Output transformer Impedance.

[Underwood]

I have a Magnatone 260A and I am not sure the OT impedance. I ordered 2 new 8 ohm speakers and just realized this. I am pretty sure it is a 4 ohm because I got a reading of 7.5 ohms on one of the speakers. Any easy way of measuring the OT ohms?The schematic does not say, but it does say that the extension speaker jack is 8 ohms. But, if the original speakers are measuring around 8 ohms and wired in parallel then it's got to be 4 ohm OT I would think. right?

[Colas LeGrippa]

well if you're swapping 2 X 8 ohm for 2 X 8 ohm speakers , OPT impedance is respected as long as you wire them as the originals are. Ifyou wire them in series, then the total impedance is 16 ohm and then you'd have to use the OPT 16 ohm tap instead of 4 ohm. When measuring a speaker, don't forget a 8 ohm speaker has a DC resistance of approx 6.5 ohm, that's the reading you'll get on your meter.

Colas

[Underwood]

One measures at 6.9 or 7 and the other I can't get a measurement on. If a speaker is blown would it not measure resistance.

[jojokeo]

2 8 ohm speakers in parallel = 4 ohms output load impedance on your amp's OT. You are correct in using the two 8 ohm speakers.
No "easy" way to measure/test your output tranny. You need to disconnect OT, apply voltage with auto transformer or similar and do math equations.

[kagliostro]

Here a little utility to calculate speaker impedance

http://www.duncanamps.com/technical/impedance.html

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about measure OT impedance if I remember well there was a method to do that without unsolder the OT

you must have the amp disconnected from AC line

take off all final tubes and rectifier

insert a jack on the speaker connection

than you must use the connection of the jack and the pin (corresponding to the OT) on the socket to have access to the OT windings

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here a couple of link that explain how operate

http://www.radioremembered.org/outimp.htm

http://www.geofex.com/ampdbug/outtrans.htm

K

[Tyrannocaster]

I have the following in my scrapbook, but unfortunately I don't know where I got it.  However, I used the technique to figure out the specs on an OT and it worked, or maybe I should say the amp has not blown up yet, LOL. For an AC source I used a wall wart that puts out 10.25 volts. (It was what I had on hand.)

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How to calculate the output transformer impedance of an OPT with unknown values

Working with tube amps you may need to calculate the output transformer impedance especially when we have an old transformer or a transformer with unknown values. All we have to do is to calculate the turns ratio of the transformer. To do this we need an AC voltage source and an AC voltmeter. So we apply an AC voltage to the primary of the transformer and we measure the output voltage to the secondary.

Turns Ratio = V in (to the primary) / V out ( measured in the secondary) e.g. if we apply 5 Volts AC and we measure 0.21 Volts AC to the secondary the Turns Ratio of the transformer is :
5 / 0.21 = 23.8 That means a turns ratio 24:1

Knowing the Turns Ratio we can calculate the impedance ratio of the unknown transformer and the impedance in a given load to the secondary. The impedance Ratio is the square of the turns ratio:24 X 24 = 576 that means an impedance ratio 576:1

So in a given load impedance of 8 ohms the transformer impedance is 8 X 576 = 4608 ohm ( it is about 4.5K)