71A Direct Heat Triode Single End Class A Amplifier

[Musik007]

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.

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


We started to make our first 71A amplifier.

 

[Musik007]

In the first, drill holes for tube sockets, transformers and knobs on top plate. The layout of 71A amplifier was complete.

 

[Musik007]

The circuit is 6SL7-71A single end class A with 5Y3 rectifier.

This is maple wood chassis.

 

[Musik007]

71A amplifier is complete. 6SL7-71A single end with 5Y3 recitifer.

 

[Musik007]

RCA 6SN7GT, RCA UX-71A and Mazda 5Y3GB.

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.

 

[Musik007]

This very rare and expensive vintage ballon shaped UX-71A triode (171A, 271A, 371A) delivers sweet, soft and smooth sound.

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.

 

[Musik007]

71A amplifier is Twin Audio Sinfonia series tube amplifer.

 

[Musik007]

The price of this 71A single end amplifier is USD900.

 

[Musik007]

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.

 

[Musik007]

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.

 

[Musik007]

115VA power transformer in EI 76x50 was used on this 71A SE. The power transformer was made in Poshan, Hong Kong.

 

[Musik007]

This maple wood frame is hand-made. It is natural color of the wood.

 

[Musik007]

The 71A SE is our unique design and was made and soldered by Thomas.

 

[Musik007]

We selected German ROE 100uF/100VDC golden capacitors as the cathode caps for 71A and 6SL7.

 

[Musik007]

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.

 

[Decker]

Beautiful! You are giving me GAS!😀

 

[Musik007]

Beautiful! You are giving me GAS!😀

Thank you.🙂

 

[kevinkr]

Tony, Thomas, very nice work. 😀

 

[Musik007]

Tony, Thomas, very nice work. 😀

Kevin,

Thank you so much.

Tony.

 

[rcdaniel]

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.