How to replace the 2A3 tube by two 45 tubes!

It is often said, the 45 sounds the sweetest of all triodes. However that works only for you, if you have enough efficiency of your speakers. Have you ever looked inside the DOUBLE plate 2A3? Inside there, you will see two triodes, each similar to the 45.

You can simple put two 45 tubes in parallel, and you get a tube that CAN replace the 2A3 in many schematics.

What you do need to know is the following:

  1. The 45 has 1.5 Ampere heater current, so you end up with a 3 Ampere total heater current. When your amplifier can deliver that, you are ok.
  2. You can only parallel tubes that are PRECISION matched for BOTH transconductance AND plate current. Only like this, you can be sure the plate current will divide evenly between the two 45's. So when you have only matching for transconductance OR plate current, or no matching at all, you can not parallel them.
  3. The 45 is 11 Watt, so the resulting parallel tube is theoretical 22 Watt. In case you want to exceed the 15 Watt of a 2A3, this is possible, and in that case it is recommended to take the usual plate voltage of a single 45, but at twice the current.
  4. Alternative to the above, you can also simply use this artificial 2A3 (made of two 45's) in ANY 2A3 schematic there is, provided you can do 3Amps heater current.
  5. Do not use Chinese 45 tubes, as these are generally lower quality, and often sold by the factory with matching data which is just written on the box, but has not much to do with the tubes. Also with Chinese tubes, the working point is too dependant of the heater voltage. All together that would go wrong if you parallel those. Good tubes to parallel are NOS tubes, but they must be the same construction, same datecode etc. Or use new made tubes from a good brand, with guaranteed matching data.
  6. The ultimate matching for PSE is when you can overlay the tube curves. Every good tube seller is able to do a curve trace for you, at a small extra cost. Overlay doesn't have to be perfect. A good and HONEST matching as described above will do the job too.

Good luck with this - you will like it!