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 Post subject: Identifying output transformer secondary wires?
PostPosted: Feb Thu 01, 2018 8:34 pm 
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Joined: Jan Thu 24, 2013 11:50 pm
Posts: 2828
Location: Bristol, Rhode Island
Hi,

I have a 35w or so output transformer from a Bell Carillon 35 pa someone gave me (I only have the transforemr, not the chassis). Anyway i haven't been able to find a schematic and I want to correctly identify the four secondary leads. Apart from black (common) they are grey, orange, green and yellow.
Reading them with my modern multimeter isn't helping; the DCR is coming in around two or so ohms each.
Is there a standardized color coding for output transformer secondaries? Of can anyone offer further assistance?

Thanks!

Ted


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 Post subject: Re: Identifying output transformer secondary wires?
PostPosted: Feb Thu 01, 2018 9:23 pm 
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Joined: Jul Mon 26, 2010 8:30 pm
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Location: Annapolis, MD
Hook up the primary to a Variac, and set the voltage to maybe 20 volts (the transformer should not get warm at this level---but keep an eye on it.)

measure the voltage on each wire with respect to the presumed common. Also do a sanity check between pairs that do not include the common. In a tapped winding, the voltages will add up algebraically. This is NOT true for the impedance.

At 20 volts in, you might get 0.5 volt on the first tap, 0.707 on the second, and 1 on the third---all with respect to common. (the sanity check would give you 0.207 from 1 to 2, and 0.293 from 2 to 3)

From the above, the voltage ratio and impedance ratio for the 3 taps:
#1: 40:1, 1600:1
#2: 28.2:1, 800:1
#3: 20:1, 400:1
the other 2 pairs give 2 more options

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 Post subject: Re: Identifying output transformer secondary wires?
PostPosted: Feb Thu 01, 2018 9:27 pm 
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Joined: Jan Thu 24, 2013 11:50 pm
Posts: 2828
Location: Bristol, Rhode Island
Excellent.
So just to clarify then, #1 would be the 16 ohm tap, #2 the 8 and #3 the 4?

And I'm assuming the 20vac would go to the primary plate leads? Leave center tap disconnected?

Thanks,

Ted


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 Post subject: Re: Identifying output transformer secondary wires?
PostPosted: Feb Fri 02, 2018 4:03 am 
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Joined: Jul Mon 26, 2010 8:30 pm
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Location: Annapolis, MD
It all depends on the desired input impedance---the transformer simply supplies an impedance ratio.

In my (arbitrary) example, the highest ratio is 1600:1. This means that--if a 4-ohm speaker is connected--the output tube(s) will see a 6,400-ohm load. With an 8-ohm speaker, they will see 12,800 ohms.....etc.

With a push-pull setup, the desired load is normally given in the data sheets as "plate to plate"---thus you would be interested in the voltage ratio input to output---using the 2 outer primary lead (not the CT)

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"It's always something". --Gilda Radner (1946 - 1989)


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 Post subject: Re: Identifying output transformer secondary wires?
PostPosted: Feb Tue 06, 2018 4:31 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 5027
Location: Rochester NY USA
The primary was likely about 4-6K for a 35W amp, so once you find the ratios, you can figure what the taps are. PA amps usually have 35V and/or 70V as well as speaker taps. If it's REALLY old, it may have 250 or 500 Ohm taps.


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 Post subject: Re: Identifying output transformer secondary wires?
PostPosted: Feb Tue 06, 2018 8:52 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2300
Location: Lakewood, California
Schematic for Bell P/A Model Carillon 35 is covered in the SED (Special Equipment Data) sheet in Sams Photofact set 1047. Output tube pair is 6L6GC, output transformer part number is listed as 32B3024. Output wire colors are:

Black: Common
Yellow: 4Ω
Orange: 8Ω
Green: 16Ω (25V)
Gray: 152Ω (70V)

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