The Hickok 539B aka the Equalizer
Updated: May 27, 2021
A vintage tube-based stereo depends on strong balanced tubes. Newly manufactured tubes make this straightforward, however vintage even if NOS (new old stock), requires testing to ensure a balance between the plates within a tube but among its companion tubes in the same amp and in the other active amps. The vintage MC275 amps use 7 signal tubes with the lead tube, a 12AX7 followed by 2-12AU7, 2-12BH7, and 2-12AZ7, (USA Tube Reference #). The tubes following the 12AX7 are paired in groups of 3 for the left and right channel, (L = 1-12AU7, 1-12BH7, and 1-12AZ7). Within each of these tubes they have 2 plates that need to be measured and must be within 3% of mutual conductance with no shorts or leaks, and then tested to within 3% of the next companion tube in the same amp and then 3% to the next 3 amps and their corresponding tubes. With 4 amps and a pre-amp having a total of 34 signal tubes and 16 power tubes the amount of testing required to keep the amps balanced for best imaging is significant and requires a large inventory of tubes. 34 signal tubes is 68 tests at about 2 min each assuming all tubes were accepted/matched; Time-wise, that’s over 2hrs of testing for a total system rebalancing. The 3% tolerance is my number determined via trial and error; 10% is too much to keep optimum imaging and less than 3% is hard to find or maintain. The 3% maybe a little tight but when running 4 independent amps it’s a must; think of it like 4 artists painting an image simultaneously where the brush strokes are aligned else the image will be blurred.
Once the amps and pre-amp are loaded with balanced tubes I check the dB of the left vs. right channels for each driver at different frequencies at the listening position. That along with reference recordings with strong imaging confirms the tube selection and balancing.
At this point, I also want to introduce my biggest support tool, the Hickok 539B. The 539 was a popular tube testing tool for many technicians who repaired TVs, Stereos, radio’s and other tube driving appliances. This 539 is also point-to-point wiring and operates on tubes requiring its own tuning and is critical. Check out the photo, it’s definitely a bit off-putting but it’s a champion for testing tubes for leaks, shorts, and mutual conductance matching.
As for the inventory of tubes, the KT88 power tubes being manufactured today can be ordered in matched lots of 16 for the setup as I have. It’s the vintage signal tubes from the 1960s that take time to find. There are plenty of tubes selling online but a knowledge of what you are buying to the detail of the plate size and color, pin layout and coating, getter shape and location, tube markings, etc. is critical. It does not take long to figure out that one may have purchased the wrong tube or the performance is below sub-optimal levels. Over time I have found great sellers and collectors and have tried all kinds of tubes to enhance the system but the selection I have now seems to be the end game. To know more details on signal tube selection, see my blog on “What makes a unique setup?"