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In Reply to: RE: Help with KT150 questions posted by daveyf on April 15, 2020 at 16:44:36
Your tech is asking a strange question- for a knowledgeable tech. Just owning a tube tester and some sort of workshop doesn't make one knowledgeable.
What did he mean by "not enough voltage"? Not the same voltage the tubes would see in the amp you are using those tubes in, or not enough to test at specs shown by the manufacturer?
Point is, the reason to test would be to establish whether tubes meet some criteria, like "do they draw current for a given anode voltage and g2 voltage and g1 bias voltage - be within close tolerance to the datasheet" and secondly, once adherence to specs is confirmed, to match tubes if needed.
Well, let's assume the manufacturer specifies anode voltage 500V, g2 voltage 400V and bias g1 -25V for a current draw of 100mA (hypothetical example). If the tester can't supply 500V, you cannot confirm that operating point, but you can choose another, let's say 30pV anode and g2... etc for a current draw of x mA... because there's a datasheet with curves. If the meter shows you just transconductance, you can calculate rather easily the transconductance expected for any given operating point in the curves - that is, if you are a knowledgeable tech.
Now, once you confirm acceptable transconductance for a chosen point, you can say that the tubes are "still good", "as new" or "bad". And match those tubes, if needed. So it's rather strange you didn't get a precise answer about the quality of these tubes from your tech, rather his question (doubts) about his ability to test the tubes.
As for sound quality (SQ) it is well known that the same tube type from different manufacturers sounds different, and even different batches from the same manufacturer sound different. Current production tubes are manufactured in small batches, particularly compared to production during the advanced tubes era. Variations are more pronounced both between tubes from the same batch and tubes from different batches. Obviously the variation in characteristics reflects a variation in sound quality.
My suggestion would be to ascertain what is (therefore, was) the transconductance group of the tubes you like (strong, medium, weaker) and buy KT150 tubes from some serious reseller who will understand your request (for instance, a matched quad of weaker testing tubes) and will be able to deliver per your request. This should provide you with spare tubes of your preferred SQ.
******
http://rh-amps.blogspot.com/
Follow Ups:
I appreciate your thoughts on this. Nonetheless, I believe my tech is quite competent. What he stated about his tester seems to be universally accepted, and that is that the Hickok cannot fully test the Tungsol KT150..because it cannot deliver enough voltage. I fail to see how ordering a less than 'spec' quad of these tubes would be something that could be beneficial. My amp is designed to self-bias, as such I would've thought that a poor performing set of new tubes would be less than desirable!
I believe you have misunderstood what I wrote.
1) If a tester can't supply enough voltage, you test with voltage it can supply. After all the tubes will operate at different operating points in different amplifiers. If they test good at let's say 300V I don't see why they would not test good at 500V, for example. Obviously the values will differ and one should be able to say if the values measured correspond to the datasheet.
2) Self-bias probably means "auto-bias" i.e. cathode resistor. But it could be a CCS instead of the cathode resistor. This only tells us for sure that the user does not adjust bias for the tubes.
If your amp is self-biasing, the only thing that matters might be having matched pairs per channel. It doesn't matter if the tubes are matched strong, medium or weaker pairs - just that they are matched.
A matched new "weaker" pair is not a pair of bad tubes, just a pair of newtubes from the same batch that exhibit lower transconductance than the average tube in the batch. A "stronger" matched pair is similarly a pair of tubes that show higher transconductance than the average tube in the batch.
I said "get a quad of matched weaker tubes" because:
a) you declared preferring the original tube complement , which is not new, and has by now lost some transconductance, and I don't think the manufacturer would have requested the strongest measuring tubes for "factory set" - they would either request a medium set or even a weaker set. Tubes age differently and the stronger new tubes tend to loose more transconductance than weaker testing tubes from the same batch.
b) it was just an example of a possible choice and of course you can order any option from a serious reseller who knows more about tubes than that they are in demand and he should get in the business of selling because it will most likely bring in profit.
I hope it is clearer now what I meant.
On another note, I understood too late that I was browsing the "wrong forum". This one is probably for audiophiles who enjoy music and factory built equipment, but know little about how it is made or what is inside and how it all works. I guess that is the reason you got my post wrong.
******
http://rh-amps.blogspot.com/
" If they test good at let's say 300V I don't see why they would not test good at 500V, for example. "
Really?
I can tell you from experience that that is not always true.
Sometimes an internal short that will not show up at 300 volts shows up at 500 volts.
Further more, just because a tube has the proper transconductance when operated at 300 volts that does not necessarily mean that it will have the proper transconductance at 500 volts.
The best tube tester, for the tube you intend to use, is the circuit you intend to use the tube in.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Question for you, tre. Would you test a KT150 tube on a Hickok tester that cannot supply enough voltage to the tube...and thereby extrapolate what Alex Kitic states?
If that's all I had to test them with I would have to.
I have a tester that applies more plate voltage that that but even it won't always tell you everything you want to know.
I look at testers this way, if a tube tests bad on a tester it is bad. If a tube tests good on a tester it might be good.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hi Tre,
You are nitpicking a little - but I don't mind because I am used to that.
This is not about hidden shorts - the tubes are obviously working in the circuit that has allegedly higher voltage than the Hickok tester can test at. The OP seems to wonder if the tubes are good or not, and why do tubes of the same type and same (in this case, the only) manufacturer sound different.
You are not supposed to get matched tubes from the factory with hidden manufacturing defects like shorts, but it can happen.
Transconductance can and will vary with voltage and bias, as well as g2 voltage in this case.
But in practice, a pair matched at one voltage (operating point) will still behave as a matched pair at another operating point. Not that matched tubes matter to me, since I am only interested in SE amps - but it's nice to know tubes are matched for left and right channel.
And, I agree with you that the best tester for the tube is the circuit in which the tube is supposed to be used. But it is my impression that "regular users" can't check how a tube is behaving in the circuit and have to rely on tube testing results and techs...
******
http://rh-amps.blogspot.com/
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?
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/
.
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/
I love to pick nits. It's what I do best. :-)
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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