Canuck Audio Mart Hifi and Audio ForumCanuck Audio Mart Hifi and Audio Forum
It is currently Thu Dec 14, 2017 8:38 pm

All times are UTC - 8 hours




Post new topic Reply to topic  [ 12 posts ] 
Author Message
PostPosted: Thu Jul 20, 2017 9:23 pm 
Offline

Joined: Thu Feb 10, 2011 2:41 am
Posts: 283
Location: Calgary, AB, CA
Hello,
I am going to buy some NOS 6550/KT88 from eBay. Since different sellers used different testers, I am not able to tell how much life of the tube left.

For example:
Seller A has a tube that rated at 77 and 50 is minimum.
Seller B has a tube that rated at 99 and 66 is minimum.
which one has longer life?

Sorry my math is sucks. Since the baseline is not 0 and it mess it up.

Please show your step for the calculation, so I can do it myself later.
Thanks
Vic


Top
 Profile  
Reply with quote  
PostPosted: Thu Jul 20, 2017 9:47 pm 
Offline
User avatar

Joined: Wed Jul 23, 2008 5:04 pm
Posts: 1764
Location: saskatoon, SK, CA
whaaat?!

if sold as new old stock they are new and unused.

even if both testers are properly calibrated you cannot accurately determine tube life.... there's a bit more to it than that.


but to answer yer question (and understand the answers are meaningless) :

50 / 77 x 100 = 65%

66 / 90 x 100 = 67%


think that's right? been a few decades...


Top
 Profile  
Reply with quote  
PostPosted: Thu Jul 20, 2017 9:49 pm 
Offline

Joined: Sat Dec 28, 2013 9:17 pm
Posts: 362
Location: Delta, BC, CA
viclauyyc wrote:
Hello,
I am going to buy some NOS 6550/KT88 from eBay. Since different sellers used different testers, I am not able to tell how much life of the tube left.

For example:
Seller A has a tube that rated at 77 and 50 is minimum.
Seller B has a tube that rated at 99 and 66 is minimum.
which one has longer life?

Sorry my math is sucks. Since the baseline is not 0 and it mess it up.

Please show your step for the calculation, so I can do it myself later.
Thanks
Vic

I don't have the answer but on a side note be careful buying tubes from Ebay..there was a thread the other day were buyers could not get their tubes if they were coming from the U.S., something about their global shipping service.
And be careful your not buying counterfeits.


Top
 Profile  
Reply with quote  
PostPosted: Fri Jul 21, 2017 3:00 am 
Offline

Joined: Sun Dec 28, 2008 7:49 am
Posts: 1672
Location: Fairport Beach (in Pickering), ON, CA
Many tube testers have a life test and for those that don't all you need to do is drop the filament voltage 10%. The needle shouldn't drop any from the test result with the correct filament voltage. If it does, avoid the tube. I don't recall any CAMmers that regularly sell tubes who will post that result. Tube sellers please correct me if I'm wrong and you do post life test results.

I also avoid any seller who posts the result as "Good". That is a useless result as the tube could test at 60% which is at the bottom end of good on many test scales. The tube was tested, why not post the actual result? Hmmmm maybe hiding something? Test results should be posted as micromhos (and the value of a new tube should be included) or %.

Keep in mind test results can vary from one tester to the next but if you buy a tube which tests at or above new and passes the life test all should be good. The seller should also post the tube was tested for shorts and gas.

New
Old
Stock

It's plain English .... I'll provide a hint: it doesn't mean used tubes.

I've never had any problem ordering tubes from the USA.

_________________
Tom

Collecting vintage tube Pilot (Pilotone) HiFi gear.


Top
 Profile  
Reply with quote  
PostPosted: Fri Jul 21, 2017 3:25 am 
Offline

Joined: Sun Dec 28, 2008 7:49 am
Posts: 1672
Location: Fairport Beach (in Pickering), ON, CA
viclauyyc wrote:
Hello,

For example:
Seller A has a tube that rated at 77 and 50 is minimum.
Seller B has a tube that rated at 99 and 66 is minimum.



