[Collins] Transformers

Thu Aug 29 16:34:54 EDT 2013

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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