Tubes Asylum

Questions about tubes and gear that glows. FAQ

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Original Message

RE: Surprising

Posted by 6bq5 on August 21, 2024 at 11:37:22:
While tube testing is 'not that complicated' as I originally said,
there are some basics that need to be accomplished before you can 'Test' the tube.

There are two basic systems inside a tube (power tube for this conversation)

1: the heater: a filament that is either directly heated or indirectly heated to act as a source of electrons for the second system. The voltage for the filament is fixed for a given tube type, and based on the design, one can look up the current required. For a KT-150, Pins 2 & 7, 6.3 Volts @ 1.7-2.0 amps.

2: the Pentode, the part of the tube that actually amplifies the signal: As the name implies, this tube has 5 elements: Plate, pin 3; Grid 1, pin 5, Grid 2, Pin 4, Grid 3, pin 8, Cathode, Pin 8. In this and many other pentodes, G3 is tied to the Cathode - hence both tie to Pin 8.

The result from your tester said that one tube had 'no output' -
this could be caused by a number of issues: a broken connection from a pin to an element within the glass envelope (not fixable), or the tube has been used and is depleted. There is also a possibility that one of the pins has lost connection with the lead from the tube - hard to test for and not easy to fix.

To test for output, you need to power up the heater circuit, and then have voltage on the plate, Grid 1 & 2 with a circuit to sense the output (meter).
[The voltages are not all the same, and depending upon the tester, are often not performed at full rated voltage]

In a push Pull circuit, having one bad tube is like having 1 flat tyre on a bicycle, you might be able to go somewhere but poorly at best, and you probably will ruin the rim that has the flat.

I encourage that you read the FAQ section of this here asylum (linky below) where you can learn about tubes, amps and testers.