[Kenwood] TL-922A question

[Bryan Swadener]

For best output and life, you should run the specified filament voltage.  The 3-500Z is specified to run at 5.0V +/- 0.25V.

On the TL922, the 3-500Z filaments are in series (requiring 10V @ 14.5A). If the voltage across each filament is 0.3V too high, you need to decrease the voltage by 0.6V.  Since the TL922 uses a common transformer for HV and filament (and HV is marginally adequate), you shouldn't drop the voltage at the primary.  Forget about using a variac.  To do it at the secondary, you need resistance in series:

R = E / I
= 0.6 / 14.5
= 0.041 ohms

All wire has resistance and, according to a nifty wire calculator spreadsheet (http://www.rst-engr.com/rst/jimsdata/wirecalc.xls), 2.5 feet of 22 AWG drops 0.59V at 14.5A, with a 98C temperature rise.  That's pretty toasty!  4 feet of 20 AWG does it with only 59C temperature rise. 6.5 feet of 18 AWG does it with only 33C rise.  If you can squeeze 10.5 feet of 18 AWG in, the rise is slightly less than 20C.

vy 73,
Bryan WA7PRC

> Date: Mon, 19 Nov 2012 10:44:37 -0800 (PST)
> From: Allen N5AG
> 
> RBIG,
> 
> I have #22 gauge > 105 degree C insulated hook up wire
> for my 922 A as per Richard Measures' recommended
> modification.
> Just replace the #14 gauge red and yellow feeding the 3-500
> Z grids. They run a little warm due to the resistance they
> are providing so don't bind them up in a wire loom but let
> them "breath". The voltage will drop to about where you
> need it. The length should be about 40cm long. I'll let you
> do the conversion to inches.
> 
> 
> 73,
> Griff....N5AG
> 
> 
> ________________________________
> From: rbig
>  
> I'm told we should have 4.9v fil voltage on the 3-500s.
> 
> We have 5.3v
> 
> Is using a 220v variac a fairly simple solution to getting
> the fil voltage down to 4.9 ?
> 
> I guess I could also put a resistance on one side of the ac
> primary of the fil transformer. That could drop it
> down slightly.
> 
> Are there any commercially available solutions?