[Collins] 75A-4 Power Xfmr

[Carl]

This chap would disagree about the benefit of going SS with the rectifier

http://www.w8ji.com/power_transformer_stress.htm

Ive built full bench bucking transformers into the station; one large 
transformer takes care of all the 120V line dropping for each bench, old or 
new gear. Using unpopular oddball voltages at high current found cheap at 
hamfests and surplus dealers I run 110-114V to everything.  Various consumer 
radios, mostly high tube count wood consoles and table models, have a small 
transformer either under the chassis or in a small box that includes a HB 
line filter and outlets that feed several radios. A simple 12.6V 2A 
secondary will handle up to a 240W load; I run mine at about half their 
ratings.

National for one commented in QST that the early HRO sensitivity was better 
with 180V B+ than 250V which is only needed for full speaker volume.

With 2 or 4 tube audio amps in the consumer sets, I hear no volume 
difference at the lower line input; they still rattle the timbers!

Carl
KM1H


----- Original Message ----- 
From: "Dr. Gerald N. Johnson" <geraldj at weather.net>
To: <collins at mailman.qth.net>
Sent: Wednesday, April 13, 2011 3:24 PM
Subject: Re: [Collins] 75A-4 Power Xfmr


> Be sure to examine the products from Hammond Transformer and others.
> Hammond has a line with 50 volts bias instead of 120 volts. Best I can
> analyze the circuits and available manual data, the 75A-4 needs about
> 225 volts each side of center at about 120 ma to a choke input filter
> (which stresses the high voltage winding less than a capacitor input
> filter and about 7 amps of heater current. I don't know which lamps are
> used for pilot lamps the manual only gives a Collins part number but I
> suspect its a type 47 or 44, a common lamp. Ahah, same part number in
> the 32S-1 manual says 150 ma at 6.3, so its a type 47. Makes the total
> heater current 6.75 amps.
>
> A transformer winding doesn't expire with a 10% overload, it just heats
> a little more, eventually charring the paper insulation between winding
> layers (in decades of operation) and it probably has a bit lower output
> voltage because the winding always has inductance and resistance. And
> not all that voltage drop is in the secondary, some of it is in the
> primary and if some other secondary is loaded less the voltage drop
> isn't quite as much on the slightly overloaded winding and the total
> transformer temperature rise is less. Some energy can be saved by using
> a solid state replacement for the 5Y3GT. More energy can be saved by
> running the line at 115 volts instead of the 120+ that modern power
> systems tend to prefer. One line of Hammond transformers comes standard
> with 115 and 125 volt taps for that purpose. The 'A4 was designed in an
> era where 115 was a typical line voltage.
>
> In transformer designs there are conservative designs (for long life and
> lower temperature rise) based on 1000 circular mils per amp, and low
> cost designs based on twice that current density, 500 circular mils per
> amp. The low cost design will operate hotter, but won't burn up.
> Probably won't last as many decades as the conservative designs, but
> fits more applications because there's half the copper and so it can be
> a smaller transformer. These days, there are better insulating materials
> for wire and paper layer insulation to allow safe and long time
> operation at increased temperature and better iron for lower loss at
> higher magnetic field intensities.
>
> Where the bias tap puts out 50 volts instead of the desired 125, I
> figure a voltage doubler can achieve adequate bias voltage. Takes adding
> a 1N4004, anode to the cathode pin of the bias rectifier, cathode to
> ground. Then adding probably a 20 to 50 mf 150 volt electrolytic between
> the transformer wire and the cathode pin of the existing bias rectifier.
> + of the capacitor to the transformer.
>
> Where there is no bias tap, it should be practical to use a capacitive
> or resistive voltage divider from the 225 volts AC plate winding.
>
> Or it would be practical to use the smallest available 6.3 volt filament
> transformer powered by the main transformer 6.3 volt heater winding
> running backwards to provide an isolated 120 volt AC supply. A practice
> for ham power supplies that has been used all my life.
>
> I see on the Hammond web page that they will do custom designs, $500
> minimum order. Its probably a dream but maybe they could sell 3 or 4
> transformers to fit a 74A4 for that price.
>
> Dave Knepper and I have been discussing these options off list.
>
> It would be nice to have my computed AC plate voltage on the main
> rectifier confirmed.
>
> 73, Jerry, Technical Adviser to the Collins Radio Association.
>
> On 4/13/2011 11:58 AM, Chris Kepus wrote:
>> Thanks to all who responded to my questions.  You guys are great! :-)
>> When I dig in further, you all will be the first to know.
>>
>> In the meantime, thanks for the leads on tranny rewinding and the
>> differences in the A2/A3 trannys and the A4 tranny.
>>
>> 73 and tnx agn,
>> Chris
>> W7JPG
>>
>>
>>
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