Understanding AUDIO Transformer Technology for Tube Amps?

[gary350]

We did not learn anything very technical about audio transformers in college 40 years ago so I am learning audio transformer design on my own. After reading a very technical book on Audio Transformer Technology, wow these things are way more technical than I ever dreamed. The author is very technical in the whole book, this is the type of book a person needs to be concentrating 100% all the time other wise you miss something. This is the type of information I love so I will understand every detail about Audio Transformers. I think I will need to read this information about 6 or 7 times to take it all into memory.

I think I have missed something but after several hours of reading I need a rest. I will read the information again in a few days.

Question. If a person needs to design an audio transformer with 5000 ohms on the primary, 5000 ohms is wire resistance only no matter how many turns it takes. Number of turns is determined by wire size and wire resistance. 5000 ohms is the tube plate load for B+. I don't recall reading anything about how to wind the secondary coil to get 4 ohms or 8 ohms. Is this determined by wire resistance too?

 

[nsaspook]

Just to be sure we are in sync, the primary DC resistance will be different from the primary AC impedance of the transformer. The AC impedance is what really matters here for matching as the actual DC resistance of the wiring will be much lower (usually).

I would suggest you read this also 1.1.2 and 1.3.5 : https://www.electro-tech-online.com/custompdfs/2011/12/Audio20Transformers20Chapter.pdf

 

[gary350]

Just to be sure we are in sync, the primary DC resistance will be different from the primary AC impedance of the transformer. The AC impedance is what really matters here for matching as the actual DC resistance of the wiring will be much lower (usually).

I would suggest you read this also 1.1.2 and 1.3.5 : https://www.electro-tech-online.com/custompdfs/2011/12/Audio20Transformers20Chapter.pdf

Yea, thats right. The book says when the amp is idle there is only DC going through the transformer so impedance is basically zero. If sound is going through the amp then you have AC impedance. So I assume DC wire resistance acts like a resistor to keep the tube from nuclear melt down. According to my wire chart #28 enamel coated copper wire is 15.41 ft per ohm. 5000 ohms of wire is 77,050. ft of wire = 924,600. inches. Wire diameter is .01264" that = 79.11 turns per inch. If the transformer has a 2" sq core 2.5" long that is about 200 turns per layer = 1600" of wire. Each winding has 200 turns but each layer is larger is diameter not sure how many total windings of wire the transformer will have. The book has some formulas for figuring all this out. The book says the hardest part is making all the wire fit on the transformer.

I don't know the ma rating of #28 wire my book does not show it.

I am just about over dosed for the day. I guess next thing to do is calculate impedance for the primary.

 

[nsaspook]

Yea, thats right. The book says when the amp is idle there is only DC going through the transformer so impedance is basically zero. If sound is going through the amp then you have AC impedance. So I assume DC wire resistance acts like a resistor to keep the tube from nuclear melt down. According to my wire chart #28 enamel coated copper wire is 15.41 ft per ohm. 5000 ohms of wire is 77,050. ft of wire = 924,600. inches. Wire diameter is .01264" that = 79.11 turns per inch. If the transformer has a 2" sq core 2.5" long that is about 200 turns per layer = 1600" of wire. Each winding has 200 turns but each layer is larger is diameter not sure how many total windings of wire the transformer will have. The book has some formulas for figuring all this out. The book says the hardest part is making all the wire fit on the transformer.

I don't know the ma rating of #28 wire my book does not show it.

I am just about over dosed for the day. I guess next thing to do is calculate impedance for the primary.

Don't worry about the DC resistance of the wire. I have no idea what book you're reading but winding a tube output audio transformer to have a specific DC resistance is usually wrong. If you have a very high DC resistance primary you will waste a huge amount of energy as heat in the transformer. You want the lowest practical DC resistance that gives the correct inductance and turns ratio for the output impedance matching of the tube plate resistance.

Examples: http://www.hammondmfg.com/125SE.htm

 

[k7elp60]

I have a Hammond transformer catalogue and the primary and secondary resistances are much lower than the actual impedance. A #146S output transformer has a primary Z of 4000Ω and a resistance of 220Ω a secondar Z 8Ω and a secondary resistance of 0.7Ω so I concure with members that say the resistance and the impedance are two different things.

 

[gary350]

I have a Hammond transformer catalogue and the primary and secondary resistances are much lower than the actual impedance. A #146S output transformer has a primary Z of 4000Ω and a resistance of 220Ω a secondar Z 8Ω and a secondary resistance of 0.7Ω so I concure with members that say the resistance and the impedance are two different things.

I found some notes I made probably 30 years ago. A 50 watt push pull audio transformer for a pair of 7027A tubes is rated 5000 ohms plate to plate. I took the transformer apart and counted the turns on the primary and secondary. The primary has 300 turns of #20 wire with center tap at 150 turns. The secondary has a total of 17 turns of #18 wire with 4 ohms at 8.5 turns, 8 ohms at 12 turns and 16 ohms = 17 turns. Power supply transformer is 600 volts RMS at 200 ma.

A 60 watt push pull audio transformer for a pair of 6L6GC tubes is rated 5000 ohms plate to plate with 4, 8, 16 ohms on the secondary. Power transformer is 600-0-600 plate to plate 150 ma. Full wave rectifier = 300 volts X 1.414 so the power supply voltage should be 424 VDC.

Another 60 watt push pull audio transformer for a pair of 6L6GC tubes is rated 9000 ohms plate to plate with 4, 8, 16 ohms on the secondary. Power transformer is 760 VAC 115 ma with center tap.