Featured VT: The 6X8

 

This is one of those odd types that were designed for a very specific purpose: a local oscillator/mixer for both VHF TV sets and FM radios. As you can see from the pinout, this type uses a common cathode for the triode and pentode. Also, it is not built the way you would expect: one triode and one pentode with the cathodes internally tied together. The 6X8 has but one flat cathode. The two types share it, with the control grids mounted on opposite sides of the single cathode.

 

 

You may be wondering: what the hell is it good for? In audio applications, not a great deal. Except for one thing, that is. The 6X8 can be used as an unbalanced differential amp for active screen regulators, and other series pass regulators. The pentode half is capable of some considerable gain, the V_HK rating isn't half bad, coming in at 100V_DC (both polarities) and could be used without heater lift if using VR-75 or VR-90 types as voltage references.

 

 

In this schemo, the triode half forms the non-inverting input, with the "signal" consisting of the voltage developed across the VR tube. The pentode forms the NFB summing node and error correction amp. This regulator topology has the advantages over the more conventional designs since there is no load on the VR tube. The noise can be reduced by a simple RC lowpass filter. This prevents the possibility of inadvertently making as oscillator by connecting a bypass capacitor directly across the VR tube. (Not like Zeners, since the VR tube depends on a glow discharge that has negative resistance.)

I have used this actual design, and it works nearly as well as its solid state counterpart. Varying the DC rail voltage by some 40V caused a 0.8V drop in the output from the regulator. When using pentodes as power finals, sonic performance is definitely improved by good screen voltage regulation, assuming you're not using Ultralinear, or pseudotrioding the pents. This is where commercial designs cut corners: very few commercial (or even industrial) designs did anything more than use a voltage divider to supply the screens. Most relied on a single series dropping resistor. YUCK! That's the advantage to DiY: we don't have to answer to the pencil-pushers in the Accounting Dept.

You can also use the 6X8 as a "quick 'n' dirty" floating paraphase, but you wouldn't want to use that anyway if you're looking for sonic excellence. It could save some Miller capacitance since the triode half, operating as essentially a cathode follower, has no Miller capacitance. However, would that save enough to make a difference? That's what pentodes were invented for in the first place. The exception would be a guitar amp, and the 6X8 just might find some good use there.

As with any high gain pentode circuit, care must be taken with the construction if instabilities are to be avoided. This includes grid and screen stoppers. If these aren't needed, no harm done, but if there are instabilities, they are definitely necessary. Keep all connecting leads short as possible, as for an RF project, use sockets that include a central pin, and connect it to the ground plane for electrostatic shielding. The screen bypass capacitor can likewise be installed across the bottom of the socket, between the plate and grid connections, for additional electrostatic shielding.

Getting more use from "useless" Tee-Vee T00bz. They're cheap, they're out there, light 'em up!