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PUSH PULL AUDIO TRANSFORMERS




 
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W2PFY
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« on: July 31, 2020, 10:44:03 AM »

How do I figure out what voltages I can expect to receive from an audio transformer when all I have is the value in ohms of the transformer?

For example I'm told on a Johnson Desk KW to drive the 810 modulator tubes to use a transformer that has a 1500 ohm secondary. ( this is an example only, the book may may call for something different) The primary could be anything up to 600 ohms.

So the object is to know how many volts I can get out of a transformer to drive the class B grids. I know that if I have the transformer all I would have to do is run 10 volts AC into it to get the turns ratio, but what if I don't have the transformer in my shop?

Inquiring dummies like me need to know:)

Thanks Terry AKA Captain Marvelous in another life..........
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WD8BIL
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« Reply #1 on: July 31, 2020, 10:54:54 AM »

What's the power rating on the transformer, Terry?
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W2PFY
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« Reply #2 on: July 31, 2020, 11:36:09 AM »

That's a good question Budly? If one were to consider the grid current of class B grids
of for example 833 or 304TH tubes one might consider 50 to 100 watt transformers. Lower wattage transformers may give the voltage but may become saturated via the grid current? So 50 watt to 100 watts should cover the current question? Please remember that I don't have the transformer here at this time.
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Chuck...K1KW
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« Reply #3 on: July 31, 2020, 01:55:10 PM »

Take the square root of the impedance ratio and that is the turns ratio.

Example:

16 ohm primary, 1600 ohm secondary.  Impedance ratio is 100, turns ratio is 10.

Chuck
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« Reply #4 on: August 01, 2020, 09:26:58 AM »

Grid current is spec'd in the tube manual for each tube running in "class 2".

Multiply the voltage x current = power.
Also the max drive wattage is often spec'd

Probably less than 10 watts for those tubes.
iirc.

                  _-_-bear
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WU2D
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« Reply #5 on: Yesterday at 06:48:19 AM »


First roughly measure the voltage ratio. Put a volt AC in with a low impedance source and see what comes out. The voltage ratio is the primary voltage divided by the secondary voltage you used. Here is the method.

To measure the reflected load to the driver, Rp-p, you need to make a voltage ratio measurement. Apply a low AC voltage like a 1 Volt, 1 kHz tone from an 8 Ohm audio amp to the secondary and measure the voltage on the primary (Plate to Plate). You can use a scope or voltmeter. Again, the voltage ratio is the primary voltage divided by the secondary voltage you used. The primary impedance will be the ratio squared x the secondary impedance. For example, if the secondary impedance is 600 ohms and the ratio is 5 the primary is 5 x 5 x 600 = 15K ohms.

Or you could use an AC Wheatstone Bridge.

Or use the substitution method using a series potentiometer at various frequencies with an audio generator and scope. You can play with with various loads you try on the secondary(s), to see what the effect is.

And probably 5 other methods!
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Patrick J. / KD5OEI
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« Reply #6 on: Yesterday at 06:56:49 PM »

That's a good question Budly? If one were to consider the grid current of class B grids of for example 833 or 304TH tubes one might consider 50 to 100 watt transformers. Lower wattage transformers may give the voltage but may become saturated via the grid current? So 50 watt to 100 watts should cover the current question? Please remember that I don't have the transformer here at this time.

This point of grid current is among those of utmost importance.
Besides saturation possibility, there is also a diminishing return as the resistance of the secondary winding becomes a significant fraction of the drive impedance of the grids. Put another way, consider the voltage drop across the grid winding's resistance at the peak grid current level. This situation may occur before saturation.

One terrible mismatch - the 40 watt CG-512 driver transformer forced to push 3-500Z grids instead of the 304 grids originally installed and previously driven effortlessly.
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