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In Reply to: RE: Help with KT150 questions posted by Alex Kitic on April 21, 2020 at 14:09:52
Thanks for your thoughts.
The tube tester seemed to indicate that only one of the four tubes tested was down in Transconductance. However, these tubes come from the amp manufacturer in matched pairs, and are supposed to be used in that configuration. In the amp, they seem to work fine. This is what is slightly confusing me.
Oh, sorry to come across with less technical knowledge, because I freely admit that is true...and don't make any bones about it...maybe a good thing, no?
Follow Ups:
If one tube was shown as down in transconductance in respect to the other three, this means it is not matched. If it is one half of a matched pair, obviously the pair is not matched.
"The tubes seem to work fine in the amp" - which is a subjective statement and is most probably quite correct because the main purpose of the amp is to amplify music, and if you are satisfied with the sound quality, that is sufficient.
From a technical standpoint, the tube that is not matching will cause higher distortion in that channel (I am assuming push-pull pairs, where one pair is matched while the other one is not). This would probably be evident on measurements. Still, the distortion as measured - might not be noticeable when listening.
Thus, since the amplifier is self-biasing, there is not way for you to either correct somehow the problem (by manual bias, although that is not the whole story because adjusting the current does not adjust the transconductance i.e. the other parameters of the tube in the circuit - only the current draw, which is beneficial for the output transformer which will see less or no DC unbalance) - or even be aware that the problem exists.
Getting back to the different transconductance for the particular tube of a presumably matched quad that came with the amp as supplied by the manufacturer - if it was tested different at any relevant voltage (let's say at 300V and not 500V) - than it is different and not matched to the rest.
How did that happen? Very probably, the tube has lost more transconductance when aging in respect to the other tubes.
Another explanation is that the tubes were never really matched, either due to a mistake in labeling or putting in the wrong box... or you can try to imagine any humanly possible mistake.
BTW, you never mentioned the manufacturer of the amp. When it comes to KT150, there is only one manufacturer - but it is a new production tube and quality is nowhere near as equalized or constant as it used to be when tubes were in mass production with serious and less serious manufacturers.
I hope this is both easily understandable and clearly readable.
******
http://rh-amps.blogspot.com/
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Have Fun and Enjoy the Music
"Still Working the Problem"
Couple of other questions. Since my amps are mono blocks, with two KT150's per side, which need to be a matched pair, wouldn't it be obvious that if just one tube was out of spec that that mono block would sound differently to the other? Which in my situation, is not the case.
Tre, you state if it measures bad, it is bad...question is did the tube truly measure bad given the inability of the tube tester to really place the tube under correct measuring conditions?
I meant bad as in shorted or very low to no transconductance.
both of your tubes should test the same if they were sold as a matched pair.
One tube is weak but not so weak as to sound bad. The only real concern is will the weak tube cause the good tube to current hog (draw a lot more current than it should). I know that your amps are auto bias (cathode biased) but you could place a 1 ohm resistor between each cathode and the common cathode resistor. That would allow you to measure the voltage across the one ohm resistor and calculate how much current each tube is drawing.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Several issues here...
1) The OP most probably is not able to solder resistors and measure the voltage across them to calculate the current. Even if he were able, he most probably doesn't want to make any changes to his amps as those were bought (New?) and should preserve their value for the next ownership change. The OP is not a DIYer.
2) Besides, there's nothing wrong with the amps, the question is about tubes, and the tech who is very skilled but cannot give a definite answer whether the tubes are good, or not.
3) Most important, the OP has not yet told us what exactly are those amps (manufacturer, model) so we cannot say anything about the output tubes circuit besides the statement, taken for granted, that the amp is self-biasing (which could mean several options). We don't know if the pair has i.e. a common cathode resistor (with or without bypass cap), 2 separate cathode resistors, or a CCS common to both or separate for each... all of these options are considered "self-bias" but only those with a common something make it difficult to measure the current draw of each tube in the pair.
4) Last but not least, and this one is nitpicking, installing 1 ohm resistors is usually not the best option because while being easy to read it is more difficult to match... I would suggest matched resistors, like 1% or better. After all, the voltage drop will not be more than 100mV for a current draw of 100mA at idle... meaning that the power rating of said resistor can be as low as 100mW... 1/4W 0.1% metal film is what I would suggest. But, as I said at the very beginning, this is the sort of advice one gives on the "other tube forum" i.e. to DIYers.
******
http://rh-amps.blogspot.com/
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