[Collins] 75A-4 Power Xfmr

[Dr. Gerald N. Johnson]

On 4/14/2011 10:45 AM, Carl wrote:
>
> ----- Original Message -----
> From: "Dr. Gerald N. Johnson"<geraldj at weather.net>
> To: "Carl"<km1h at jeremy.mv.com>
> Cc:<collins at mailman.qth.net>
> Sent: Wednesday, April 13, 2011 4:29 PM
> Subject: Re: [Collins] 75A-4 Power Xfmr
>
>
>> 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.
>
>
> Agree altho Collins wasnt known for sourcing the best transformers in those
> years. Since not all receivers use choke input I thought that article would
> be of more general interest and not model specific.
>
>
>> 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.
>
>
> This is mostly conjecture

No conjecture. But based on considerable transformer analysis and design 
over the past 50 years.


>
>
>>
>> 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.
>
>
> That is done often, especially with consumer type sets. Hams seem to be very
> slow on adapting the bucking concept to counter the high line voltages.

The topic has been on the forum and some have applied the bucking 
transformer.
>
>>
>> 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.
>
> Hallicrafters are the most prone to transformer problems; Ive measured
> several models that are already starting to saturate at 123V. A few
> Nationals are similar...unfortunately they are the top of the line models.
>
>
> 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.
>
>
> If you use a dual trace scope and also monitor the secondary you will see
> the effect of line spikes that arent pre filtered. With todays line
> pollution there are sometimes spikes well above what would be considered
> safe. For that reason I use the 3 capacitor line bypass ( .01 both sides of
> line to ground and up to .1 line to line, leakage current wont affect GFI's)
> plus another at the secondary.  National used one at the secondary on a few
> models to counter noise on 3rd world power lines. Unfortunately paper caps
> deterioated and took out transformers.
>
> SS diodes also have their own switching signatures that can create noise. A
> single .01 across each string and another to ground at the output eliminates
> it.

That's true.
>
> I prefer the vacuum tube rectifier plus the bucking transformer, when not
> pushed hard they have an amazing life expectancy. Switching a 5U4 to a 5R4
> reduces filament load by 1A, has a higher PIV and drops a little more
> voltage. Except for filament current they also are a good swap for the 5Y3
> and other 2A octal rectifiers.  I use the regular ST shaped GY's in most
> radios and the WGB potato mashers in several transmitters.....prices are
> still well below the 5U4 and 5Y3. Some rebase them and use in place of the
> 5Z3.

Motorola liked the 80 for receiver rectifiers because it had more 
voltage drop and was more tolerant of bad loads than the later rectifiers.

Typically OEM application used 5U4 for higher current, 5R4 for higher 
voltage (like in the 516F-2) and 5Y3 for receivers. 5U4 was most common 
in TV sets.
>
> Carl
> KM1H
>
>
73, Jerry, K0CQ