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USING THE HEATHKIT TT-1A TUBE TESTER
I think that it would be safe to say that there is a tube renaissance occurring
in home audio equipment today. Short of returning to the 1950s and early
1960s, there has never been a time when so much tube audio equipment
was available to the music lover. The even better news is that much of it is
very reasonable in price and all of it is of high quality.
The only downside to tubes is that they, like vinyl records, start wearing out
from the first moment that they are used. Being able to ascertain the
condition of the tubes in your audio equipment can insure that the
equipment is kept working at its absolute best. Whether having a good
quality tube tester around is a luxury or a necessity depends somewhat on
just how much tube equipment you have and how fanatical you are about its
performance. Me, I want to know how good the tubes in my equipment are
performing and I also want to be able to do things like match them to each
other. I guess that tells you where I am coming from.
I have been using a very basic Lafayette tube tester for years. It does tests
for both shorts and emissions. I used it for matching tubes based on their
output reading on its 0-100 scale. Click here for a photo. The problems
with a tester like this are that it does not test the tube in a way that
simulates actual usage and, also, it offers no baseline for what is a “good”
reading other than the relative “Replace?Good” markings on its scale, which
is the same for every tube being tested. This scale readout leaves the
question of how actually “good” a tube is up for grabs. The only way to test
tubes is relative to each other. Test ten tubes and you will find where they
generally fall on the scale and you can weed out those that are way out of
line. Test only one tube, though, and you have no idea where you are with
it. For example, a good 12AX7 will go well into the good range, around 70-
80 on the Lafayette’s scale, but an equally good 6SN7 tube will barely make
it past 60. If you do not know that this reading indicates a good 6SN7, you
would likely conclude that is was barely usable since it only just gets into the
good range of the scale.
While finding a more sophisticated tube tester than the Lafayette was not a
high priority for me, it was something for which I kept my eyes open. I
finally found the upgrade I was looking for about a year ago.
I bought a Heathkit W-5 tube mono amp and mono preamp from an older
gentleman holding a garage sale in the fall of 2006. While I was there, I
asked him if he had any tubes or a tube tester. He said that he had both
and invited me to come back later in the afternoon. I went and I bought the
TT-1A from him for $50.
I tried testing some tubes with my TT-1A shortly after I got it. I could not
make sense of my results. I finally called my friend, Tom Boffa, in Colorado
with some questions that I had. Tom ended up offering to go through the
tester for me to insure that it was working properly. The TT-1A went west
shortly after the New Year. I got my restored TT-1A back in June of 2007.
It has had all of the capacitors and resistors replaced; some bad solder
joints redone; and some outright mistakes in its construction corrected.
What I made selling the Heathkit mono amp and preamp just about covered
restoring the TT-1A. Click here for a photo.
Should you find any tube tester of this vintage, it would be prudent to
assume that, at the very least, its capacitors and any tubes in it are bad.
THE HEATHKIT TT-1A
The Heathkit TT-1A tube tester is a sophisticated, late 1960s tester. Click
here for a photo. Believe it or not, these were sold as kits, as mine was.
When you see the maze of wiring in one of these units, you have to wonder
who, in his right mind, would even consider building one of these. I got the
Operational Manual with my tester and could not begin to follow the
schematic that is in it.
The TT-1A is distinguished from the TT-1 tube tester by virtue of having an
additional set of sockets in the lid of the unit. So far, I have not found a
tube that these additional sockets actually test, but should there be one, I
am ready. Click here for a photo.
The TT-1A tests for five types of leakage and for mutual conductance (Gm).
Mutual conductance testing comes closest to testing a tube under actual
operating conditions as is practical, yet even it cannot replicate the actual
conditions under which a tube operates. The Heathkit manual says that the
best tube test is simply to replace the tube with another and if it works,
assume the first tube was bad. Simple yes, but also potentially expensive,
depending upon the tubes involved. Most of us want some method of
determining a tube’s condition even if it has to be done at something other
than actual operating conditions.
At first, the controls of the TT-1A looked only slightly less daunting than its
interior. There appears to be an awful lot of things to set. Fortunately, the
Operational Manual explains both the theories upon which the TT-1A’s tests
are based and how to perform them. I have referred to the manual
numerous times as I have taught myself to use the tester.
