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KT150 puzzle (longish)

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Posted on August 20, 2024 at 18:42:39
tlea
Audiophile

Posts: 701
Location: New Orleans
Joined: January 20, 2002
Contributor
  Since: October 29, 2006
 I'm a tube noobie and am very much in learning mode. About 4 yrs ago I acquired my first tube amp after 40+ years as a SS guy. I am loving the music but freely admit that I am ignorant about the tech.

The amp is Ayon Spirit III, an integrated that uses 4 x KT150 power tubes. Bought it used, so I don't really know the age of the tubes, but seller claimed less than 200 hrs. Everything sounds wonderful. No problems in 4 yrs of moderate use.

Recently I went out on a limb and bought a quad of KT150 as backups through USAM. Seller claimed low hours, and after talking to him I took the risk. The Ayon user manual has instructions for running an initial pass/fail test on newly installed tubes and an "auto fixed bias" program, whatever that means. I plugged in the backup tubes and they passed the test. Listened for a couple hours and everything sounded fine, so I closed the sale.

I decided to send the backups out to be tested by a vendor that advertises testing services online. I was very pleased with the service. The price, including return shipping, was reasonable. Communication was friendly and collegial. I got the test results in one day, paid the invoice, and got my tubes back a few days later. No complaints.

Three of the KT150s tested pretty good, but one tested bad. The report said, "Heater lit. No output.". I corresponded with the vendor, who replied, "It could be that it will work in your amp but it did not on the tester". I got the tubes back today, plugged them in (including the bad one), ran the test and auto-bias, and everything sounds fine.

That was all preamble. Here is the puzzle for you experts:

How can the tube test "no output" but work okay in my amp? I would expect that if a power tube was bad, there would be no sound from one channel or no sound at all. Help me understand what's going on.

Let me stress that I have no beef with the testing vendor. They did what they were paid to do. The guy generously offered to throw in at no cost an older KT150 that still tests at 70% life expectancy. That is great customer service. I'm disappointed with the results, but I am not blaming the vendor. He simply reported the results from his equipment.

I'm trying to use this as an opportunity to learn something about the alchemy of tube amplification. Thanks in advance.




. . . in theory, practice and theory are the same; in practice, they are different . . .

 

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RE: KT150 puzzle (longish), posted on August 20, 2024 at 20:13:08
6bq5
Audiophile

Posts: 4601
Location: SF Bay
Joined: August 16, 2001
 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

 

RE: KT150 puzzle (longish), posted on August 21, 2024 at 01:09:10
Nunki
Audiophile

Posts: 658
Joined: November 27, 2010
 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.

 

Surprising, posted on August 21, 2024 at 07:14:22
tlea
Audiophile

Posts: 701
Location: New Orleans
Joined: January 20, 2002
Contributor
  Since: October 29, 2006
 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 . . .

 

RE: KT150 puzzle (longish), posted on August 21, 2024 at 11:05:35
6bq5
Audiophile

Posts: 4601
Location: SF Bay
Joined: August 16, 2001
 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

 

You'll need to crank it up a bit!, posted on August 21, 2024 at 11:12:44
Ralph
Manufacturer

Posts: 4870
Location: Minnesota
Joined: April 24, 2002
 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!

 

RE: Surprising, posted on August 21, 2024 at 11:37:22
6bq5
Audiophile

Posts: 4601
Location: SF Bay
Joined: August 16, 2001
 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

 

Lessons Learned, posted on August 28, 2024 at 18:13:03
tlea
Audiophile

Posts: 701
Location: New Orleans
Joined: January 20, 2002
Contributor
  Since: October 29, 2006
 I'm posting a summary of what I've learned from this experience because some other tube novice might find it informative.

I bought four backup KT150s from a questionable source, at my risk. Had them tested. Three tested strong (current 86%-102%/transconductance 95%-109%) but one tested bad (no output). The tester generously gifted me one KT150 that was serviceable (77%/91%).

I switched those in and had my "1st string" KT150s tested. Three tested strong (80%-86%/87%-93%) but one tested lower (73%/87%). I've shuffled them around and now have the highest testing four running in the amp.

So now I own 9 KT150 tubes: 1 probably bad, 2 OK, and 6 pretty good. I'm beginning to see why you tube guys tend to have so many tubes.

Here are the lessons learned:
The "auto-bias" feature seems like a good, low maintenace concept, but the drawback is that if you don't manually check your tubes occasionally, you can have a dead tube in your amp and not realize it. Who knew ???

There is no option to manually bias with this amp. Supposedly it has a built-in tube testing program that automatically shuts it down if a tube goes bad or biases low, but that didn't happen with the tube that tested "no output". I don't see that as a big deal - after all, it's an amplifier, not a tube tester. But the lesson is: Beware. Unlike solid state, it's not "set it and forget it". Which makes things kind of fun and interesting, IMO.

All of this may seem routine to you experienced tube folks, but it is a big mystery to the rest of us.





. . . in theory, practice and theory are the same; in practice, they are different . . .

 

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