Calculating output transformer impedance

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Calculating output transformer impedance

Submitted by costas sarris on Wed, 10/12/2008 - 23:42.

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)

costas

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If my tube amp design asks

Submitted by szanti (not verified) on Sat, 17/10/2009 - 07:35.

If my tube amp design asks for a 7500 ohm primary output transformer, what would happen if i can't fond one and replace it with a 6000 ohm or a 8000 ohm. Would i have to compinsate the difference with resistors.

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Generally when we reduce the

Submitted by costas sarris on Sat, 17/10/2009 - 23:21.

Generally when we reduce the primary impedance of the OPT this action increases the output tube current flow. So check your circuit and your tube data and find which one fits best. I don't know what kind of tube you are planing to use but I think the 8000 ohm is better choice.

costas

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The amp I am building is on

Submitted by Salvo on Mon, 26/10/2009 - 04:59.

The amp I am building is on the web site mentioned below. In my previous e-mail I asked about using a different output transformer. Also can not find 2.2m ohm pots. Can i incorperate 1m ohm pots into the circuit. Your help would be appreciated. I don't want to give up on this amp. have invested too much time and money.

http://www.wkinsler.com/radios/5tubeamp.html

Thanks
Salvo
Montreal Canada.

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He has undated the

Submitted by Jason Katz (not verified) on Sun, 22/11/2009 - 04:54.

He has undated the schematic, it doesn't have the 2.2m pots anymore....

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hi guys, clarify something

Submitted by docguitar (not verified) on Sun, 14/03/2010 - 00:22.

hi guys, clarify something for me.

i am using a signal generator to test my old o/t i have here. but i am getting different final impedances on the primarys,

i do not know the tap specs for the color codes. this is a old conn organ o/t 4 wire output/secondary side. blk/gray/green/yellow.

i did play around with dvom positions and did get 2 of the wires at .034vac..

but just do not know the correct points.

i did read that it did not matter which tap, but to me it does since i get different readings.this is by marstran testing.

thanks , doc

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Hey Doc, I don't know if

Submitted by Eddie (not verified) on Fri, 30/04/2010 - 19:07.

Hey Doc,
I don't know if this has been answer yet but here's a little trick I use and it works good. Try it this way. OK you say the Sec/windings are Blk/Gray/Green/Yellow, well lets start with that.We'll say the black is ground since most are. Take a low volt power tranny (1 to 12 volts, a heater tap on a tube power tranny is great)and connect the sec/winding to the primary of the output tranny. Since we asumming that black is ground take a volt meter and connect the ground lead to it. Now plug the power tranny in. take the hot lead of your volt meter and touch it to the Gray wire, write the voltage reading down. Now take the red lead and touch the green wire write the voltage down. Now touch the yellow wire, write the voltage down. now you have a low volt reading, a med volt resding and a high volt reading. Now go back to
http://sarris.info/main/calculating-output-transformer-impedance
calulate all three tabs as 4 ohm, then calculate all three tabs as 8 ohm and then all three tabs as 16 ohms. now that you have done all the math there sould be a impedance average between 4, 8, 16 ohms if there is then you have found the 4, 8, 16 ohm taps if one tap is still off the that tap was most likly a feedback tap.