TESTING A TUBE
To test a tube, you first scroll through the tube data roll chart in the center
of the unit until you find the tube you want to test. This list provides all of
the information needed to set the tester. With this information, you then
work from upper left to lower right on the TT-1A itself setting the
appropriate knobs and dials. You end by setting the nine Selector knobs at
the lower middle of the unit. If the last three digits of the Selector settings
are zeroes, you are done. If there are additional numbers shown, then you
need to test using the sockets in the lid and set the switches there
accordingly. While I would not characterize setting up the TT-1A for testing
as idiot-proof, it is pretty straight forward. I am just in awe that someone
could even design this machine.
Having completed the setup, you insert the tube into the socket which fits
it. Make sure the Leakage knob at the left is set to its maximum
counterclockwise position. Now you can turn the unit on. Once the TT-1A
is warmed up, shift the lever at the extreme lower left from Disconnect to
Normal. The tube will start to glow.
The most critical setting on the tester is the Line Check setting. This setting
insures that the testing voltages are the same for every test. The Line
Check is set by holding the Line Test switch at the lower right in its “on”
position and simultaneously using the Set Line knob at the upper left to
adjust the meter needle exactly over the red Line Check indicator on the face
of the meter. Different tubes draw different amounts of current so the Line
Check setting fluctuates tube to tube, test to test, and minute to minute
depending on mains voltage. I found myself checking it constantly as even a
very small amount of drift of the needle affects the test results substantially.
Let the tube warm up. Then move the Leakage knob through its five
leakage measurement positions. Leakage is indicated by the movement of
the meter needle, which acts as an Ohm meter and indicates the isolation of
the tube elements. At rest at the left of the dial, the resistance between
elements is greater than ten MegOhms. If the meter needle moves to the
right, even slightly, from its rest position you have leakage. If it moves way
to the right, the tube is obviously bad. The Heathkit manual suggests 500
KOhms as the arbitrary division between usable and bad tubes.
Having finished the leakage tests, you can actually test the tube for Mutual
Conductance. Move the Leakage switch to the Tube Test setting. Then hold
the Gm test switch on the lower right in its “on” position and watch the
needle move across the meter. Note where it stops. You want to compare
this value to the R.P. (Reject Point) value noted in the tube data.
The Reject Point is nominally 65% (call it 2/3rds) of the value that a good
new tube should test at. If, for example, the R.P. is 1000, the meter should
read somewhere in the 1500 range.
You have now tested for both leakage and mutual conductance. You can
perform addition tests for Grid voltage and Tube Life if you wish. Testing
varies somewhat for diodes and rectifier tubes and I am still not clear on
how to do the test for low-power Thyraton tubes. For now, I am hoping it
just never comes up.
Actually testing a tube is less intimidating than reading about it. There is a
lot of setup involved in testing any tube, though, so it makes a lot of sense
to test all of a given type of tube while you have the tester setup for it.
Between the tubes warming up enough to test and then cooling down
enough to remove, it is a fairly slow process. What becomes evident fairly
quickly after doing some actual testing is that, while the testing itself is
straightforward, figuring out what the results are telling you is not.
REALITY SETS IN
I have accumulated a pile of 12AX7 tubes, many of which had been
represented to me as new. I assumed that I would use the TT-1A to quickly
sort through them and find out exactly what I had. Well, things did not
proceed quite as directly as I had hoped.
First, none of the 12AX7 tubes I had were even reaching the R.P. point
(1040), much less going beyond it to the sort of reading that would be
expected from a good tube. Perhaps none of them were actually in good
shape? I also had some new and near-new tubes from my preamps that I
checked. These did not test much above the R.P. either. Were they all
these tubes bad? Was the tester working correctly? I was not sure.
I retested quite a few of the tubes and reread the Operational Manual several
times and my results stayed the same. I considered calling Tom Boffa but I
hesitated doing so for several reasons. First, I had only worked with one
type of tube and thought it might be useful to test some other tubes and
see what sort of results I got from them as well. Two, I know how Tom
fixed this tube tester. He started with page one of the Assembly Manual
and went through every single step in the manual. Operator error was a far
likelier possibility than a problem with the tester. Finally, even I get tired of
asking dumb questions. So I came up with an alternative strategy.
I went to eBay and bought eight new JJ 12AX7 tubes. I would test them
and see what sort of results I got, if these results were weird, then I would
call Tom.
