[Amps] 3-1000Z - Filament Voltage

[w6ah at comcast.net]

Hi Rob

-------------- Original message -------------- 
From: "rob" <VE6TR at 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