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In Search of the Perfect 300B Tube:
Now that we've discussed possible lurking hazards (life is never easy), let's take a look at the seductions posed by over-specified 300Bs. Some manufacturers imply that you get a higher power output from your amp when you install a high-dissipation 300B (eg, 18Wpc output instead of 9Wpc from a typical SE amplifier). This is not true. The amplifier would have to be redesigned to fully take advantage of a tube's higher maximum anode dissipation. The output generated is a strict function of the available power-supply voltage, the specified anode current, and the output transformer's impedance (including load). If, in an existing amplifier, an increased anode current should occur and result in higher power output (rarely more than one or two extra watts), this means that the output transformer wasn't correctly designed in the first place, and that there would be power loss using normal, traditionally spec'ed 300Bs to which it wasn't optimally matched. A "heavy" 300B might sound more powerful, though, because of its more rugged construction. Before we turn to the evaluation itself, a few more warnings: I don't have to tell you that 300Bs are expensive, so treat them with care—and I mean more than just avoiding breaking the glass envelope. The 300B is directly heated, so the heater is not safely tucked away inside the cathode as in a 6CA7, a 6550, or most other power tubes—in a 300B, the heater is the cathode. It's hung from springs or wires inside the anode, in order to keep it from touching the grid. This kind of filament is brittle when warm, so let it cool completely before handling the tube—or the amplifier. If you should ever see one or more segments of your 300B not glowing while the others are, beware of that tube: if a part of a broken filament should bend and touch the grid, the anode current will quadruple, which can damage your amplifier. Also, there are rumors that acids from your fingertips can migrate through the glass and slightly change the tube's performance, which is why certain people always use gloves while handling expensive tubes. I have never seen any proof of this assumption, but it's better to be safe than sorry. The 300B has two thick pins for the filament and two thinner pins for anode and grid. This should exclude the possibility of inserting the tube into its socket in the wrong orientation, but I've seen sockets that don't care about pin size and will accept the tube in any orientation; needless to say, this could damage the amplifier. So study the socket before inserting a new 300B. Better still, while removing your old 300B, note which holes its thick pins were in. And this article about the heavenly sounds of our beloved 300Bs was supposed to be fun... The Roundup The test field was composed of the following (footnote 2): The filament current of each tube was measured at 5.00V DC precisely, and checked with three different digital multimeters (one of them a calibrated professional meter) to eliminate errors. Table 1 shows the results. Tubes with filament currents above 1.5A are a potential threat to amps equipped with standard 7805 filament regulators. Verify that the amplifier's power supply can handle a tube's filament current before you try it (footnote 3). Table 1: Minimum & maximum measured filament currents
Footnote 1: After the review, four of the six Svetlana samples developed a short between grid and cathode after some 300-400 hours of use. Svetlana has since made changes to its line. I know that the Western Electrics, JJ Electronics, and Golden Dragons are reliable; I cannot tell what the others would do in an endurance test. Once Svetlana sends me the new samples, I'll report my findings. Footnote 2: I decided not to include the AVVT. I've repeatedly seen VAIC or AN/AVVT (the AVVT's predecessors) 300Bs die before my eyes. I prefer to grant the AVVT 300B, now in its third incarnation, some more time to prove its reliability. Footnote 3: If an amplifier develops a soft hum through the speakers following insertion of a high-filament-current 300B, this does not necessarily mean that the amp is damaged. It could also be that the extra current draw results in the 5V regulator being left with too little voltage headroom on the power-supply side, and no longer fully suppresses the 100/120Hz ripple.
Article Continues: Page 3 »
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