With the L3-3 we are lucky to have a tester that is extremely well suited to test 300B. Still, testing a 300B stays difficult, because it was never specified by Western Electric what is the way to do an incoming inspection. However Western Electric gives only data of a "typical tube". When it comes to reject limits, they do not say what or how to measure. The result is a lot of confusion. Some tube testers say the tube is good, other says it's bad.

Another situation is, some tube testers like the AT1000 are testing the tube with DC Voltage, and ground the heater for this with one end. The other end is at +5V. They did not realize, this lifts up the heater effectively by +2.5V. As the plate current responds to the grid-to-heater voltage, the effective grid voltage is now 2.5V larger, so the tube will draw approximately 11mA less current. Also it makes the effective plate voltage 2.5 Volts lower, which makes the tube draw even less current. This mistake was initially overseen. However the correction of the software was not communicated to buyers of previous models. So we have still the situation that earlier earsions of the AT1000 can l reject a specific tube, wheras later versions will say the SAME tube is good. Other examples are the AVO tube testers, which are not capable of testing tubes with such low plate inpedance as the 300B, and tubes read 25% lower as they really have. Hickok testers are able to keep the good and the bad tube apart, but that is all they do. On the other hand, such wonderful testers like the Funke W19 can test a 300B like a piece of cake, and it has no cards for it. (So we designed one...). All of this is mentioned to point out that testing a 300B is subject to a lot of confusion, because the manufacturer never instructed how to do this.

With the L3-3 there are more ways to test this tube. The first is the best. Transconductance should only be measured with the tube set for 300V/60mA.

Test results: As a minimum the tubes should have the value indicated on the card above "bad", but this means not everything. If the tube is above "good" it must work in all amplifiers. If the tube is between "bad" and "good" this means what it is. It may fail or it may work, depending what you require from the tube. This is the yellow range, or "?" range. Read more about this here.

1. This tube is tested at standard settings of 300V, 60mA. You need to adjust the grid voltage to get this 60mA. In this condition, the transconductance is measured
2. Also compare the grid voltage you measure this way, with the factory value for an average tube. Though your tube may as well be not average at all, that doesn't mean a not-average tube us a bad tube. So, better is to compare the grid voltage you need, with the original factory value this particular tube had when it was new. In case this informatipn got lost, you can at least re test it now and wrire it on the tube. Don't forget the test date, otherwise it is less meaningful later.
3. You can use the standard setting of 300V Anode, -60V Grid, and measure the plate current. Use good/bad limits as on the card. Some amplifiers work still with a 42mA tube, others don't. Wheras below 36mA the tube is a reject for most amplifiers. Note new tubes can be above average with plate current, and such will be below average with transconductance. Or, vice versa, a tube can be below average with plate current, but is above avarage with transconductance. Both would be a "new" tube, so a tube with full lifetime still on it. Some part of the lifetime is lost, when both plate current AND transconductane are below average. This does not really apply for small differences, so that means nothing. It's just when plate current is for instance 30% lower, and even at plate current adjusted to 100%, the transconductane is still 30% lower, this means the tube is very weak.
4. The scale is chosen at 150mA, and you can bias the tube at maximum 130mA, representing full 40 Watt for a 300B, which is the limit for a 300B. It is a matter of taste of you want to do this to your 300B tubes, but they should be able to take it for some time. To spare the tubes and the tester, better use maximum 100mA.
5. Grid leakage is important with 300B, because many amplifiers are designed with a wrong and too high grid to ground resistor. Test grid leakage at 300V, 100mA or even125mA, and let the tube get fully warm, also the socket should be warm. 300B must be able to take this at 125mA and have no more grid current as on the card. When testing tubes with little lifetime left, better test at 100mA only.
6. Mesh (wire) tubes are no direct 300B replacments, but lighter versions. They should not be tested above 86mA, also not very short, even when this seems possible, and the tube doesn't protest. (There is no "protest buzzer" on the tube anway.... so it gets damaged in silence)
7. Chinese so called mesh tubes (that are no mesh tubes, but punched plate tubes) are specified too high, and you will see use the maximum "so called" possible plate dissipation, the plate will glow red at one side, and this means the tube is already damaged when you see this. Even when the tube still works, it will develop grid current by such a red-glowing action.
8. We are working on a 300B grid-current repair card. Also a burn-in card is under development, with this you can try to wake up a retired tube. Please ask later.

A curious and wrong method is set the tube for the standard grid voltage, and measure the transconductance like this, regardless the plate current. This is wrong and meaningless for the transconductance, as it varies strongly with the plate current.