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In Reply to: RE: KT150 puzzle (longish) posted by tlea on August 20, 2024 at 18:42:39
Tube testing is not that complicated
even the simple 'suitcase' testers can determine if the tube will amplify voltage.
I think your tube seller may be trying to preempt you from asking for a return.
Yes, there are variations of circuit, and different topologies - however tubes are made to a specific spec. and as such is capable of being used in ANY amp where the designer/manufacturer has designated that tube acceptable.
Having a spare set is a Great Idea, and I encourage that you make sure you have a set!
Happy Listening
Follow Ups:
MY thoughts: if one tube in a pair (push pull) fails, the other still works with reduced power. right? so if you operate the amp at moderate volume, you would not even notice a failure.
Tubes do not work like that in a Push:Pull circuit.
The need for the tubes to be matched is based on how the signal is shared between the different halves of the circuit.
If the two tubes are not matched, or at least close (sounds like this amp has an auto bias circuit which can compensate for slight differences)
Then the amplification is uneven, and can create serious problems
There will be audible aspects as well as electrical.
Happy Listening
Thanks for the response. That doesn't seem logical to me, but as I said I am out of my lane when it comes to tubes. If one tube is completely dead (no output) I would expect one or both channels to be silent. I did not test at high output, only at my normal listening volume which I would describe as moderate.
Sounds like the homebrew test for this would be to use a dB meter to check for SPL imbalance between channels at high volume. I could connect a laptop to my DAC to generate a consistent test signal and use an iPhone app to roughly check dB output from the speakers. I assume the imbalance would be easily detectible. Speakers are reasonably efficient (95dB) and the amp is running in Triode mode (45 wpc), if that makes any diff.
I also have an inexpensive, basic multi-meter if that is a better way to test. Looking for advice.
. . . in theory, practice and theory are the same; in practice, they are different . . .
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.
Happy Listening
At lower power levels you many not notice that one tube isn't doing its job.
Run one channel at a time as loud as you can and measure the sound pressure.
Then do the other channel with the suspect tube. You may notice before you get to full volume that it doesn't sound right.
OTOH if both channels can play nice and loud with equal sound pressure, it may be that the person who tested the tube has a bad connection in the base of the socket used to test that tube. It would not be the first time I've seen that problem!
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