RCA red base 5691 / 5692

[marknoir]

Hello. I just got a few RCA red base tubes. Some are clearly marked 5692 and 5691, but on one letters are faded and there is no 5691 nor 5692 printed anywhere. Question: is there a way to tell which is it, by measuring or some other way? I looked at them and they look identical construction-wise.

[SY]

I had the same problem!

The best way to tell is to do a quick measurement. The one I did was to put a red LED between cathode and ground, grid to ground, and a 1k resistor between plate and a variable power supply. I increased the supply voltage while monitoring the voltage across the resistor. At more or less 120V supply, there's about a volt across the 1k resistor for 5691. By 80V or so from the supply, the 5692 already showed 6 or 7 volts across the resistor.

[HollowState]

The best way to tell which is which it by testing them in a tube tester. The 5691 (6SL7) will have a much higher gain then the 5692 (6SN7). These are the ruggedized versions designed ot have less microphonics. Physically, the 5691 will have a tighter wound grid structure then the 5692. But you probably won't be able to see inside the plate to verify this. And remember, if they're RCAs, the number will be inside of a stopsign (octagon).