D.C. heaters and tube life

[TUBEDUDE]

Found this gem in an old Audio Express article:

But DC heater power in itself is detrimental to tube life.

At switch-on, the large inrush currents and the mechanical spiral construction of the heater always cause a very small mechanical movement of the heater structure inside the cathode. With a DC supply, these movements are always in the same direction and of similar magnitude, causing a kind of scraping. Over many switch-on cycles, this scraping can lead to weakening of the isolation between the heater and the cathode, causing crackling sounds and eventually failure. So unless you have an issue with hum, AC powering the heater is better for tube life.

[LOUDthud]

DC on a heater can't come up any faster than the AC Mains. If there is a DC regulator, it will probably go into Current Limit, but that depends on the design.

IMHO, more internet nonsense.

[TUBEDUDE]

I don't think speed was the issue.

[LOUDthud]

Heat is heat. The wire is always going to move the same direction when it gets hot.

[mhuss]

I remember that paper magazine article (from way back in the day). The writer was electrically competent, and this was way before the internet. This was a hobbyist magazine, though, not an industry journal.

A couple of points.

Most (all?) tubes were designed for AC heater power, so probably little to no testing was done using DC power.

The person espousing this opinion may have been an electrical engineer (I really can't remember) but was unlikely to have practical experience actually manufacturing tubes, and is also unlikely to have done any statistically significant failure mode testing. It was probably just an opinion.

I don't think using AC or DC makes a huge difference in cathode or heater life. You're more likely for a tube to fail because of the less-than-optimal manufacturing being done today. Not throwing shade at anyone, but the manufacturing machinery is very old at this point, and the experts who designed the machines and tubes are long gone.

[trobbins]

I agree that the rate of temp rise in the heater would be slower for a dc powered means than for ac powering, as the peak surge current capability into a cold heater is highly likely lower (especially if the dc method has an inherent current limit).

One 'advantage' of initial/post movement of the heater assembly within the cathode tube at turn-on/off is that age-related metallic migration and micro-bonds from the heater to the tube may be repeatedly fractured and perhaps even slightly relocated, so one form of demise (heater to cathode shorting or lowered resistance) may be being suppressed.

[R.G.]

This dredged up a memory. Cathodes get hot enough that there is a degradation mechanism from the voltage difference between the heater and the cathode. This is worst on either the high voltage or low voltage end of the cathode, so the cathode fails on one voltage-end. AC cathode drive flips the voltage many times per second, so wear is spread out. I think this was in ... er, "How To Get The Most From Your Tubes", maybe. I'll have to look it up.

In any case, the advice was to periodically reverse DC cathode current for longest life if you don't use AC heater drive.

[solderhead]

> Most (all?) tubes were designed for AC heater power, so probably little to no testing was done using DC power.

I'm old enough to think that there was extensive testing using DC power.

From a guitar amp perspective, where we focus our attention on miniature 12A?7 types of tubes and plug our devices into an AC wall power source, the view that the world revolves around miniature tubes and AC power makes sense because it verifies our experience with guitar amps and maybe tube televisions if your experience goes back that far. But the reality is that guitar amps and TVs are relatively new technology. In the beginning the high tech electronics industry focused on radio. In the early 20th Century, Radio Corporation of America was the world's biggest company.

Mobile tube radios were designed to run on DC. A good (and more "recent") example of this is the Motorola tube radio that we used to have in our cars. Those radios were designed to run on the vehicle's 12VDC electrical system, or if you go back far enough, some cars ran on 6 VDC electrical systems.

Portable tube radios have always run on batteries. In fact, the old portable radio units had multiple batteries, one for the heaters and one for the main supply. The multiple batteries were comprised of an "A" battery for the filaments and one or more "B" batteries for the plate voltage. The "A" battery provided low voltage but high current, while the "B" battery supplied higher voltage, often around 90 volts. The voltage rail derived from the "B" group of batteries carried high positive voltage, which is why we refer to the PSU rail today as "B+".

The industry had LOTS of experience with DC heaters. We tend to forget about that because it's not part of our experience or our area of expertise. But if you look closely at the GE 12AX7A data sheet, it lists AC or DC power both being specified for the heaters.

https://frank.pocnet.net/sheets/093/1/12AX7A.pdf

I think it's the norm, rather than the exception, for tube data sheets to list AC or DC voltage ratings. I'm sure if you look at older data sheets then you'll find that DC heating was very common.

Another thing to consider is that back in the really old days, when Tesla was battling Edison, there were competing AC vs. DC standards for utility power. The rivalry between Nikola Tesla and Thomas Edison, known as the "War of Currents," centered on their competing electrical systems: Tesla championed alternating current (AC), while Edison supported direct current (DC). Although the first large scale central power plant, built by Edison in London, used 110 VDC power distribution, Tesla's AC system ultimately proved to be more efficient for long-distance power transmission, leading to its widespread adoption.

Looking back to the 1940s-1960s many tabletop radios were designed to run off of either 110 VAC or 110 VDC power. These types of radios are commonly known as the "All-American-Five" design because they used 5 vacuum tubes. The earlier designs used large octal tubes (50L6, 35Z5, 12SA7, 12SQ7, and 12SK7) and the later designs used seven-pin miniature tubes (12BA6, 12BE6, 12AV6, 50C5, and 35W4). These sets were actually quite dangerous, as they often had a hot chassis insulated in Bakelite and were designed to have their tube filaments powered directly off of the power line with no transformer required.

[TUBEDUDE]

And the -C batteries were for bias voltage.
Thank the car and portable radio popularity for the abundance of NOS 6V6's we enjoy nearly a century later

[martin manning]

Another thing to consider is that back in the really old days, when Tesla was battling Edison, there were competing AC vs. DC standards for utility power. The rivalry between Nikola Tesla and Thomas Edison, known as the "War of Currents," centered on their competing electrical systems: Tesla championed alternating current (AC), while Edison supported direct current (DC). Although the first large scale central power plant, built by Edison in London, used 110 VDC power distribution, Tesla's AC system ultimately proved to be more efficient for long-distance power transmission, leading to its widespread adoption.

Not to digress further, but It was Westinghouse (who adopted Tesla's ideas and acquired his patents) Edison was competing with. In 1892 Thompson-Houston merged with Edison General Electric to form General Electric (GE), and Edison and his DC system were sent packing. GE brought Charles Steinmetz on board from Eickmeyer, and acquired their AC patents to catch up.