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Thursday, December 23, 2021

Tube of the Month : The 71A

 Hi!

This months tube is the queen of flea power triodes. The 71A.



The 71A is a directly heated triode developed for use in the output stages of audio amplifiers or radios. As mentioned above it is the queen of the flea power triodes delivering output in the sub 1 Watt range only.

The 71A has a 4 pin UX4 base like other triodes. The pinout is shown on the left. It has a coated 5V filament which only needs 250mA of filament current. The maximum plate voltage is 180V. As such the tube was well suited for battery powered applications. The RCA data sheet lists a maximum obtainable output power of 790mW but closer to only half a Watt for typical operating conditions. The plate resistance is 1750 Ohms which narrows the selection of suitable output transformers. A minimum of 5k primary impedance should be used while I would rather use 7k or more. With its low amplification factor of 3 the tube still needs considerable voltage swing at the grid to reach maximum output power. In the range of up to 80V peak to peak. Nevertheless a very interesting tube and I have to admit that I never built anything with it. Could be a nice tube for a tweeter amplifier in a an active set up or a headphone amp. Of course could also be used in a full range amp with suitable speakers which would have to have a sensitivity well above 100dB.




As always we have a look at the plate curves which are very linear as expected. Here the curves of an actual tube:




Nice and evenly spaced. Maybe some day I will develop an amp with this tube. Let's have a look at my stash of 71As.




Here a heap of Westinghouse 71A which came in generic boxes.




Some more views:












A bunch of 71A in military packaging:




















A pair of RCA 71A in sealed boxes:





Truly NOS NIB as the seals are perfectly intact. Not going to open these.






















Before the introduction of the ST shaped 71A the tube was made in ballon shaped glass and named UX171A.







Another UX171A:








And lastly photos of a 71A in operation:




The low current filament is rather dull and the glow is barely visible.




An interesting tube. I hope to realise a project with it sometime.

Best regards

Thomas



2 comments:

  1. I have only recently started collecting ancient triodes. I am behind all you guys. You mention batteries so with the major advances in battery tech what is the possiblility of using them as power supplies? My speakers are 107db1w1m so I can get down with the fleas.

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  2. Early radios were battery powered and for those the low filament current was helpful. You can power any amp with a battery but the question is why would you want to. I don't see a benefit

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