[AMRadio] Re: TL/TH & more

D. Chester k4kyv at charter.net
Tue Feb 26 11:43:12 EST 2008

> Wow Barrie ....6C21s modulated by 304TLs.  I actually had the idea to 
> build a transmitter using the same line-up.  I don't know if I will ever 
> proceed with it, but thought it would be really cool and have "strap" to 
> spare.  I have a pair of new 6C21s and several new(?) 304TLs.  The big 
> challenge would be to find mod iron to handle the chores.  The 304s will 
> run nicely at low plate voltage, but then current is really high and mod 
> iron very low Z.  Anyway...will be interested in how your rig progresses. 
> Like I say, I don't know whether I will proceed with mine...I don't need 
> TVI/RFI neighborhood problems!  Those 6C21s sure are neat looking though!
>  73,  Jack, W9GT

The 6C21 is listed to have an oddball filament voltage, 8.2v I believe.  I 
once read somewhere that the tube is really a 450TH or TL, that has been 
re-rated for radar pulse service.  The 7.5v filament is jacked up to 8.2v to 
gain extra emission, at the expense of tube life.  The tube will last much 
longer in normal  transmitter service if run at the same 7.5 volts filament 
rating as the 450T's.

I would test the tube at full rf power out and modulated 100%, or at full 
modulator output, by reducing the filament voltage until the power begins to 
drop off or you begin to lose positive peak output in either service, 
indicating filament emission is beginning to starve.  Don't run them at 
starved filament voltage any longer than it takes to get meter or scope 
pattern readings.  Then, it is safe to run the filament just a fraction of a 
volt above the minimum that gives full output and positive peaks, in fact, 
that will give maximum tube life.

Don, k4kyv 

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