[Amps] Matching tubes by exhaust heat?

Rich 2@vc.net
Mon, 16 Dec 2002 22:07:05 -0800


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>Question?  As the emission of a tube drops, does the efficiency % (ratio 
>of output to input) stay relatively constant?  If the plate current simply 
>decreases while the efficiency remains constant, then a simple way of 
>comparing two tubes in a parallel amplifier would be to measure exhaust 
>temperature of each stack.  
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**  A better engineering solution might be to select one tube that is 
large enough to do the job.  It is my opinion that doubling the 
feedback-C by using two tubes is only an advantage if one is trying to 
build a VHF oscillator.  

>Even if the efficiency decreased a bit, the increase in exhaust 
>temperature should not offset the decrease in plate input.  Thus a weak 
>tube would always have a lower exhaust temperature than a good one.  
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**  However, a full-emission tube with a higher-Mu and a lower ZSAC  
would also have a lower exhaust temperature.
 
>Do any commercial amplifiers monitor exhaust temperature for more than 
>simply determining if the blower is working (like the Collins 30S-1)?  
>Perhaps efficiency could be estimated with such a temperature probe.  
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>Not sure if the digital temperature probes would be affected by the strong 
>rf field above the tube - even if outside of the amplifier cabinet.
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**  Since IR thermometers will read temp. though an exhaust screen, an 
anode-cooler temp, reading should be accurate.

>Colin  K7FM

-  Rich..., 805.386.3734, www.vcnet.com/measures.