[AMPS] TL-922 Filament Transformer Protection

[Jon Ogden]

>I also question whether
>overheating of parasitic suppressors isn't down to them getting very hot
>because of physically where they are, and to them being under rated for the
>amount of dissipation of the fundamental they get, especially on 10, 12 and
>15 metres.

MY POINT EXACTLY!

Let's nor forget too, that as you increase in frequency, the output capacitance of the tube becomes a greater percentage of the total first capacitance in the tank circuit (C1 = Ctune + Ctube).  This means that as you go up in frequency, more and more tank circulating current will flow through the tube which means more current through the suppressors, blocking caps, etc.  In my 4-1K, the tube capacitance on 10m is roughly the same value that I use for tune C.

Resistors lose dissipative capability when they get hot.  At some temperature they will dissipate zero watts.  The heat they are exposed to comes from radiated heat from the tube, heat conducted down the anode line and heat generated by current flowing through the resistors themselves (yes, *some* RF current does flow through the resistors in a suppressor since the inductor has some reactance.

And any electronic component regularly exposed to a high temperature environment has a shorter MTTF than one used at room.  All resistor specifications are done at room temp.

I first connected the dots on all this when with my amp stable, I kept on burning up Rich's resistors (and nichrome).  When I yanked them out, my amp was still stable.  Now I have NO resistors in the anode line and still stable.  No parts to worry about!

Resistor manufacturers also play games with numbers regarding dissipation.  The company I sell for used to spec their resistors at 25° C on an infinite and ideal heat sink.  That's an industry standard.  They now spec them at a film temp of 150° C and a maximum heat sink temp of 100° C.  Big difference.  A part that used to be spec'd at 100 W is now spec'd at 10W!

73,

Jon 
KE9NA

--------------------------------------------------------------------------
The Second Amendment is NOT about duck hunting!


Jon Ogden

jono@enteract.com
www.qsl.net/ke9na

"A life lived in fear is a life half lived."


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