In the broadcast industry, where I worked for 25 years as a transmitter
tech besides studio jobs, the transmitters usually had a pot or variac
to adjust the filament voltage. My normal procedure was to set it at
the nominal voltage for 200 hours after installing a new tube. Then the
voltage would be backed off, while observing power output and
distortion. At the point the output or the distortion started to
degrade, the voltage was bumped up slightly, say 0.1 volts. I have
never heard of a case where running filaments low caused decreased tube
life. Of course I never tried to do something dumb like running a 5.0
volt tube on 4 volts. After the initial burn-in, I would expect one
could start by reducing the voltage 5%.
Is it possible on the Harris the 7.8 volts was read before the drop in a
filament choke, assuming a grounded grid configuration? You would think
that a measurement point, if provided by the manufacturer, would be
right at the filament, but who knows. In the case of GG operation,
maybe an OEM would not want to hang a bunch of chokes off the filament
that was getting the RF drive, and decided to measure it before the choke.
73, Rob, NC0B
w6ah@comcast.net wrote:
>Hi Rob
>
>-------------- Original message --------------
>From: "rob" <VE6TR@telus.net>
>
>
>
>>Greetings,
>>
>>I have a Harris RF-104 with a 3-1000Z tube. I have several questions
>>regarding
>>the filament voltage.
>>
>>The Eimac spec. sheet that I have lists the filament voltage as a nominal
>>value
>>of 7.5 volts with no min or max values
>>shown (unlike the 3-500Z spec. sheet which has min/max values). The Harris
>>RF-104 that houses and powers the
>>tube has a spec. of 7.8 volts. As I've had to adjust the filament transformer
>>taps to achieve 7.8 volts (due to different
>>line voltage here), I was wondering if:
>>
>>1. There a preferred voltage to run the filament at?
>>
>>
>The tube is rated for 7.5 volts. 7.8 volts will dramatically shorten the life
>of the tube. The only reason for running it that voltage is if the tube is
>proven to be soft and near the end of it's service life.
>
>
>>2. There a loss of tube life running the filament voltage too low or high?
>>
>>
>Loss of tube life is dramatic if run at voltages even slightly above its
>specified rating. The first effect noticed if the tube is running below it's
>rated voltage is lack of emission resulting in decreased plate current. Output
>distortion increases soon there after. It's very easy to shorten the life of a
>tube by running the heater with a few extra tenths to give it some "extra
>umph". Nothing could be further from the truth. Extra emission beyond what is
>needed to do the job is pure waste.
>
>
>>3. There a practical tolerance for the filament voltage?
>>
>>
>That depends on the size of the bank roll in your pocket. There is a way of
>determining the optimum voltage to run any Thoriated Cathode Tube. The
>procedure is outlined in Eimac's Application Bulletin 18 and can be found at
>http://www.cpii.com/eimac/PDF/AB18.pdf Though initially tedious this prcatice
>is SOP in the Broadcast Industry. Quite a few reduce the advance margin from 5
>% to 2.5 %.
>
>73s andEnjoy
>Bob W6AH
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>
>
>
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