Well, ya better buy eight!
Or maybe nine or ten, like this guy found out. These six "strong" Fleabay tubes included three that tested good enough. One that's probably too weak to use. And two with what we call the "Blown Fuse" failure.
Seems that using tubes meant to use in a 23-inch color TV can reveal quirky faults when used to amplify high-power RF energy.
In particular, we see the tiny ribbon at the bottom of the tube's cathode becomes a weak link. Leaves a broken circuit that looks like the failed element in a glass fuse that has tripped from a slow-to-medium overload. Leads me to think that RF current will release more heat where the RF resistance is high, or reactance creates a voltage drop. Turns out that operating an amplifier out of proper tune will speed up this exact failure. Seems to afflict the compulsive knob twisters. They ones you hear tuning up, turning the Tune and Load knobs every which way looking for a little higher wattmeter reading.
The tube has one pin connected to the cathode. In this case, the 6LF6 and 6KD6 tubes used in the Palomar 300A use pin two.
The pin itself is spot-welded to a tiny thin metal ribbon. It leads to the base of the cathode sleeve that encloses the heater.
I suspect it's just a tail that's part of the pattern when they cut the cathode blank from thin sheet stock.
It melts just below the cathode, leaving a visible gap like the element of a glass fuse.
I have nothing to say about the seller of this batch of tubes. But any prospective buyer should consider the "yield", or percentage of good tubes you may expect.
In this guy's case, the yield was 50 percent. Figured that way, he should have ordered twelve tubes to end up with the six tubes he needs.
Sounds expensive.
OTOH, there are dealers who can be trusted to supply a 100 percent yield. Be confident that their prices are typically a lot higher.
Caveat Emptor!
73
Or maybe nine or ten, like this guy found out. These six "strong" Fleabay tubes included three that tested good enough. One that's probably too weak to use. And two with what we call the "Blown Fuse" failure.
Seems that using tubes meant to use in a 23-inch color TV can reveal quirky faults when used to amplify high-power RF energy.
In particular, we see the tiny ribbon at the bottom of the tube's cathode becomes a weak link. Leaves a broken circuit that looks like the failed element in a glass fuse that has tripped from a slow-to-medium overload. Leads me to think that RF current will release more heat where the RF resistance is high, or reactance creates a voltage drop. Turns out that operating an amplifier out of proper tune will speed up this exact failure. Seems to afflict the compulsive knob twisters. They ones you hear tuning up, turning the Tune and Load knobs every which way looking for a little higher wattmeter reading.
The tube has one pin connected to the cathode. In this case, the 6LF6 and 6KD6 tubes used in the Palomar 300A use pin two.
The pin itself is spot-welded to a tiny thin metal ribbon. It leads to the base of the cathode sleeve that encloses the heater.
I suspect it's just a tail that's part of the pattern when they cut the cathode blank from thin sheet stock.
It melts just below the cathode, leaving a visible gap like the element of a glass fuse.
I have nothing to say about the seller of this batch of tubes. But any prospective buyer should consider the "yield", or percentage of good tubes you may expect.
In this guy's case, the yield was 50 percent. Figured that way, he should have ordered twelve tubes to end up with the six tubes he needs.
Sounds expensive.
OTOH, there are dealers who can be trusted to supply a 100 percent yield. Be confident that their prices are typically a lot higher.
Caveat Emptor!
73
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