This turned out to be a reasonably good idea.
The new JJ tubes, with one exception, tested in the sort of range that was
expected of a new 12AX7. Finally I was getting some results that made
sense. The new tubes were also testing consistently enough to leave me
feeling that I knew what I was doing.
Here are the results that I got. A 12AX7 is a dual triode tube so there are
two sections of the tube to test.
1. 1800/1700
2. 1400/1750
3. 1725/1750
4. 1800/1800
5. 1950/1750
6. 1750/1850
7. 1900/2050
8. 1700/1800
These results are fairly consistent, with the exception of tube 2, which has
one low side and tube 7, which tests somewhat high. With an R.P. of 1040,
good tubes should test at least 1600 on the meter. Overall, these results
look pretty good.
Even with these tubes, however, a very slight change in the Line Test
setting would make a large difference in the results. More confusing still, re-
testing a tube half an hour after I first tested it would give me a slightly
different result. This result would still be in the expected range but it was
disconcerting to see how much the value changed with even slight use on
the tube. Note: Buying tubes that have been burned-in is a good idea.
I ended up chatting with Tom anyway and he was surprisingly sympathetic
to my situation. It turns out tubes will test differently right out of the box
than they will with some usage on them. It turns out that two properly
calibrated tube testers will get different results. It turns out that testing
tubes is still more about comparing them to each other than to any sort of
absolute values. It turns out that new, good quality tubes can still test all
over the map.
It turns out tube testing is not an exact science. Damn.
I used tubes 1 and 5 in my Rogue 90 amp and used tubes 3, 4, and 8 in my
preamp.
Unfortunately, the one thing that a tube tester cannot tell you is whether a
tube is noisy. Tube 3 was crackling like an open fire within a minute of
turning the preamp on. I replaced it with tube 6. All of these tubes have
subsequently proven to be very quiet and there is a definite improvement in
the sound of my system with them in it.
I would strongly suggest that when buying new tubes that you buy more
than you need so that you can sort through them. I still have tubes 2 and
7 new in their boxes and I am not sure what I will do with them. I may put
7 in the Rogue amplifier with 5 and keep 1 as a spare for the preamp. In
fact, the more I look at it, the better I like that idea.
The results that I got with these 12AX7s left me feeling that I understood
what I was doing and that the tube tester was working properly. So I went
back to look at some used tubes that I have.
I have two sets of used 12AT7s from my VTL amp and wanted to select the
best four of them to use in that amp. Again, I tested and these are the
results:
1. 1800/1500
2. 1750/2300
3. 1450/1350
4. 1450/1600
5. 1450/2000
6. 1700/2750
7. 1750/>3000 (off the scale!)
8. 1775/1800
The R.P. for a 12AT7 is 1190, so a good tube should test in the 1800 range.
In looking at these results, the question becomes just which of these tubes
are good, if any? Tube 3 tests low on both sides, so it is probably fairly
worn. But is it worn out? Tube 4 also tests low but the 1600 side is
probably very usable. Tube 5 tests somewhat low on one side but like new
on the other. What is the deal with tube 7? It pegged the meter!
These are the sort of results that I have gotten from used tubes. There is
not a lot of consistency and the test values fall into a gray area that is
neither clearly good nor obviously bad. The low testing tubes are not
necessarily unusable but I would not want to put them in my equipment,
especially not knowing which side of the tube is being used in the circuit.
I ended up putting tubes 1, 2, 6, and 8 in the VTL. I paired them as 1/6
and 2/8. They sound fine but I am already thinking about investing in some
new tubes.
HERE IS WHAT I FEEL THAT I MAY HAVE POSSIBLY LEARNED FROM ALL OF
THIS
Assuming you own, or have access to, a working, properly calibrated tube
tester:
Invest in some new tubes of the types you use a lot. Get more than you
actually need. Test these tubes and let them determine what constitutes a
good test result. Keep notes. Use those results to evaluate used/unknown
tubes. It is not a bad idea to set aside a tube that tests close to ideal as a
standard for future testing. These “standard” tubes will also let you know if
there is any change in the function of the tester itself.
Having some tubes test in the expected range helped put the performance
of the other tubes I had in perspective but even then it is a judgment call as
to what level separates good from fair or poor condition. This is further
complicated by not knowing exactly how the tube is being used in a piece of
equipment. A tube might test somewhat weak but will be perfectly fine in
some applications and inadequate in others.
