71A Direct Heat Triode Single End Class A Amplifier - diyAudio
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Old 11th April 2010, 06:32 PM   #1
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Default 71A Direct Heat Triode Single End Class A Amplifier

71A is a vintage direct heat triode power tube for audio amplifier. 71A is firstly made by Cunningham as CX-371A in USA in 1930s. Although 71A is a low plate voltage (180V max.) and low ouput power (0.8W in SE class A) triode, it can drive loudspeakers in high sensitivity. Nowadays, more and more audiophiles like the sound of vintage 71A.

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http://www.mif.pg.gda.pl/homepages/f.../029/7/71A.pdf


We started to make our first 71A amplifier.
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Old 12th April 2010, 06:05 PM   #2
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In the first, drill holes for tube sockets, transformers and knobs on top plate. The layout of 71A amplifier was complete.

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Old 13th April 2010, 05:05 PM   #3
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The circuit is 6SL7-71A single end class A with 5Y3 rectifier.

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This is maple wood chassis.
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Old 18th April 2010, 05:07 PM   #4
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71A amplifier is complete. 6SL7-71A single end with 5Y3 recitifer.

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Old 18th April 2010, 05:49 PM   #5
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RCA 6SN7GT, RCA UX-71A and Mazda 5Y3GB.

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Old 19th April 2010, 06:06 PM   #6
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This very rare and expensive vintage ballon shaped UX-71A triode (171A, 271A, 371A) delivers sweet, soft and smooth sound.

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Old 20th April 2010, 04:08 PM   #7
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71A amplifier is Twin Audio Sinfonia series tube amplifer.

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Old 20th April 2010, 05:26 PM   #8
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The price of this 71A single end amplifier is USD900.
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Old 21st April 2010, 05:18 PM   #9
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We adjusted 71A under AC filament supply and would be a little bit hum, but maintain the sound of 71A warm and smooth.
The plate current of 71A is 20mA.
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Old 22nd April 2010, 09:23 AM   #10
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This is the output transformers for 71A single end. The primary impedance is 5K and secondary impedance is 8 ohms and 16 ohms. The output power is 7W. The steel core used is Z11. The transformers were made in Poshan, Hong Kong.

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