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Author Topic: AL-811 Tube Recommendations  (Read 4589 times)
W7MJM
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Posts: 109




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« Reply #15 on: December 04, 2012, 11:32:18 AM »

After experimenting with 572Bs in the AL-811 amp, I have come to the conclusion that it's best to stick with 811A tubes.

I bought a used AL-811 amplifier about 8 years ago. The seller had just replaced the original 811A tubes with a new set Chinese-made 811A's. They continued to put out full power, but after about 3 years of use, when I opened up the amp to clean out dust and check on the tube condition, I noticed those telltale welts on the plates, indicative of my having maybe taken a little too much time when tuning them up. So I decided to replace them with 572B's, not for additional power, but to give me a little more margin of error when tuning up. They worked fine, but after a few years, the power output began to drop. I examined the tubes and they looked fine physically. I then swapped the original 811A's back in and the power output came right back up to full power, even with the welts on the plates.

After lots of discussion online, I've come to the conclusion that it comes down to the relative difficulty of manufacturing 572B tubes as opposed to 811A's. The graphite plates in 572B's allow them to tolerate much higher heat dissipation than the metal plates in 811A's. However, the tendency of graphite to provide lots of nooks and crannies in which air can "hide" makes it much harder to pump out all of the air during the manufacturing process. In the "good ol' days" when tube manufacturing had higher quality control, this wasn't as much of an issue. But now, given poor quality control, a tube like the 572B, during use, will be more likely to leak air trapped in the graphite. That will eventually make the tube gassy and gradually reduce output. In the worst case, the tube will fail catastrophically, though that has not been my experience.

Sure, you'll find 811A tubes that fail prematurely, due to poor manufacturing quality control. But being easier to manufacture, at least in terms of pumping out air, there's one less point of failure. And of course, if they do fail, they're cheaper to replace.

Yes, you need to be a bit more careful when tuning up 811A's. Using an electronic keyer set to its highest speed along with a good peak reading RF wattmeter allows you to tune up at 50% duty cycle, which is much easier on the tubes than tuning up "key down" with a continuous carrier. Follow the manufacturer's instructions for tuning up your amp, or take a look at W8JI's excellent website. Properly cared for, those 811As will last a long time. And if they do fail, they're cheap to replace.

73, Martin, W7MJM
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