[Collins] Transformers

Thu Aug 29 18:49:00 EDT 2013

Hi Gerry:
Might consider a polypropylene film capacitor in series with the primary. 
Provides about the same effect as a series resistor without the heat loss. 
Works well in receivers that use gas voltage regulation for the oscillators. 
Use 30uf per 100 watts for a 6 volt reduction in line voltage.  Reduces in 
rush, has about the same line voltage regulation as the bucking transformer 
but regulation is sensitive to load like the series resistor.  Can't be used 
in a transmitter where the load is volatile.  There, the bucking transformer 
or autotransformer are the answer.

Kindest regards Jim K9AXN

-----Original Message----- 
From: Dr. Gerald N. Johnson
Sent: Thursday, August 29, 2013 3:34 PM
To: collins at mailman.qth.net
Subject: Re: [Collins] Transformers

I would probably explain the need for reduced voltage and perhaps sell a
suitable transformer from my transformer shelves.

73, Jerry, K0CQ

On 8/29/2013 3:32 PM, antqradio at sbcglobal.net wrote:
> Touche'
>
> But when you tired of the radio and either traded or sold it off, I guess 
> it is now becomes the problem of the new owner?
>
> I can see it now, "Hey, what is with the smoke?  That so-n-so sold me a 
> lemon."
> Jim
>
>
> ________________________________
>   From: Dr. Gerald N. Johnson<geraldj at netins.net>
> To: antqradio at sbcglobal.net
> Sent: Thursday, August 29, 2013 3:15 PM
> Subject: Re: [Collins] Transformers
>
>
> I'd go to my transformer shelf and choose an appropriate filament or
> buck/boost transformer and wire it to an outlet box with power switch
> and line cord. I might purchase a short extension cord to cut apart to
> make the line cord and a receptacle.
>
> 73, Jerry, K0CQ
>
> On 8/29/2013 3:10 PM, antqradio at sbcglobal.net wrote:
>> Jerry, you are so over the top!
>> If I had an $800 radio and I was worried that much about the power
>> transformer, then I would be powering it with, as you put it, a Sola.
>> But if the radio is only worth say $50, and a replacement power
>> transformer is "unobtanium", then how much would you spend on protecting
>> the power transformer?
>> Jim
>>
>> ------------------------------------------------------------------------
>> *From:* Dr. Gerald N. Johnson<geraldj at netins.net>
>> *To:* collins at mailman.qth.net
>> *Sent:* Thursday, August 29, 2013 2:14 PM
>> *Subject:* Re: [Collins] Transformers
>>
>>
>>
>> On 8/29/2013 1:33 PM, antqradio at sbcglobal.net
>> <mailto: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.
>>
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