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Discussion in 'Ham Equipment' started by Mudfoot, Jul 2, 2018.
Nice tubes. I have a pair of NOS 4CX250B's laying around here somewhere looking for a home to heat. One of these days I may make a 2m amp......one of these days..........
Just cross your fingers the vacuum is still good. Eimac tubes probably have the best record for holding a good vacuum after decades on a shelf. But nobody's perfect. We have seen them flunk the HV breakdown test, or go "SNAP!" in our tester more than once.
Odds are they're good. And if one of them flunks the smoke test when installed in an amplifier, the damage it does might not be too bad.
I would have thought "Damage might not be too good".
Just for my own education, could you please explain? (In all seriousness. I would think it could wreck a bunch of components.)
There is more than one way for a transmitting tube to break down inside. The most impressive is an arc inside the tube. If there are enough gas molecules contaminating the tube's vacuum, you get a tiny bolt of lightning inside. It's metal, so you won't see inside when this happens, but a burst of high voltage comes shooting out of the tube's low-voltage terminals when that vacuum loses its ability to insulate the elements inside from one another.
A burst of 2000-plus Volts into the 300-Volt screen-grid circuit can cause damage to that function of the power supply.
A burst of positive-polarity high voltage coming out of the tube's control-grid (input) terminal will almost certainly damage the circuit components that normally feed a NEGATIVE-polarity voltage into the tube. We learned to install a reverse-polarity protection diode in amplifiers using this tube. It doesn't prevent damage, but limits the casualty list to a couple or three parts only.
Kinda like the reverse-protection diode on a mobile radio's DC power jack.
There are ways to protect the amplifier's components from a tube breakdown. But they add cost to the build and usually get left out to minimize the sale price.
Some tubes will become "gassy" after decades on the shelf, and some tubes won't. Only way to tell them apart is to test before it's installed in an amplifier. A tube that shows leakage current from gas will sometimes improve after being powered with the filament only. Don't quite understand the chemistry of this, but we have seen it work with 1980s-era ex-soviet tubes. We check grid leakage at the start, and let the tube cook, filament power only. If the leakage reading falls after a couple of hours, we keep running it until the leakage reading stops falling and levels off.
Some tubes will arc over just once, the first time you put them under load. This is a part of broadcast-transmitter folklore. The official explanation is that the factory doesn't always get the vacuum perfect in the 10, 20 or 30 kW tubes. That one "Bang" causes gas molecules to be chemically bonded to the copper-metal surface inside the external anode. This is called "chemical pumping" and improves the tube's vacuum. If the tube does it once only, that was considered a good thing.
Or so they say.
Of course, if your 4CX250B does this, the sound is more of a "snap!".
I do know that when replacing the monster CRT in analog color TVs you would always want to run it all day once before sending it back to the customer. Turns out this same 'feature' applies to a CRT that has 23,000 Volts on it. It was routine to see the TV go "SNAP!" a time or three the first few hours of running. The picture would shrink, then spring back to normal. Always stopped before turning off the lights to go home for the day.
Conventional wisdom was that you don't want a TV with a new picture tube doing this trick in the customer's home. Might give them the wrong idea. Sending it home the day after you fixed it or later would reduce that risk.
High-priced ham amplifiers like the Acom have a built-in computer that monitors the tube's operating voltages. If anything is out of whack, it won't let you key the thing.
Sure does boost the price.
Thanks for the education.
I guess I forget that the "commercially" designed and available amplifiers over the years were built to make a profit and sell parts. Any extra protective circuits were quickly laughed out of production. The "normal guy" with a thin wallet would try to protect his investment if building his own.