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Friday, March 23, 2018

Tube of the Month : The 2A3


Since I just finished an amplifier which uses this tube, I thought it is a good time to present the 2A3 as tube of the month.

The 2A3 is a directly heated output tube which enjoys continued popularity over the decades and is even still made to day by various manufacturers.

It shares the same UX4 base and pinout with other triodes like the 45, 300B, 10Y and many more. In single ended operation it is good for 3.5W of glorious output power. Up to 15W can be obtained from a pair 2A3 in Class AB push pull. It has the same heater voltage as the 45: 2.5V but needs almost double the current (2.5A). It has a rather low plate impedance of 800 Ohms which makes it usable with single ended output transformers of 2.5k ohms and upwards. Since it has about half the plate resistance of the 45, delivers twice it's power and needs about twice the filament current it is often regarded as two 45s connected in parallel in one bottle. Most 2A3s have a bi plate structure, two systems mounted side by side and connected in parallel which also led to this impression. In early samples the tube was probably even constructed this way by mounting two 45 elements in one tube. The 2A3 was also made with single plates, the so called mono plate 2A3 which is regarded as the best sounding 2A3 by many. Unfortunately I do not have any mono plate samples in my stock, so only bi plate 2A3s will be shown in this post. For detailed technical information please see the RCA data sheet. As usual lets start with a look at the plate curves, here the set of curves from the data sheet:

And for comparison curves from an actual tube on the curve tracer:

The plate curves are as linear as you can expect t from a directly heated triode. Ideal for no feedback amplifiers.

RCA probably made most of the 2A3s.

Here we see the typical bi plate structure, the most common variant of 2A3.

Some details:

Another variant from RCA:

Slightly different construction as can be seen on the top:

An older RCA with engraved base:

Again different construction:

Some close ups:

A version with red printing on the base:

Similar construction as in one of the tubes shown above:

But with grey plates:

A 2A3 made for the military:

2A3 produced by Sylvania, probably the second largest maker of this tube after RCA:

Here we have a dual plate variant in which the two anodes are separate (but internally connected of course:

Here the two separate plates can be clearly seen:

Yet another Sylvania:

Here we have a special version made by Sylvania called 2A3W:

This is a ruggedised version made for the military or industrial applications.

It  comes in a straight sided bottle. It had also a 4-digit industrial designation: 5930. Here in comparison to the regular 2A3:

Zenith and Philco usually sourced their tubes from Sylvania:

Next we have a bunch of 2A3s made by General Electric:

This one looks suspiciously similar to one of the Sylvanias shown above:

Cross branding was very common, manufacturers sold tubes to each other.

The next one also looks like it's been made by another company and simply rebranded:

This one has the black coating in the middle as was mainly done by Ken-Rad.

Another two GE branded tubes:

These two look like RCA made.

Next we have some rarely seen 2A3, branded Philips Miniwatt:

Both in sealed boxes, so truly NOS NIB:

I broke the seal of one of the boxes to have a look at the tube:

Seems to have been made by RCA.

Also says made in USA:


A Marconi with the same black coating as the GE shown above:

Two National Union 2A3:


Different style Arcturus 2A3:



And lastly Tung-Sol which also used black coating:

The rather dim oxide coated filaments of the 2A3 do not produce much of a glow:

A nice triode which deserves it's popularity!

Best regards