Relative results are more important than absolute results. If you test four
new tubes and they test very close to each other in value, they are probably
all very good tubes, even if none of them tests in precisely the range
predicted by the Reject Point.
If you are thinking of investing in a tube tester, look around for one that is
being offered as restored or rebuilt with, hopefully, some sort of guarantee.
If you get one for free, that’s great but you will probably need to find
someone who can, at the least, calibrate it, or more likely go through it and
get it working properly. Like a lot of tube audio equipment itself, most of
these testers are getting old and you should assume that they are not
going to be accurate if they are completely original.
No tube tester is of any value if you do not have the data sheets for setting
it. Thanks to eBay finding a lot of this information is fairly easy. In the
course of locating an Assembly Manual for the TT-1A, I found Vintage
Manuals, Inc. of Batesville, Indiana. I cannot recommend them highly
enough. They are on the web at
www.w7fg.com. Check with them first if you are in need of manuals, data
sheets, and so on for any type of older equipment.
IN SUMMARY
This is not the article that I thought I would be writing when I got my TT-1A
back this summer. I thought I would be telling you how I plug in a tube, run
the test, and, bingo, here is the result. It does not work quite that way.
Very good tubes are obvious; very bad tubes are obvious (and fun to test
just for the novelty of seeing the meter do something different) but the
80% of the rest of the tubes are largely judgment calls. Should I have just
stuck with the Lafayette tester? No, even though there is less
“definitiveness” involved in testing tubes with the TT-1A than I had
expected, it is light years more informative than the Lafayette was.
I hope this article does not dissuade anyone from getting a tube tester.
They are useful, informative, and kind of fun to work with. It is just that a
tube is not a light bulb. It is not obviously good or bad. Determining where
a tube falls on the scale between new and unusable is much trickier than it
first appears; but it is well worth spending some time to try to understand.
I feel much better knowing something about the performance of the tubes I
am using in my equipment and that, in turn, has led to an improvement in
the sound I am getting. And that’s the whole point.
Kent Johnson
October 25, 2007
I think that it would be safe to say that there is a tube renaissance occurring
in home audio equipment today. Short of returning to the 1950s and early
1960s, there has never been a time when so much tube audio equipment
was available to the music lover. The even better news is that much of it is
very reasonable in price and all of it is of high quality.
The only downside to tubes is that they, like vinyl records, start wearing out
from the first moment that they are used. Being able to ascertain the
condition of the tubes in your audio equipment can insure that the
equipment is kept working at its absolute best. Whether having a good
quality tube tester around is a luxury or a necessity depends somewhat on
just how much tube equipment you have and how fanatical you are about its
performance. Me, I want to know how good the tubes in my equipment are
performing and I also want to be able to do things like match them to each
other. I guess that tells you where I am coming from.
I have been using a very basic Lafayette tube tester for years. It does tests
for both shorts and emissions. I used it for matching tubes based on their
output reading on its 0-100 scale. Click here for a photo. The problems
with a tester like this are that it does not test the tube in a way that
simulates actual usage and, also, it offers no baseline for what is a “good”
reading other than the relative “Replace?Good” markings on its scale, which
is the same for every tube being tested. This scale readout leaves the
question of how actually “good” a tube is up for grabs. The only way to test
tubes is relative to each other. Test ten tubes and you will find where they
generally fall on the scale and you can weed out those that are way out of
line. Test only one tube, though, and you have no idea where you are with
it. For example, a good 12AX7 will go well into the good range, around 70-
80 on the Lafayette’s scale, but an equally good 6SN7 tube will barely make
it past 60. If you do not know that this reading indicates a good 6SN7, you
would likely conclude that is was barely usable since it only just gets into the
good range of the scale.
While finding a more sophisticated tube tester than the Lafayette was not a
high priority for me, it was something for which I kept my eyes open. I
finally found the upgrade I was looking for about a year ago.
I bought a Heathkit W-5 tube mono amp and mono preamp from an older
gentleman holding a garage sale in the fall of 2006. While I was there, I
asked him if he had any tubes or a tube tester. He said that he had both
and invited me to come back later in the afternoon. I went and I bought the
TT-1A from him for $50.
