The ECF80 Triode-Pentode Vacuum Tube: The Integrated Circuit Vacuum Tube?

Given that it contains two different thermionic amplifiers in a single enclosure, does the ECF80 triode-pentode vacuum tube qualify as an “integrated circuit vacuum tube?

By: Ringo Bones 

Multiunit vacuum tubes may not be able to compete in size and component count with their solid-state integrated circuit counterparts, but if your hankering after that good old robust tone vacuum tube sound, post World War II era subminiature vacuum tubes offer those in a more reasonable sized package. The ECF80 triode-pentode vacuum tube is unusual in that this multiunit vacuum tube can run both its triode and pentode sections at about the same current. The ECF80 makes a great Mu-follower with the pentode strapped as a triode on top and it is often described as a medium Mu triode sharp cut-off pentode. 

Even though it is not featured in the 1959 Mullard Tube Circuits For Audio Amplifiers, Mullard says that the ECF80 was designed for operation up to 220-MHz and it was first introduced back in 1954. As 1950s era television receivers tend to use the PCFnn 300-milliampere filament heater vacuum tubes, the production of the ECF80 with a 6.3 volt heater filament suggests that it was aimed at the 1950s era professional VHF communications market. In a typical VHF application of the ECF80, the triode section is often used as a Wien-Bridge oscillator and has to produce a 5-volt peak-to-peak signal which could allow the ECF80 to be used as the active transceiver tube in 1950s era walkie-talkies. The screen pentode is the mixer. The vacuum tube has two cathodes so the two independent amplifiers are enclosed in a single envelope. The twin anode construction is clearly visible with the triode occupying the much smaller anode. The two sections can be used separately if required and thus extending the versatility of this vacuum tube. 

Given that post World War II subminiature vacuum tubes are primarily used as a cost saving preamplifier and phase-splitter sections of power amplifiers that use 1930s era output power tubes, the World Audio Design K6L6 Integrated Amplifier is probably the most famous application of the ECF80 triode-pentode vacuum tube where it was used as a “novel” phase-splitter. The problem with conventional double-triode phase-splitter, like the 12AX7 double-triode phase-splitter section of the ubiquitous Mullard 5-20, is that its high output capacitance caused by the Miller Effect. This causes high frequency loading on the input vacuum tube and reducing bandwidth, making it very difficult to use appreciable amounts of negative feedback without instability due to the phase shifts incurred. A pentode vacuum tube has a very low output capacitance and high gain due to the shielding effect of the screen grid. This means that the loading on the input vacuum tube is greatly reduced, increasing bandwidth and decreasing troublesome phase shifts. And the WAD K6L6 Integrated Amplifier’s use of the ECF80 triode-pentode vacuum tube as a phase-splitter also eliminated the need of an interstage phase-splitting transformer which also kept costs down.