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Sunday, April 19, 2015

45 / 2A3 Mono Amplifiers


Here is a set of just finished Mono Blocks which can be used either with 45 or 2A3 as output tube. The driver is also selectable between 10Y and 45.

The amps share the same circuit and transformers as a previous set of similar amps. Just a different amplifier chassis layout as used in my most recent builds.

The wood is smoked eucalyptus which is a very dark brown, almost black. All transformer covers and oil capacitors are varnished in a graphite grey metallic tone.

The two amplifier chassis with 45 in the driver tube sockets:

Interstage and output transformers are Tango of which I still have some stock. The input transformer is the silver version of Lundahl LL7903 which I use in most of my all DHT amplifiers.

The power supply chassis:

Top view:

The amplifier chassis:

Top view:

These amplifiers reminded me again what a great tube the 45 is.

It simply is the queen of tone. Those who are blessed with efficient speakers should give this tube a listen. Especially in globe fashion.

I already described the sound of the different tube combinations here. So I won't go into that again.

The amp with a 10Y in the driver socket:

Besides the 10Y, a 801A can be used as driver as well.

2A3 as output tube:

45s are still available as NOS or good used tubes, despite many claims that the supply dries up. This tube has been produced in large quantities over decades and is still easy to find. Also in my experience they last quite long if run conservatively. The situation with 2A3 is a bit different. Bi-plate versions can be found rather easy, but the mono plates got very expensive and scarce.

So no excuse not to consider such an amp if speakers with a matching sensitivity are available.

Best regards



  1. Absolutely beautiful ! Best regards. Tony

  2. Hi. I have a svm pset 2a3 amplifier. What must I observe to replace 2a3 for 45? Thanks

    1. You need to contact the manufacturer or designer of your amp and ask them to help you.