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71A Direct Heat Triode Single End Class A Amplifier

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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.

71A.jpg


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


We started to make our first 71A amplifier.
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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.

An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.
 
The capacitors using at the coupling stage between 6SL7 and 71A is CDET 0.33uF/630VDC film caps from USA. The quality of CDET caps is very good. We're using them in some of our tube amps.
 

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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.

I think you mean the output power is ~ 0.7W

I am looking forward to using some of these in an amp project - supposedly, they present music with a nice sense of delicacy and nuance, though can sound (visually) flat at times... We will see.
 
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