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Application Notes:
1. All pentodes and beam tetrodes are
triode-connected.
2. For dual-section tubes, ratings are for
one section.
3. Maximum ratings normalized to
"Design-Maximum Ratings" system.
Triode-connected pentodes and beam tetrodes can perform well, but seldom have published data. An interesting project would be to characterize these tubes in triode mode and publish the results. In the mean time, here are some tubes that have shown good promise as series pass tubes:
6AR6 / 6098
6AU5GT / 6AV5GA
6CD6GA
6CU5 / 50B5 / 50C5
6CW5 / EL86
6DB5
6EH5
6DQ5A, 6JE6, 6JS6, 6KG6 / EL509,
and other color TV sweep tubes
6W6GT / 12L6GT / 25L6GT / 50L6GT
6Y6G
6550 / KT88
7591A
8417
Using Pass Tubes
Since the pass device is the main heat-dissipating device in the series regulator, it needs good ventilation. If a semiconductor pass device is used, it should be mounted on a big enough heat sink to keep the junction temperature well below 100 degrees C. A rule of thumb is that if the heat sink gets too hot to touch, it needs to be bigger or have more air moving across it. Since the case of the pass device is often at a high voltage, it needs to be protected so that no one gets shocked. Power tubes need either to be mounted so that they can get good air convection or have forced-air blown across them.
For positive-voltage supplies, the cathode of pass tubes is at the output voltage, which may be several hundred volts or more. The maximum heater-cathode rating needs to be observed and in most cases the pass tube needs its own heater supply that is connected to the cathode. Since most
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modern tubes have a maximum continuous heater-cathode voltage rating of 100 volts (+/-200V peak), the use of dual tubes such as the 6EM7 or 6DE7, where one section is the gain stage and the other the pass tube, can be problematical. The only reliable way to use these types is to float the heater voltage about half-way between the cathode potentials of the two sections. This still limits the maximum output voltage to about 200 volts. Of course, filamentary types, such as the 6B4G or 300B need their own filament supply. In all but the simplest cathode-follower regulators, the pass tube is inside a feedback loop that reduces hum, so the use of a DC filament or heater supply generally isn't needed, although the use of a center-tapped filament winding or "hum-pot" is necessary with filamentary tubes.
Many pass tubes have high transconductance and are susceptible to parasitic oscillations. These occur when the wiring connected to the tube looks like a tuned circuit. Even several inches of wire can be resonant at several hundred megahertz, and if the tube has gain at these frequencies, it can turn into a high-power RF oscillator! Due to inherent frequency limitations, bipolar transistors seldom have this problem, but power MOSFETs can have parasitic oscillations. The key to suppressing parasitics is to lower the Q of the parasitic resonant circuit, without affecting low frequency operation. The usual technique is to place a small resistor, say 47 to 1K ohms, in the grid lead, right at the tube socket. These can be low-power resistors, say 1/2 watt. In triode-connected beam tetrodes, it is customary to put a 100 to 200 ohm, 1 to 2 watt resistor in the screen lead. Ferrite beads can also be used. Depending on wiring layout and circuit design, these suppression resistors may not always be needed, but they are cheap insurance against a difficult-to-diagnose problem.
// JA
In the next installment: series regulator topologies
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