Avoid both sellers ... posting the minimum doesn't help you as the buyer in any way. That is for the person testing the tubes. You need to know the value of a new tube which is on the same line on the tube test chart as the minimum value!! Posting tubes results in that manner is an easy way to sell tubes which could have less than stellar results.

_________________
Tom

Collecting vintage tube Pilot (Pilotone) HiFi gear.


Top
 Profile  
Reply with quote  
PostPosted: Fri Jul 21, 2017 4:24 am 
Offline

Joined: Mon Jan 31, 2005 8:37 am
Posts: 501
Location: Quebec, QC, CA
I never trust usage or tubelife percentages, so my expectations are always low, and I expect to pay a very low price and throw the tubes away if needed in buyer beware fashion with no regrets.

Consequently for me, for power tubes, it has always been buy new or NOS or nothing.

For small-signal tubes, I am more open to taking chances.


Top
 Profile  
Reply with quote  
PostPosted: Fri Jul 21, 2017 5:09 am 
Offline

Joined: Sat Sep 06, 2014 10:16 am
Posts: 126
Location: london, ON, CA
I hope this isn't taking things off topic, but I've been wondering about tube testing with regards to the life span of brand new tubes. There are several companies out there now that, for a very premium price, sell new Asian tubes that have supposedly been carefully selected and tested so that you get only the very best. Sophia Electric, Northern Electric, and Grant Fidelity spring immediately to mind, although they are not the only ones.

And yet, having spent a fair bit of time reading about this lately, I find that the failure rate of these tubes, anecdotally at least, is not low. There are all kinds of reports of failures of these carefully selected and tested tubes. Sophia Electric in particular seem to fail at a surprising rate. To be fair, from what I have read, all three of the aforementioned companies are very good about replacing failed tubes. But, if the tubes are so carefully screened, shouldn't failures in the first month or two of use be just about non-existent? Clearly there is something about tube testing I don't understand.

How much can you actually tell about a tube by testing it? Does testing give you any real idea about the likelihood of failure or of how the tube will sound? If not, why would anybody pay more for a Sophia Electric or a Northern Electric than a plain old TJ Fullmusic? Similarly, to stay closer to the original topic, are used tubes almost entirely a crap shoot?


Top
 Profile  
Reply with quote  
PostPosted: Fri Jul 21, 2017 7:24 am 
Offline
User avatar

Joined: Sun Aug 15, 2010 11:02 am
Posts: 1075
Location: Kingston, ON, CA
Lethargy wrote:
Similarly, to stay closer to the original topic, are used tubes almost entirely a crap shoot?

Absolutely.

_________________
"It is the mark of an educated mind to be able to entertain a thought without accepting it." (Aristotle, 384-322 BCE)


Top
 Profile  
Reply with quote  
PostPosted: Fri Jul 21, 2017 8:59 am 
Offline
User avatar

Joined: Tue Dec 07, 2004 4:18 pm
Posts: 2541
Location: GTA, ON, CA
Lethargy wrote:
I hope this isn't taking things off topic, but I've been wondering about tube testing with regards to the life span of brand new tubes. There are several companies out there now that, for a very premium price, sell new Asian tubes that have supposedly been carefully selected and tested so that you get only the very best. Sophia Electric, Northern Electric, and Grant Fidelity spring immediately to mind, although they are not the only ones.

And yet, having spent a fair bit of time reading about this lately, I find that the failure rate of these tubes, anecdotally at least, is not low. There are all kinds of reports of failures of these carefully selected and tested tubes. Sophia Electric in particular seem to fail at a surprising rate. To be fair, from what I have read, all three of the aforementioned companies are very good about replacing failed tubes. But, if the tubes are so carefully screened, shouldn't failures in the first month or two of use be just about non-existent? Clearly there is something about tube testing I don't understand.

