[Collins] 516F-2 Query

[Dr. Gerald N. Johnson]

The fixed tap is called a buck/boost transformer. For this application 
the common ones have a pair of 120 volt primary winding that can easily 
be wired in series for 240 or parallel for 120 and a pair of 12 volt 
windings that can be easily wired in series for 24 or parallel for 12. 
Wiring the primaries in parallel and the secondaries in parallel and 
then the two pairs in series, if polarized correctly you can apply 120 
volts to the primaries and between neutral and the end of the 
secondaries you have 108 volts. Or 125 and have 113 volts. Or with the 
other polarity with 120 on the primary you have 132 on the series 
combination. I prefer that connection with 125 applied to the 132 volt 
connection taking off 113 from the primary winding. In this auto 
transformer configuration, the transformer handles only 10% of the power 
to the load so a 50 VA transformer will support a 500 VA load, enough 
for a S-line need something bigger to include the 30L-1. A 12 volt 5 or 
6 amp filament transformer works just as handily.

73, Jerry, K0CQ Technical Adviser to the Collins Radio Association

On 1/23/2012 8:57 AM, Bruce H McIntosh wrote:
> On Sun, 2012-01-22 at 07:16 -0800, Jim Whartenby wrote:
>> I have read this topic many times on various lists with most replies agreeing
>> with what is found below.
>>
>> If you look at the problem from the standpoint of "heat is the enemy of
>> electronic equipment" then anything you do to reduce the heat load will extend
>> the life of the equipment.  If you use either resistors or voltage regulators to
>> lower B+, the heat load will not change.  The power transformer will still
>> provide the same power to the equipment.  For the most part, the power
>> transformer is unobtainium so anything done to reduce the load on the power
>> transformer is a good thing.
>
> That sounds like you're back the variac approach, but like Jerry pointed
> out, that variac knob is a point of vulnerability to visitors with
> twiddly fingers (file under "kids who can't keep their hands to
> themselves  :-)").  Maybe a fixed isolation transformer with suitable
> taps to drop the secondary below line voltage just a tad?  Just thinking
> out loud.  Don' know how many holes the idea might be full of. :)
>