testing tubes? can it be done

[sinnerboy]

Hi,
Is there anyway of testing tubes to check if they are working correctly? Apart from putting them in an amp and judging them by sound. I noticed that one of my poweramp tubes isn't glowing as brightly as the other three. Is this a sign that it is not working properly?
thanks.

[markh]

The best way is a tube tester, although they're getting expensive on eBay. The brightness of the heater glow does not usually indicate the health of the tube (except in extremes, e.g., dark!)

--mark h

[merlin]

The best way is a tube tester, although they're getting expensive on eBay. The brightness of the heater glow does not usually indicate the health of the tube (except in extremes, e.g., dark!)

--mark h

Yes the brightness of the heater glow is, is, is, is, is, an indication of the health of the tube.
One test I make is with the tube in circuit. For example, if the tube is a pre-amp tube, like a 12ax7, the plate current is usually biased for about 1 milliamp. Now, 1 milliamp flowing through a 1.2 K cathode resistor should drop a voltage of -1.2 volts dc with respect to ground. This voltage can be measured with a multimeter. If the tube current is low, one cause is the filaments not glowing hot enough to warm the cathode.

v/r
-g