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Tuesday, April 27, 2021

Making of 6CB5A Mono Amplifiers - Part 1


I just started assembly of 2 single ended 6CB5A amplifiers and will share the construction process.

Readers of my blog will know that I have a long history with the 6CB5A tube and always proposed it as an interesting alternative to the more mainstream tubes.

I have built numerous versions of this amplifier which uses a 6N7 as driver and a rectifier bridge with 6AX4 TV Dampers.

This time it is a fully fleshed out pair of mono blocks built in the floor standing tower style chassis, as used in recent amplifiers like the last Stereo 300B amp I built.

Once the bare metal plates arrive the assembly can begin. Here we see the 2 top plates which are mirrored to each other this time, and all the bits and pieces needed for the internals:

Mounting the tube sockets and some initial wiring:

The top plate with all parts mounted and wired:

The next level below the top plate carries the interstage and output transformers and some capacitors:

Below that another capacitor bank and filter chokes:

And on the bottom the mains power transformer and another 2 chokes:

All the sub assemblies of one channel:

Completion of the amps will continue when some missing parts arrive. Stay tuned for part 2 which will show the final steps of assembly and finished amplifiers.

Best regards


Thursday, April 15, 2021

Tube of the Month : The 274


This month is rectifier time. Meet the 274 family of tubes.

The 274 is a classic full wave rectifier tube with two diodes in one bottle sharing the same cathode. It was introduced by Western Electric in the 1930ies.

The 274 comes in two flavours, The 274A and 274B. Both share the same technical specs but have different bases. While the 274A came out with a 4 pin UX4 base, the 274B was later introduced as well with an 8 pin Octal base (of which only 4 pins are used). The respective pinouts can be seen on the left. The 274 is a directly heated rectifier which means the filament serves directly as the cathode. The filament operates at 5V and 2A. The tube was designed for full wave rectification with the plates connected to either end of a high voltage winding of a power transformer with center tap. The output

voltage can be obtained from either end of the filament or from a center tap of the filament winding on the power transformer. The high voltage winding center tap carries the ground. Like all tube rectifiers the 274 is best used working into a choke input filter. It can work also into a capacitor input filter, however the Western Electric data sheet specifies a maximum of 4 uF of load capacitance. While the original WE274A data sheet lists a maximum rectified output current of 140mA, this was later uprated to 225mA when the WE274B was released. In 2017 Deutsche Elektronenröhren Manufaktur GmbH released the ELROG ER274A and ER274B as modern versions which can replace the original tubes. While the Western Electric 274s used oxide coated filaments the ELROG 274s use thoriated tungsten.

Here we see the ELROG ER274A and ER274B

Both versions share the same construction and only differ in the base.

The ELROG rectifiers use a vertical arrangement of the two plates while other manufacturers mount them side by side.

They come in the straight sided glass bulb and construction which is characteristic for ELROG tubes.

Of course a post about the 274 would not be complete without also showing a Western Electric tube.

Here we have a WE274A with engraved base, a very rare item.

Some details of the tube:

Datasheet included in the box:

And now as always some photos with tubes glowing.

The thoriated tungsten filament of the ER274A is clearly visible at daylight.

And more so at night:

I hope you enjoyed this tube of the month post with both vintage and modern versions of the 274 rectifier. You need more output current or want to load the rectifier with more capacitance? Stay tuned for next months tube presentation.

Best regards