[Collins] Transformers

[Dr. Gerald N. Johnson]

On 8/29/2013 1:33 PM, antqradio at sbcglobal.net wrote:
> Carl, welcome to the fray.
> Like you, I have more then my fair share of old radios.  I have seen the same issues with power transformers and share your concerns when transformers are too hot to touch.
>
> At my workbench, I have rewired a 1 KVA, 220 to 115 volt auto-transformer to be a one-to-one isolation transformer so that I have less of a chance for shock and other calamities while working on AC/DC All American Five radios.  Interesting to note that the output voltage is around 117 volts when the input line voltage is at 125 volts.  In the early morning, this voltage can go as low as 113 volts as the power company backs off on the line voltage during low demand.
>
> Yes, the bucking transformer method is superior in efficiency to using a power resistor to lower the operating voltage for power transformers that run hot.  A bucking transformer doesn't get as hot as the power resistor when it is doing it's job.  But in reducing the heat stress on a power transformer, the series resistor still gets the job done.
>
> For small power transformers, let's say under 120VA the series resistor is dissipating under 10 watts to drop about 8 volts from the ac main and it will allow the power transformer to run 10 to 15 (or more depending on core saturation) degrees F cooler.  There is an added benefit in that the peak current in a tube rectifier is also reduced because the series resistor is acting as a current limiter to large current spikes.  Although a better job is done in reducing peak rectifier current by adding 100 or so Ohm power resistors in series with the rectifier plates.  But this discussion point is for another time.
>
> In looking at the Mouser online catalog, a Hammond 266VB6 dual primary and dual secondary transformer goes for $11.90 each.  It has a two 3.15 volt 1.2 amp secondaries so it could be configured as Jerry mentioned below and wired to buck from 6.3 volts down to 1.6 volts.  It will require a 3 X 2 X 2 inch mounting area and weighs just under 3/4 pound.  Other voltages are available but at higher cost.  Also in Mouser is a Xicon 280-CR10-13-RC 10 watt power resistor (I looked at a 13 ohm unit) that goes for $0.64 in unit quantity.  This would work well for a 70VA transformer and drop around 8 volts from the AC line.
>
> Jerry mentioned that the voltage regulation is not as stiff as a bucking transformer and I can see his point.  But with the series resistor, as voltage drops so will the current demand and the associated voltage drop across the series resistor.  So one would think that voltage regulation would be a bit better with the series resistor then with a fixed ratio bucking transformer.
> Jim
>
Its not voltage regulation with varying line voltage that's my concern, 
its voltage regulation for the varying load of a radio with class AB 
audio or PA and switched to standby. The last thing you want is those 
varying loads varying oscillator voltages and so causing FM (sometimes 
at a subaudible rate).

Remember I suggested a 25 KVA distribution transformer for a 100 amp 240 
volt service had .09 ohms impedance.

The 6.3 volt 1.2 amp transformer has a 5.25 ohms full load rating, and 
with EI core should have about 4% impedance or 0.2 ohms. In the bucking 
transformer that's all that adds to the line impedance giving .29 ohms. 
While your series resistor dropping a 1.2 amp load 6.3 volts is 5.25 
ohms making the line impedance at the radio 5.34 ohms so the effect on 
radio voltage is 18 times greater for the same current change.

You are comparing spending $11 or 74 cents on protecting a transformer 
in a $800 radio? Me thinks you aren't looking at the cost of replacing 
that vintage radio transformer.

Vacuum rectifiers are quite efficient and limiting peak currents to a 
capacitor input filter. Silicon replacements not so. Plate windings in 
transformers are also pretty good at limiting peak plate currents else 
the tubes with limited peak currents would not have survived. If you 
really want to limit peak rectifier tube currents the venerable type 80 
is quite emission limited and very good at limiting peak rectifier 
currents. Considerably better than a 5Y3 at the cost of poorer load 
regulation from varying load currents.

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