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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


Thursday, April 8, 2021

Making of a 10Y Line Preamplifier


Here some photos of a recently finished 10Y line preamp and it's assembly steps.

This preamp is equipped with ELROG ER801A tubes which are compatible with the 10Y family.

As always the power supply is external and uses a full wave bridge with 4 6AX4 tubes for the rectification of the high voltage.

Above all the metal plates for the preamp section. And for the power supply below.

Installing the oil capacitors and filament chokes:

The underside:

Trabsformers and B+ chokes are mounted on a separate plate:

This plate is mounted to the underside of the top plate and wired to the rest of the circuit:

This preamp uses Lundahl output transformers and Intactaudio AVCs. Optionally the preamp can also be built with Tribute OPTVCs.

The AVC is wired with teflon insulated solid core silver wire to the switch.

Finished preamp section:

The power supply:

Almost finished PSU:

Finished power supply:

The finished preamp, tested, and ready to ship:

Power supply and preamp are packed in separate wooden shipping crates. This unit ships to the US.

Drop me an email if you want me to build your very own 10Y preamplifier for you.

Best regards