator

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 

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 the
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 bulb
> 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 to
> 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 failure.
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