[Milsurplus] TBW Transmitter power-ON - filament wiring

[WA5CAB at cs.com]

Greg,

The rectifier-modulator originally had three transformers in it.  On for 
high B+, one for low B+ and one for filament.  The filament transformer had a 
105 VAC primary and a rheostat in series with it.  You used the rheostat to 
set the filament volt meter to 10.0.  According to the TBW, TBW-1 and the 
TBW-5 manuals, the winding for the 837's in the HF transmitter was 9.3 VAC 
each side of center-tap, and rated at 1.4 amps.  The 1.4 makes sense as the 837 
filament current is 0.7 amps at 12.6 volts.  There is a 4.5 ohm resistor in 
series with the filament of the IPA but it would only drop the applied 
filament voltage to about 15.2 volts.  Actually, it would probably be a little 
higher than that as the filament current will decrease a little as you raise 
the voltage.  There is no voltage and resistance chart in any of the manuals 
but my guess is that the 9.3 is wrong.  The same transformer shows the 2.5 
volt winding for the 843 modulator as rated at 205 amps.  I believe that the 
205 should be 2.5.  The 837 VFO has a bifilar RF choke in its filament 
circuit but its resistance is not given.  From other information it would 
probably be possible to calculate it within say +/- 20% but I didn't do it.  

Perhaps someone with a working TBW (I don't mean just the HF transmitter) 
could measure the RMS voltage on T-202 between terminals 9 and 11.  
Alternately, you could measure the resistance of the two choke windings and add them. 
 If close to 4.5 ohms, then I would guess the filament winding output was 
around 16 volts.  If significantly lower than 4.5, then I would guess it was 
a 12.6 volt winding and for some reason they ran reduced filament voltage on 
the IPA.

In a message dated 6/3/2009 7:46:33 PM Central Daylight Time, 
bluebirdtele at embarqmail.com writes: 
> Thanks to all for your input.
> Some folks say don't modify it if it's in NOS condition.  Others say do 
> what 
> you need to do to get it on the air.
> I think I will split the difference and try to fire it up without cutting 
> or 
> soldering stuff in.
> A home brew supply is  nearly finished and out putting 1.6 kvdc and 500 
> vdc 
> unloaded.  Also, 10 vac @ 5 amps and 12 vac is available onboard.
> All I have to work with is a smallish system schematic and I looked at the 
> 
> filament wiring for the HF unit.  The two 836's are wired together at the 
> filaments and what looks like a resistor array for both.  Since the 836 
> lists as a 12 v tube, I wonder what uncle Sam  used for filamant power.  
> Any 
> ideas?
> Thanks
> Greg
> WA7LYO
> Kinston NC 

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