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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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« 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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« Reply #5 on: August 02, 2020, 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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« Reply #6 on: August 02, 2020, 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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« Reply #7 on: August 03, 2020, 11:23:03 PM »

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..........

What transformer are you referring to, the interstage driver transformer for driving the grids of an 810 or the modulation transformer?

If you are referring to a driver transformer for driving the grids of an 810 you need about 175 Volts peak audio voltage X 65 mA peak audio current from the center Tap to each outside winding.

Phil
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« Reply #8 on: August 04, 2020, 12:25:02 AM »

Quote
What transformer are you referring to, the interstage driver transformer for driving the grids of an 810 or the modulation transformer?

Well in this case I am looking to drive a couple 100TH tubes in my BC-610. I am already doing that but I wanted to get more exact on my approach in driving the tubes.
I haven't sat down and done any of the figuring out of the turns ratio etc with the information everyone has sent me on the board as yet.

I already have at least by the people who have heard my transmitter a very descent sounding 610 but I wanted to tailor it a bit more, with a transformer that that meets the specs rather than a transformer that I just happened to have, on hand that I think is not quite what I need to meet the voltages and current for the tubes. Its just an experiment to see if I can get more positive peaks than what I get now with the same settings on my PA amp. Just playing around here to see what's what Tongue Tongue
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« Reply #9 on: August 04, 2020, 02:56:38 AM »

Quote
What transformer are you referring to, the interstage driver transformer for driving the grids of an 810 or the modulation transformer?

Well in this case I am looking to drive a couple 100TH tubes in my BC-610. I am already doing that but I wanted to get more exact on my approach in driving the tubes.
I haven't sat down and done any of the figuring out of the turns ratio etc with the information everyone has sent me on the board as yet.

I already have at least by the people who have heard my transmitter a very descent sounding 610 but I wanted to tailor it a bit more, with a transformer that that meets the specs rather than a transformer that I just happened to have, on hand that I think is not quite what I need to meet the voltages and current for the tubes. Its just an experiment to see if I can get more positive peaks than what I get now with the same settings on my PA amp. Just playing around here to see what's what Tongue Tongue

Here ya go!

BTW, the OX and OY resistors are the Ohmite non-inductive ceramic resistors:

https://www.ohmite.com/catalog/ox-oy-series?filter=category%3A2%3AOX%2FOY%20Series#cds-attribute-category

and highly recommended.

Changed resting grid bias according to Eimac Constant Current Characteristics.

Phil

* 100TH Modulator circuit.pdf (29.01 KB - downloaded 9 times.)
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« Reply #10 on: August 05, 2020, 02:04:16 PM »

Here is the actual problem I am having and this has been verified with two good sets on 100TH tubes that give the same results.

When I idle the tubes at 40 ma which is 20 ma per tube,when speaking normally into the microphone the modulator will rarely do any positive peaks and the energy as seen on a friends scope is mostly negative, I have discovered that by idling the tubes at 80 ma or about 40 ma per tune, the positive peaks are wonderful and I seldom go to 100% negative. 40 MA per tube is about 80 watts dissipation on 100 watt dissipation tubes so that probably has me closer to class A operation. Since I am 79 years old I am not worried about short tube life as they still may out live me but there must be someone out there who knows more about this than me as to why the modulator seem to work so much better at a higher resting current? I have at all times an REA mod monitor in line as a reference but I would not have picked up the mostly negative energy as seen in my friends scope. I don't have anything where I can measure distortion so I guess I am somewhat handicapped. So some ideas about whats going on may be helpful? Is there a simple way to measure distortion that one cannot hear? Whats needed a scope and a signal gen?

 
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« Reply #11 on: August 05, 2020, 09:02:24 PM »

Here is the actual problem I am having and this has been verified with two good sets on 100TH tubes that give the same results.

When I idle the tubes at 40 ma which is 20 ma per tube,when speaking normally into the microphone the modulator will rarely do any positive peaks and the energy as seen on a friends scope is mostly negative, I have discovered that by idling the tubes at 80 ma or about 40 ma per tune, the positive peaks are wonderful and I seldom go to 100% negative. 40 MA per tube is about 80 watts dissipation on 100 watt dissipation tubes so that probably has me closer to class A operation. Since I am 79 years old I am not worried about short tube life as they still may out live me but there must be someone out there who knows more about this than me as to why the modulator seem to work so much better at a higher resting current? I have at all times an REA mod monitor in line as a reference but I would not have picked up the mostly negative energy as seen in my friends scope. I don't have anything where I can measure distortion so I guess I am somewhat handicapped. So some ideas about whats going on may be helpful? Is there a simple way to measure distortion that one cannot hear? Whats needed a scope and a signal gen?
 

What is the voltage at the plates of the modulator tubes? The tube specs state that at 1500 volts and at a grid voltage of -20V the plate at idle would be 80 mA.

Your original question, the best I could determine, was related to transformer impedances, so what are the impedances of the transformers being used as you seem to have implied that you were not using the original transformers. Your transformer impedances could very well affect the way things operate.

Phil
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« Reply #12 on: August 06, 2020, 12:13:32 AM »

The plate voltage I measured with some good quality probe and meter are Fluke.

The B+ is 2100 VDC. Are far as the transformers impedance are concerned, both the drive xformer and the modulator are not original, I'll have to get back to you on that which will take two weeks.

I'll be going back up to my camp 125 miles north of Albany, NY and that is where my ham activities take place as far as on air operations are concerned. I do however, have a work bench here for making smoke and sparks Shocked Shocked
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« Reply #13 on: August 06, 2020, 12:48:07 AM »

The plate voltage I measured with some good quality probe and meter are Fluke.

The B+ is 2100 VDC. Are far as the transformers impedance are concerned, both the drive xformer and the modulator are not original, I'll have to get back to you on that which will take two weeks.

I'll be going back up to my camp 125 miles north of Albany, NY and that is where my ham activities take place as far as on air operations are concerned. I do however, have a work bench here for making smoke and sparks Shocked Shocked

I assume you have changed the polarity of your mike wiring to see if the modulation shows some asymmetrical behavior.

If you can determine the transformer brands and part numbers I am sure someone here can identify it and we can then consult the specs to determine just what impedances and turns ratio's we have, and whether they are appropriate for the Class B 100TH tubes circuit.


Phil 
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« Reply #14 on: August 06, 2020, 05:55:35 PM »

Hi Terry,

I'm wondering if one 100TH is much weaker than the other, specifically the one that has to do the work to drive the modulated B+ positive.  I just looked at the schematic and there is no way presently to monitor the tube current individually (directly heated cathodes).  Try switching the two 100Th between the tube sockets and run the lower bias current setting again and see if the behavior changes.  The only other convenient way to see the individual tube current I think of is to have a clamp-on ammeter that does dc at 10 mA or better resolution and have enough slack in the filament wires to put the wires of one tube's filaments in the clamp. Not ever being a BC-610 owner, I don't know if this is possible while running the whole thing.

Is the B+ voltage the same for the modulator and final?  More bias current gives less modulator current variation.  Filter capacitances?
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