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Switcher Test Results

Posted by Triode_Kingdom on June 7, 2017 at 07:07:51:
Yesterday I tested a few switchers and an eBay DC-DC buck converter. The converter is rated at 8A, and it uses a different regulator than the more common 3A units that sell for a dollar or two. I still need to try those, but they haven't arrived yet. The converter I used is adjustable for both output voltage and current limiting and is pictured below. I used a 811A tube for all the tests. The filament draws 4A at 6.3V, and cold resistance is very low.

The first test was a 5V/4Amp switcher that I've converted to 6.3V. When AC power is applied, it produces a series of three short output pulses, then it starts and heats the tube. Total time to stabilize is about four seconds.

The second test was a 12V/2Amp switcher driving the buck converter. I adjusted the converter in advance to 6.3V and set the current limit just above 4 Amps. The switcher wouldn't run properly in this situation, even with the converter's current limiter reduced to near-zero. Output of the switcher would slowly creep up to a maximum of 2 or 3 volts over a period of 10 or 15 seconds. That was enough to get the buck converter working a little, and the 811A would light dimly. Despite the heated filament, the switcher's output would not increase further.

Next, I tried a 12V/5A switcher driving the buck converter. This worked fine, but the switcher is much larger than the other two. It won't fit on a 2" sidewall like the others, so it would take up a lot of space under the chassis.

Finally, I powered the buck converter from a linear 10A bench supply operating at 15V. This also worked well, and it provided the opportunity to test efficiency. Comparing input power to output, I calculated the efficiency at 85%. This agrees with the datasheet for the regulator IC used in this particular converter. It also means the 12V/2Amp switcher would need to output 2.5A to heat the tube, even after the filament is warm. So, in addition to the failure to start, that's simply the wrong way to get this done.

A single large switcher is more difficult to mount than two small ones, and this seems to be leading the charge in determining the best way to resolve this. I now think the two best approaches to dual voltage heating are A) two small switchers, a 6.3V and a 12.6V, or B) a linear 12.6V supply driving a 6.3V buck converter. A third possibility would be a 6.3v/4A switcher (same small size as the 12V/2A version) along with a boost converter to power a few 12V preamp and driver tubes. The best configuration will obviously depend on how much current needs to be supplied at each of the two voltages. In any event, I'm very encouraged by these results. They do seem to indicate that no linear, heat-producing regulators will be needed in order to power all the 6V and 12V tubes on a single chassis.

Incidentally, 120 Hz ripple from these switchers was less than 20mV pk-to-pk, and HF/RF noise was even lower. No external output filtering will be required to use any of these devices for heating.