This months tube is a dissimilar double triode, a kind of tube which was not presented in the Tube of the Month series yet. So it is about time since there are many interesting tubes of this species out there. Here is the 6GL7.
These kinds of tubes have been developed as combined TV vertical oscillator and amplifier applications. For this they contain a high mu triode along with a low mu power triode section. What makes these interesting for audio is the possibility to build an amplifier with a single tube (nick named spud or potato amplifier). The high my section can be used as driver and the low mu section as output triode.
The 6GL7 has an octal base which is shown on the left. It is especially interesting for audio since the low mu section has a plate resistance of only 780 Ohms at an amplification factor of 5. With it's maximum allowable plate dissipation it can provide almost 2W output in single ended configuration with an output transformer of 2.5-3.5kOhm primary impedance. The high mu section has an amplification factor of 66 with a plate resistance of 30kOhm. Just right to drive the output section with a well usable input sensitivity. All this combined with a moderate 1.05A heater current. See the datasheet for all technical parameters. The low plate resistance of the low mu section also makes it very suitable as a line stage output tube. RC coupled it will yield a low output impedance with a useful and not to high gain. In case only one section is used, it is recommended to ground the electrodes of the unused half. That's how I used it in the recently introduced Octal Line Preamplifier.
I have not found a datasheet with plate curves. So I measured them with a tube tracer:
High mu section:
This is a very linear triode system.
Low mu section:
Not as linear as the other section but still well usable if the operating region os carefully chosen.
This makes it very usable for audio and I already presented a line stage with this tube, the new version of my Octal Line Preamplifier. It performs very well in that preamp which encourages me to also develop a simple power amplifier with these tubes. The 6GL7 is surprisingly small considering the 10W plate dissipation rating of the power section. Including the base pins it is just about 70mm high. The diameter is the same as that of other Octal tubes. This would allow for a very compact power amp with just one of these in each channel. Earlier versions of the 6GL7 have the typical Octal base, which is about 20mm high (excluding pins). Later versions use the so called coin base also called wafer base. This only has a small plastic wafer attached to the glass which holds the pins. This wafer looks like a coin, hence the name. Below a photo of two Sylvania 6GL7 one with the old style base and the other with the coin base:
Another comparison between the two:
6GL7 manufactured by Lindal (Japan):
Now let's have a look at the internal construction. Removing the glass tube reveals more details.
Clearly visible the different plate size of the high mu section (left) and power section (right):
The high mu section from the side:
The getter ring attached to the plate of the power section:
The top mica disc:
A small strip of metal welded to the rods of the grid of the power section provide some extra cooling.
The connections from the electrodes to the base pins:
Another view, showing the heater wire going into the cathode of the high mu section:
The heaters of both sections are wired in parallel:
The heater wires removed from their cathodes. Since the power section has a stronger cathode it needs more folds of the heater to get it to the right temperature:
A close up:
The plates removed from their triode sections:
The grids and cathodes in comparison, power section on top and high mu section below:
Note the gold grid wire of the power section. The cathodes removed:
The grids in comparison:
A close up:
The high mu grid: