Power Tube Life Question

[Paul Litwinovich]

Most broadcast tubes are Thoriated Tungsten direct heated cathodes and your 
amp is really no different than an AM transmitter's modulator. The standard 
procedure that we follow is to run the tubes at rated filament for the first 
100 hours when installing a new tube. This helps the getter degas any 
remaining O2 that may be in the tube. We then reduce filament voltage by 
5-10%, on a FM transmitter we check to see that we can make at least 110% 
rated power so that we have sufficient head room to allow for dips in line 
voltage etc. On AM transmitters we look to see that we are making the proper 
positive modulation peaks. If not, we increase filament voltage enough to 
make it happen. So I would translate this into checking the amp's output 
with a scope for clipping at your required power output. That done, run it 
that way until you see an indication that the tube is nearing the end of 
life, then increase the filament voltage in steps up to +10% until you have 
squeezed all of the useful life out of it

Below is a link to a very good white paper on the subject:

http://www.bext.com/filament.htm

Hope this helps
Paul A. Litwinovich
Director of Engineering
WSHU AM&FM WSUF FM
PG-GB-039245
N1MUC

----- Original Message ----- 
From: "J. Forster" <jfor at QUIK.COM>
To: <BOATANCHORS at LISTS.TEMPE.GOV>
Sent: Tuesday, March 16, 2010 8:43 PM
Subject: [BOATANCHORS-TEMPE] Power Tube Life Question


> This relates to the 500 Watt audio power amplifier for the 1MC PA system
> on the Massachusetts again.
>
> The amp uses 4 type 838 tubes, push-pull and parallel connected, with
> directly heated cathodes (flaments) presumably Thoriated Tungsten,
> although the RCA data sheet does not say specifically.
>
> Since the tubes are somewhat precious, I'd like to run them at reduced
> filament voltage (maybe 5-10% less) to extend their life. However, I'm
> concerned about loss of Gm and/or emission from prolonged operation with
> the reduced voltage.
>
> The amp has separate plate and filament transformers, BTW.
>
> Can anyone provide some guidance, please?
>
> Thanks,
>
> -John
>
> ================
>
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