Sorry to be so darned late in answering your question about the 572B tube life. There are some problems with Chinese 572B tubes not giving as much service life as the good old American made tubes. They also have significantly reduced ratings (e.g., they are not made as ruggedly) compared to American tubes, also inferior glass-to-metal seals which allows them to go gassy very quickly, and do tend not to give as much service life as American tubes.
So far so good, until:
There are also some serious problems with parasitic oscillations in Chinese 572B tubes because they are designed to have a higher transconductance than the American tubes, which tends to make them oscillate easily in the 70 Mhz range. (I refer you to the excellent work done by Richard Measures for taming down parasitic oscillations in the 572B, which tends to be a serious problem when operating them in the 15 and 10 meter band.)
That's not true at all.
The problem is feed-through power caused by the long very thin grid lead and the base and socket. The tubes will oscillate right on the operating frequency if not neutralized. If you look back through history you will see Gonset, Heathkit, and everyone else who made 4 tube amps with the 811/572 family neutralized the amps. Collins and Dentron were two of the very few that did not, and they had stability problems from the day they were built. Yaesu also had that problem in a two tube 572B amp, mostly because someone though they could add a feedback cap from the end of the pi-net back to the cathode. Two 572's, because the voltage is generally higher, are right on the edge of stability and won't tolerate mistakes like that.
As for the transconductance being higher, it is not. The transconductance is LOWER in the Chinese tubes. Just look at the idle current. It is higher for the same bias. That means transconductance is lower, not higher. The "higher transconductance" thing is a ploy to sell nichrome when the real problem is the lack of neutralization and adequate cut-off bias caused by the slightly lower mu of the Chinese tubes. The nichrome sometimes works by lowering the HF Q of the tank circuit slightly, especially when used with other gain reducing mods, but you can still often make the amp oscillate even with the nichrome.
A better cure is to fix the real problem, the lack of neutralization.
As for gain and current we all know a lower mu tube, for the same family and plate voltage, draws more current and a higher mu tube for the same bias has less current.
More good stuff:
Chinese 572B tubes are also not manufactured to be conservative in their plate voltage ratings, and while American 572B tubes are operating conservatively, the Chinese tubes are operating just about at full maximum capacity. As for the Ameritron AL-811, I recently serviced one of these amps running Chinese 572B tubes and found the filament voltage to be 6.38 volts at the tube sockets. That is a teeny bit high, but probably not worth intervention for the casual AL-811 user. Early failure of Chinese 572B tubes seems to be more of a problem with Chinese quality control than anything else. Keep them cool and clean (so they can dissipate heat, don't stick the amp where air can't circulate around and behind the cabinet) and don't try to run them at the upper range of their ratings (which should probably be derated by about 20%), and you'll get REASONABLE tube life from them. (Of course, the 572B is operating very conservatively in the AL-811 or AL-811H or similar amplifiers designed to use 811A tubes.)
If they've been sitting idle for more than a month, lift the plate caps and burn them filament-only for a couple of hours to burn off any oxygen accumulation due to poor glass-to-metal seals. (UNPLUG the amp and short out the high voltage before you stick your fingers in that cabinet!
The primary gettering on the 811 and 572 is on the anode and sometimes some flash is left on glass, depending on the type of processing. The filament really does no degassing at all. The anode has a gray powered looking zirconium coating and when the anode is hot, nearly red, it degases.
There are tubes that degas from filament heat, but they are generally only tubes like external anode tubes. The gettering is caused by use of certain metals on the filament structure, like tantalum in the 8877.
Lock the cat and kids out of the room!) Meanwhile, pray hard that some American tube manufacturer decides to brush off their manufacturing equipment and make a run of American-quality 572B's. Frankly, I don't understand why someone hasn't done that already. The old manufacturing jigs have to be sitting in a warehouse SOMEWHERE. Where did the Cetron or Taylor factory disappear to, anyway?
The US Government allowed Richardson's Electronics to buy up all the competing tube sources. They even closed a deal with Eimac where they became the sole distributor of Eimac tubes. Once Richardson's had control of all the tube manufacturing and distribution there was no reason to keep multiple vendors open. As a matter of fact the owner of Richardson's announced in a trade magazine his intention was to control the market because people with existing equipment would pay anything he wanted for tubes. That was when 572's went from $20 to $70 overnight.
When Richardson's was finally sued, after the market was shot, they divested some of their holdings. Old Richardson managers left Richardson's and took over the companies they were ordered to dispose of. Richardson's remained the primary distributor of the divested companies, that were run by former Richardson's people.
This is how our government solved an anti-trust violation and a lawsuit brought to force enforcement of laws.
It's a lot like when we sued the Japanese for dumping consumer electronics here and we won, and our government didn't allow or help with the collection of damages.
There is no turning back the clock now. It is too late.
Tom
