[Hallicrafters] Power transformer temperatures

[Gary]

Well, Here is some more thoughts on tranny temps. With no load, filament and
high voltage windings are going to be higher. One thing to consider is the
"service factor" of the tranny. Yesteryears insulation was more like 105
degrees but today's insulation is rated at 155 degrees, or higher, depending
on application. True, running a tranny at today's voltages will increase the
heat and will eventually destroy insulation. Other factors to think about is
current draw. This also effects the temp. on the xfrmr. Its easy to blame
high voltage house receptacle but what about a leaky filter cap, a audio
transformer short, a cathode resistor that has gone down in value. These all
take part in the efficiency of the tranny. I rewind transformer in the
winter time and have done quite a few for the 20's and 30's radios along
with ham radio gear. I always use 10 or 15 mil nomex paper for insulation
and double coated magnet wire for the windings. Just for
s__ts and giggles I wound up a tranny for one of my old tombstone radios. I
put a load across the trannys secondarys, brought up the variac watched the
amp gauge. At 125 volts, it was drawing 6 amps. After an hour and a half the
HV winding opened up. Filaments were still running high. A thermo spot
thermometer was used to measure iron temps. it was running at 173 degrees.
Needles to say, paint was starting to peel of and had to open door to shop
because of the smell. This whole experiment was to see just how long a
tranny would last of this size. The next day I tore it down and examined the
insulation and the magnet wire. There were burn spots on the HV winding and
the insulation. I was surprised that the filament windings looked normal and
also the primary windings had little fatigue. I only done this as an
experiment. I am very satisfied with today's insulation. My 2 cents worth.
Regards
Gary...WZ1M


----- Original Message -----
From: <k6uuz at juno.com>
To: <magnuson at mac.com>
Cc: <hallicrafters at mailman.qth.net>; <magnuson at mac.com>
Sent: Thursday, April 03, 2003 11:28 PM
Subject: Re: [Hallicrafters] Power transformer temperatures


> Let me get on my soap box. I feel that high voltage leads to excessive
> temperatures and subsequent failures. Most of the tube-type equipment was
> manufactured when the line voltage was 110 to 117 VAC. Running it on
> today's 122 VAC can lead to failures. Some people think that because your
> equipment may be rated for something like 105 to 125 VAC you are within
> limits and safe. When a range is listed it usually is the design voltage
> +/- some amount. In this case the design voltage would be 115 VAC. If you
> have any doubts, measure the filament and B+ voltages and compare to the
> service manual. A quick and easy way is to pull a tube and measure the
> voltage on the socket filament pin. If it is more than 6.3 or 12.6 volts
> you are running over rating. You can get info on building or buying a
> simple but efficient voltage reducer from the BAMA site, about 1/2 way
> down the page. Thanks for the bandwidth.
>
>  Ed Richards
>
> On Thu, 3 Apr 2003 18:46:02 -0800 Waldo Magnuson <magnuson at mac.com>
> writes:
> > Does anyone know what the normal range of temperatures is for power
> > transformers?  I've been monitoring the temperature on an audio
> > amplifier (a Craftsmen RC-2 which drives a pair of 6V6s).  I measure
> >
> > the temp at the mid-point on top of the power transformer with my
> > wife's digital food probe and it has been reading up to 120.0 deg. F
> >
> > after a couple of hours.  Is this normal, or hot, or cool, or ?  Is
> > there any guess on how much the cover on my SX-42 power transformer
> > increases its temperature.  I've heard that over time the
> > temperature
> > tends to weaken the insulation amoung the transformer windings much
> > in
> > the same way as it dries out capacitors.  Any thoughts?  Thanks.
> > Skip Magnuson
> >
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