LC10 Printer power transformer for (sub-)miniature tubes

[maarten]

Recently I found a powertransformer in my junkbox, which I apparently salvaged from my old Star LC 10 printer. This printer was quite popular some 20 years ago, so there might be still quite a few around....
Anyway, the secondaries give 30 and 9 volts AC, and the size of the iron used and the thickness of the wire indicate that this transformer was meant to deliver quite some current. After rectification and filtering the AC 30 volts winding was at 37 volts DC; the 9 volts AC provides a nice 6V3 after rectification and regulation (a 7805, the adj. connected through 2 diodes - 1n4148 - to earth).  I was able to run 3 preamp tubes (5672) and 1 power tube (5902), without this transformer getting very warm - if i am right, the word for this is lukewarm. Total filament requirement of these tubes together is 600 mA. My conclusion: a nice transformer for playing around with subminiature tubes - well worth to pay a visit to your attic or basement!

From the 6V3 (for the 5902) I regulated the current further down to 1V25 by using a LM 317T. You also can use 100 Ohm resistors to drop the 6.3 voltage down to 1V25 for each of the other tubes.

Take care of proper isolation of the primary side! Also, you will find that on the primary side there is a fuse (very well) hidden under the coil: use it, when connecting to your power outlet.

Regards,
Maarten

[frequencycentral]

Sounds like a plan Maarten! Well worth keeping an eye out for one.

From the 6V3 (for the 5902) I regulated the current further down to 1V25 by using a LM 317T. You also can use 100 Ohm resistors to drop the 6.3 voltage down to 1V25 for each of the other tubes.

There's no need to use a 100 ohm resistor, just hook pin 2 of the 317 directly to pin 3 of your 5672's, you'll get 1.25v. Someone over at AX84 put me onto the idea, I've been using it ever since - I too used to use a 100 ohm resistor.