This months tube is a high mu triode with Loktal base, the 7B4.
This is the 4th Loktal tube covered in this series. For some more background on Loktal tubes see the posts about the 1LE3, 7F7 and 7N7.
The 7B4 contains a single triode with an amplification factor of 100, internal plate resistance of 66kOhms and a transconductance of 1500 micromhos. For all technical data please see the Sylvania data sheet. The 7B4 is electrically equivalent to the 6SF5 and could replace it in the Octal Phono Preamplifier. Like other Loktal tubes the 7B4 is still available in good quantities and at modest prices. Loktals are still largely overlooked by amplifier builders and are a good alternative to Octal tubes. As always let's start with a look at the plate curves. This set of curves is from the data sheet:
And these curves are taken with a 7B4 on the curve tracer:
Since most of the Loktal tubes were produced by Sylvania and just a few by some other manufacturers, there is not a lot of variety of tubes to show.
All the tubes I have in stock appear to be made by Sylvania.
Like all Loktal tubes the 7B4 has the characteristic metal base.
Two of the three unused pins are omitted.
Most of the glass is covered by getter.
Philco branded 7B4:
Another one with red printing on the glass:
These are all the 7B4 varieties I have to show.
Since the getter covers the internals, let's break one open to see more of the construction details.
With the glass off we see the plate structure which is perfectly cylindrical.
And the holder for the getter at the top.
A view of the top:
Side view:
The bottom side with the heater wires:
The plate removed:
Nice circular cross section.
The grid and cathode:
Close Up:
Some more views of grid and cathode:
The cathode removed:
The grid on a ruler to get a feel of the tiny spacings of the grid wires:
With all the getter inside, there is not much glow visible when the tube is in operation:
That's all for the 7B4. Stay tuned for another Loktal tube in next months post.
Best regards
Thomas
The tube pins rust and the tube can sound " dumpy ". Clean the pins with a sharpened paring knife, every six months. Be very careful not to put micro-cracks into the glass holding the pins. 7B4s with badly rusted pins, usually can not be salvaged.
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