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Wednesday, July 31, 2019

Tube of the Month : The 6BL7

Hi!

This month we take a look at a double triode. Meet the 6BL7.




The 6BL7 was developed for TV vertical deflection oscillator and amplifier applications. It is similar in structure and pinout to the 6SN7 and 6BX7 but they have different amplification factors and other parameters.


The octal base pinout is shown on the left. The 6BL7 has an amplification factor of 15 which classifies it as medium mu triode. It has a hefty 60mA maximum cathode current and each of the two triode systems can dissipate 10W. Albeit not both at the same time. Total maximum dissipation for both triodes together is limited to 12W due to the compact size and small bottle. Still impressive and both systems connected in parallel would be able to work in an output stage of a small power amp or headphone amp. Capable to deliver about 2W in single ended operation. The plate resistance is about 2k per system so 1k when operated in parallel which is in the territory of typical output tubes. Such parameters require a rather high filament current of 1.5A. It would also make a great driver for large output tubes since it can deliver quite impressive voltage swings. For complete technical specs refer to the General Electric data sheet. Lets have a look at the plate curves:




And some curves taken from a tube on the tracer:




An interesting tube which gets little attention nowadays.




I haven't really used this tube myself and the small heap of 6BL7GTA above is all I got.




So lets have a closer look. Here is the tube in all its glory:







The rather large plate structures (for such a small tube) are immediately apparent.






Being designed for TV deflection service the tube had to be quite rugged.







The tube in operation:






A nice little gem!

Best regards

Thomas



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