[AMRadio] Class C modulation

[Donald Chester]

>From: "John Coleman" <jec at pctechref.com>
>Reply-To: amradio at mailman.qth.net
>To: <amradio at mailman.qth.net>
>Subject: RE: [AMRadio] Class C modulation
>Date: Tue, 19 Mar 2002 11:53:01 -0600
>
>One of the most missed issues is that a lot of folks think that changing
>tubes will give them more power, but the increased capability of the tube
>upgrade is only realized if the voltage or load is changed to make use of
>that capability.

I have verified that with  my HF-300 rig.  It uses a pair of them in 
pushpull in the final.  Once,  I wanted to check out a single tube for 
performance, so I took one of my many HF-300 duds and cut off one of the 
filament pins and used the crapped out tube as a neutralising dummy.  The 
first time I fired up the rig with only one tube in the f inal,  the meters 
read almost exactly the same as when I had two tubes.  It immediately became 
obvious what was  happening, so I quickly decreased the loading until the 
one operative tube drew normal plate current (1/2  the normal total plate 
current to  the final).

That explains why it is often observed that one section of a 304TL can go 
dark, or one or more strands of an 833A filament can open up, and the tube 
will appear to still work normally.  Of course you are severely overloading 
the other tube (or fraction of a tube) that is left to run that way.

You can, for example, unknowingly ruin a good tube in a final if you operate 
it for an extended period, unaware that  the filament on the other one has 
opened up.  That's a good reason to  have a window on the transmitter panel 
that allows you to see  both tube plates, or else use a separate plate 
current  meter for each tube, when you run two or  more tubes in pushpull or 
parallel.

Don K4KYV


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