[Swan] SW 240 lineage questions

[Robert Nickels]

Bill Clarke wrote:
>
> On the PA (6DQ5) pin 3 has a 5 watt 5 ohm ceramic resistor to ground.
> It does not look original and is not on the diagram I have. What
> might the reason for this modification have been?
Bill, that's a hack to re-balance that leg of the series-parallel heater 
string for use with the "dark heater" version of the 6DQ5.  RCA 
introduced this version of the tube in the 60s as a "running change" 
i.e. no change in marking or designation.  However the dark heater 
version draws substantially less current than the original 6DQ5 and thus 
the Swan 240 filaments will be really unbalanced if one is just plugged 
in.   I did the same thing to prove to myself that this was  the case 
before learning about the dark heater issue and have been trying to 
spread the word ever since.    In most applications with 6.3 volt 
filament wiring, the new version was a plus as it required less filament 
current, but not for the Swan 240!

By the way, I'd recommend restoring it.  Several of us use them 
routinely on the Vintage Sideband Net and I like the broader sounding 
audio.  It also puts out a heck of a signal for having only one 
inexpensive final tube!  I also have a spare TCU remote VFO console 
available if anyone's interested.

73, Bob W9RAN