How much can you actually tell about a tube by testing it? Does testing give you any real idea about the likelihood of failure or of how the tube will sound? If not, why would anybody pay more for a Sophia Electric or a Northern Electric than a plain old TJ Fullmusic? Similarly, to stay closer to the original topic, are used tubes almost entirely a crap shoot?


Tube testing results will tell you nothing about potential premature tube failure. IME, premature tube failure is more to do with quality of the construction and manufacturing.


Top
 Profile  
Reply with quote  
PostPosted: Fri Jul 21, 2017 9:12 am 
Offline

Joined: Sat Sep 06, 2014 10:16 am
Posts: 126
Location: london, ON, CA
brf wrote:

Tube testing results will tell you nothing about potential premature tube failure. IME, premature tube failure is more to do with quality of the construction and manufacturing.


Can it tell you anything about the relative sound quality the tube is likely to produce?


Top
 Profile  
Reply with quote  
PostPosted: Fri Jul 21, 2017 9:19 am 
Offline
User avatar

Joined: Wed Jul 23, 2008 5:04 pm
Posts: 1764
Location: saskatoon, SK, CA
brf wrote:
Lethargy wrote:
I hope this isn't taking things off topic, but I've been wondering about tube testing with regards to the life span of brand new tubes. There are several companies out there now that, for a very premium price, sell new Asian tubes that have supposedly been carefully selected and tested so that you get only the very best. Sophia Electric, Northern Electric, and Grant Fidelity spring immediately to mind, although they are not the only ones.

And yet, having spent a fair bit of time reading about this lately, I find that the failure rate of these tubes, anecdotally at least, is not low. There are all kinds of reports of failures of these carefully selected and tested tubes. Sophia Electric in particular seem to fail at a surprising rate. To be fair, from what I have read, all three of the aforementioned companies are very good about replacing failed tubes. But, if the tubes are so carefully screened, shouldn't failures in the first month or two of use be just about non-existent? Clearly there is something about tube testing I don't understand.

How much can you actually tell about a tube by testing it? Does testing give you any real idea about the likelihood of failure or of how the tube will sound? If not, why would anybody pay more for a Sophia Electric or a Northern Electric than a plain old TJ Fullmusic? Similarly, to stay closer to the original topic, are used tubes almost entirely a crap shoot?


Tube testing results will tell you nothing about potential premature tube failure. IME, premature tube failure is more to do with quality of the construction and manufacturing.


but a good tester can still give you some valuable info.

Commenting on premature failure I'd include operating parameters as well. And yes, some of these new tubes, particularly from the Asian market, prefer to be run how can I say, conservatively.


Top
 Profile  
Reply with quote  
PostPosted: Sat Jul 22, 2017 3:17 am 
Offline

Joined: Wed Sep 14, 2005 11:47 am
Posts: 3837
Location: Ottawa, ON, CA
It seems tube testing and understanding the life of a tube is an entire topic.
From what I can figure out, the main parameter to look at is gm (transconsductance).

Recall when biased, a change in voltage at the grid, controls a change in current flowing through the plate.

gm = di (plate) / dv (gate)

It seems that as a tube ages, gm decreases. However, I've only be able to find one graph
showing the decline in gm with tube use - and this was for transatlantic telephone transmission.

Looking at the graph, if gm is down about 10 % - then the tube still has about 67% to 75% of a usable life remaining.

The graph also shows that the higher the bias current, the shorter the tube's life.
Which is important to remember when setting the bias for power tubes.

The problem is that for a new tube, gm is within a minimum and maximum range.
For example for a new 12ax7, gm can range from 1250 to 1600 umhos.


Attachments:
img_0871_2.jpg
img_0871_2.jpg [ 239.83 KiB | Viewed 1002 times ]
Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 12 posts ] 

All times are UTC - 8 hours


Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group