[Collins] Transformers

[Carl]

----- Original Message ----- 
From: <antqradio at sbcglobal.net>
To: <Collins at mailman.qth.net>
Sent: Thursday, August 29, 2013 4:32 PM
Subject: Re: [Collins] Transformers


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.


Jim,
1. Ive yet to pay more that $1.50 for a 1-2A transformer and under $10 for 
the bigger ones mentioned prior.

2. You bounce around from an AA5 to no real details but a power resistor for 
a set that draws 70-140W or more wont be .74.

3. Adding a 10W resistor in an already packed to the limit AA5 chassis is 
asking for trouble. Putting a 50W resistor in a big radio is no different.

4. Adding a resistor to a radio with PP AB1, AB2 or B audio will just add 
distortion due to the added lack of regulation plus what Jerry mentioned.

5. Using a resistor is the cheapskates way out (-; and reminds me of those 
that reform a 50-70+ year old electrolytic or leave the paper caps in since 
the radio "plays well".  I wonder how many 75A4's were junked with bad 
transformers? I still use the one I bought in 1965.

While I realize that Collins receivers never had PP audio or sounded good 
either that doesnt mean that some of us dont have radios that sound good for 
ham AM, SWLing,  the BCB, or to play old fashioned phonograph records thru a 
radios audio section as I do. Some include a Scott 800B with PP 6550's 
(originaly 6L6G's), or want to rattle windows with a Philco 38-690 and 
6L6G's, Scott AW 23 with four 2A3's, or one of many senior Hallicrafters and 
Nationals with just PP 6V6's or 6F6's.
Even my AC/DC NC-46 with PP 25L6's qualifies and sounds great thru the 
matching speaker.

I do have several AA5's and a complete Zenith T/O collection but I dont use 
them other than as shelf queens. Im into more useable radios.

And dont forget that your transmitters also benefit with a bucking 
transformer. That 32V whatever already has a history of transformer problems 
and keeps rewinders busy.

Transformer manufacturers used to sell bucking or boost transformers, I have 
and use several in the 500-2500W variety with home brew amps and modulators 
and built in as part of the PS as intended back when built. Some are older 
than me (-;

Carl
KM1H