[Collins] Transformers

Thu Aug 29 16:32:32 EDT 2013

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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