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RE: Matching current on signal tubes

Posted by JKT (A) on January 25, 2013 at 15:24:38

In Reply to: Matching current on signal tubes posted by jburtt on January 25, 2013 at 00:51:11:

Hi, I have a few differential amps/preamps that require dual triodes with current matched sections (6N30/6N6P, 6SN7, 6BX7/6BL7 etc.). I built a "box" similar to what you propose. It uses a 0.75A variac for a transformer and a voltage doubling rectifier arrangement for the B+ which is variable from 0-300v or so. Bias is supplied to the common cathode connection by two, switchable constant current sources (K&K Audio on this forum sells a nice little CCS kit) set to 10mA and 40mA as these are the bias currents that I am interested in. They have adjustment screws and can be changed if needed. Each cathode is in series with its own 10R resistor. This is there to monitor the current. The CCS is connected to the other end. I have tip jacks on the front of the box for ground, B+, the CCS and both cathodes. By plugging the probes of a DMM into the cathode jacks mV/10="offset current" in mA (ie a reading of 15mV means there's a difference of 1.5mA in the two sections. This box has proved to be very useful to me over the years. If you need dual triodes with internally current matched sections a jig similar to this would be useful.


"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln

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