Cheap output trans making by cheap audio autotrans

A project log for Vacuum tube game in 21st century

Playing vacuum tube is not so difficult as expected. Let's play great heritage of human being by modern technology!

kodera2tkodera2t 08/10/2019 at 12:361 Comment

If you are rich enough to buy audio output trans as you wish, this log may not be useful at all. Just before I've completed 6SN7 stereo amplifier, which needs two output trans, and I have just two trans, so my 14GW8 amplifier lost its output trans. This time I tried to make "cheap output trans" by audio autotrans (matching trans) and succeeded to revive my 14GW8 amp with this.

Autotransformer is very quick way to realize impedance conversion (or AC voltage conversion), but the primary and secondary side are not isolated. As you know, the plate voltage of output tube is applied through transformer, and if the secondary is not isolated, your speaker will get fire (or burn) by the applied B voltage.  One quick way to isolate DC is inserting capacitor of one side of speaker out, but I want to avoid any additional insertion of R, L, C, because they will limit (put color) the sound taste. (I am not audio maniac, though..)

I quickly recheck the trans in my hand (available at aliexress or else), if we cut the connection between pin 2 and 3 in the above image, at least we can use #1 and #2 for speaker out (8-ohm), now the #3 becomes new 0-ohm terminal of the primary side, and we can choose primary impedance as we wish! Of course, primary-secondary are isolated and the plate voltage can be applied through "new 0-ohm" terminal. The price of this trans is just around $5 and we can get very low price output transformer! (If you are not pious audio maniacs..) The movie below shows the actual operation. Indeed its sound quality is not so bad (not extreme, I think. I have no will to fight with $$$ transformer.) and this quick hack will be very useful for making low price vacuum tube amplifier.

Yes it is working (and also now the end of my vacuum tube game?)


Ken Yap wrote 08/10/2019 at 14:55 point

Thanks for your very educational and entertaining series!

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