[Collins] 75A-4 Power Xfmr

[Dr. Gerald N. Johnson]

But the 75A4 has a choke input filter so the low drop and hard 
conduction of the silicon rectifier doesn't stress the transformer. He 
says that too. Tubes aren't as happy with the high peak currents of a 
capacitor input filter and silicon rectifiers pass that stress on to the 
transformer. Good transformer vendors rate a plate winding at a 
significantly greater DC current with a choke input filter than with a 
capacitor input filter because the peak currents to the capacitor input 
filter are much higher and of much shorter duration.

I count on a transformer of typical construction have about 4 to 5% 
impedance, and only part of that is resistive making heat. But 10 watts 
less load on the 5 volt winding is 10 watts less load on the primary and 
should allow 10 watts more load on the 6.3 volt winding without changing 
the primary current or the total transformer heat dissipation. And since 
part of the voltage drop is in the primary and part in the secondary for 
a particular load I expect the 6.3 volts to stay up a bit better when 
slightly overloaded by having removed the 5 volt load.

Of course one can confuse the whole matter by using the freed 5 volt 
winding as a buck winding in series with the AC line to the radio. And 
gain in energy dissipated in saturating the transformer core as well as 
lowering the operating voltage through the radio.

Point is, unless run with way to high line voltage, transformers are 
resilient and not quickly damaged with a fairly large overload. 
Transformers designed to run at low line voltages and designed to be the 
most economical of construction (minimum copper and iron) will heat the 
core rapidly as well as the primary winding from the effects of being 
driven too far into saturation by the high line voltage which cause high 
peak primary currents. If one looks at the transformer primary current 
with an oscilloscope, the maximum proper line voltage shows up easily as 
the current peaks rise much more rapidly than line voltage rises.

On 4/13/2011 2:54 PM, Carl wrote:
> This chap would disagree about the benefit of going SS with the rectifier
>
> http://www.w8ji.com/power_transformer_stress.htm
>
> Ive built full bench bucking transformers into the station; one large
> transformer takes care of all the 120V line dropping for each bench, old
> or new gear. Using unpopular oddball voltages at high current found
> cheap at hamfests and surplus dealers I run 110-114V to everything.
> Various consumer radios, mostly high tube count wood consoles and table
> models, have a small transformer either under the chassis or in a small
> box that includes a HB line filter and outlets that feed several radios.
> A simple 12.6V 2A secondary will handle up to a 240W load; I run mine at
> about half their ratings.

The transformer won't smoke run at nearly twice their ratings but the 
voltage regulation won't be very good.

Most classic radios had no line filter, just a couple .05 mfd caps from 
each side of the line to chassis which often puts enough voltage on an 
ungrounded chassis to tickle and draws enough current when the chassis 
is grounded to trip a GFCI.

All that is good for your radios and will go a long ways to get a very 
long transformer life.
>
> National for one commented in QST that the early HRO sensitivity was
> better with 180V B+ than 250V which is only needed for full speaker volume.

In the 75A4, the 6AQ5 runs from the first filter capacitor at about 220 
volts while the rest of the radio is supplied through another choke and 
capacitor combination at about 180 volts. With only 4 pins on the power 
supply connector, National limited the possibility of running different 
voltages on different stages.

75S varied over their lifetime from running 120 volts up to about 185 on 
the output stage and the 6BF5 ran hot the whole time, hotter in the 
later radios at higher voltage, but they did add a bit of unbypassed 
cathode resistor for tube protection from a failed bias supply and 
distortion reducing inverse feedback in the latest versions. A 6AQ5 
draws less current but at the low voltage and wrong impedance plate load 
has more distortion and lower power output.
>
> With 2 or 4 tube audio amps in the consumer sets, I hear no volume
> difference at the lower line input; they still rattle the timbers!

The simple baffles of the console radios make for very efficient 
speakers, unlike some modern "hifi" speakers that absorb all the back 
wave to get a more uniform response vs frequency.
>
> Carl
> KM1H
>
>

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