Use 2A3 with AC Heating
How to measure 2A3 correctly and precise.
The heater current of 2A3 is 2.5 Ampere, and like with almost any tube, there is a non-specified tolerance on the current. Non specified means, when it is 10% deviation, there is nothing to complain about.
Heater voltage however is more important, as this is specified as +/-5%. Plate current with new tubes will respond to that not significantly. However with used tubes, the difference will be larger than this 5% by itself. It can be up to 10% even for much used tubes. Whereas such a tube may well serve some time if only the heater voltage is correct.
The main purpose of a tube tester is not so much to find out if new tubes are good. Most of the time the condition of used tubes is more important. Care should be taken to use a heater voltage within 5%, to make sure the tubes are measured by the book.
Here comes a problem which is specific for 2A3, because the heater voltage is so low. The voltage drop across bad socket contacts can be 50mV easily, and the same is for the wiring. Now 50mV may seem very little, but we have two contacts, and two wires. So we talk about 200mV, which already is 8% of 2.5V. The practical solution for this is not to use extremely thick wire, and extremely stiff socket contacts, but rather use 0.2V more voltage at the transformer winding. Professional builders of tube transformers all know this, but when people order exactly 2.5Volts, 2.5Ampere transformers, that's what you will get, and the poor 2A3 is at -8% under heated, which will impair lifetime, and may even lead to cathode poisoning, which means permanent damage, if discovered too late.
The situation with tube heaters that work at higher voltage and lower current, is less critical. . There is a 2A3 version with a 6.3V heater, called 6A3. It has only 1 Ampere heater current. Not only will the voltage drop across socket and wiring be much less (only 80mV) but this 80 mV compared to 6.3V is only 1.3%.
In the end, under heating of a 2A3 may cause problems in uncareful designed amplifiers.
At least while testing, we want to do a good job, meaning:
- Test at the correct voltage
- Find out how the tube responds to heater voltage variations.
The L3-3 tester has some tolerance on the transformer windings, which is just normal. Also, the unloaded voltage of the 2.5V winding is a little higher. The designers of L3-3 did their job well. Yet, the voltage is only an average and it is no specific 2A3 winding. To deal with this, it is possible to change the mains voltage setting of the whole tester. That will only change the voltage of the 2A3 heater winding, since all power supplies are stabilized anyway. Even the internal oscillator runs on it's own 250V stabilized power supply.
Because of the (unknown) tolerance of the transformer and the (unknown) mains voltage, the range of the mains adaptor switch may not high enough for this purpose. So, for reducing the voltage very far, there may not be enough steps on the switch, and when going up to far, the transformer may begin to hum too loud.
To overcome this, two cards were made. One uses the 2.5V tap of the heater transformer, and one uses the 3V tap. It is really impossible to say which card produces the 2.5V best, and sometimes you need to change from one card to the other, depending on the mains voltage.
On the card itself are instructions how to measure the heater voltage. Simply connections to another socket are made, which has two holes which fit exactly banana plugs.