I tried testing some tubes with my TT-1A shortly after I got it. I could not
make sense of my results. I finally called my friend, Tom Boffa, in Colorado
with some questions that I had. Tom ended up offering to go through the
tester for me to insure that it was working properly. The TT-1A went west
shortly after the New Year. I got my restored TT-1A back in June of 2007.
It has had all of the capacitors and resistors replaced; some bad solder
joints redone; and some outright mistakes in its construction corrected.
What I made selling the Heathkit mono amp and preamp just about covered
restoring the TT-1A. Click here for a photo.
Should you find any tube tester of this vintage, it would be prudent to
assume that, at the very least, its capacitors and any tubes in it are bad.
THE HEATHKIT TT-1A
The Heathkit TT-1A tube tester is a sophisticated, late 1960s tester. Click
here for a photo. Believe it or not, these were sold as kits, as mine was.
When you see the maze of wiring in one of these units, you have to wonder
who, in his right mind, would even consider building one of these. I got the
Operational Manual with my tester and could not begin to follow the
schematic that is in it.
The TT-1A is distinguished from the TT-1 tube tester by virtue of having an
additional set of sockets in the lid of the unit. So far, I have not found a
tube that these additional sockets actually test, but should there be one, I
am ready. Click here for a photo.
The TT-1A tests for five types of leakage and for mutual conductance (Gm).
Mutual conductance testing comes closest to testing a tube under actual
operating conditions as is practical, yet even it cannot replicate the actual
conditions under which a tube operates. The Heathkit manual says that the
best tube test is simply to replace the tube with another and if it works,
assume the first tube was bad. Simple yes, but also potentially expensive,
depending upon the tubes involved. Most of us want some method of
determining a tube’s condition even if it has to be done at something other
than actual operating conditions.
At first, the controls of the TT-1A looked only slightly less daunting than its
interior. There appears to be an awful lot of things to set. Fortunately, the
Operational Manual explains both the theories upon which the TT-1A’s tests
are based and how to perform them. I have referred to the manual
numerous times as I have taught myself to use the tester.
TESTING A TUBE
To test a tube, you first scroll through the tube data roll chart in the center
of the unit until you find the tube you want to test. This list provides all of
the information needed to set the tester. With this information, you then
work from upper left to lower right on the TT-1A itself setting the
appropriate knobs and dials. You end by setting the nine Selector knobs at
the lower middle of the unit. If the last three digits of the Selector settings
are zeroes, you are done. If there are additional numbers shown, then you
need to test using the sockets in the lid and set the switches there
accordingly. While I would not characterize setting up the TT-1A for testing
as idiot-proof, it is pretty straight forward. I am just in awe that someone
could even design this machine.
Having completed the setup, you insert the tube into the socket which fits
it. Make sure the Leakage knob at the left is set to its maximum
counterclockwise position. Now you can turn the unit on. Once the TT-1A
is warmed up, shift the lever at the extreme lower left from Disconnect to
Normal. The tube will start to glow.
The most critical setting on the tester is the Line Check setting. This setting
insures that the testing voltages are the same for every test. The Line
Check is set by holding the Line Test switch at the lower right in its “on”
position and simultaneously using the Set Line knob at the upper left to
adjust the meter needle exactly over the red Line Check indicator on the face
of the meter. Different tubes draw different amounts of current so the Line
Check setting fluctuates tube to tube, test to test, and minute to minute
depending on mains voltage. I found myself checking it constantly as even a
very small amount of drift of the needle affects the test results substantially.
Let the tube warm up. Then move the Leakage knob through its five
leakage measurement positions. Leakage is indicated by the movement of
the meter needle, which acts as an Ohm meter and indicates the isolation of
the tube elements. At rest at the left of the dial, the resistance between
elements is greater than ten MegOhms. If the meter needle moves to the
right, even slightly, from its rest position you have leakage. If it moves way
to the right, the tube is obviously bad. The Heathkit manual suggests 500
KOhms as the arbitrary division between usable and bad tubes.
Having finished the leakage tests, you can actually test the tube for Mutual
Conductance. Move the Leakage switch to the Tube Test setting. Then hold
the Gm test switch on the lower right in its “on” position and watch the
needle move across the meter. Note where it stops. You want to compare
this value to the R.P. (Reject Point) value noted in the tube data.
The Reject Point is nominally 65% (call it 2/3rds) of the value that a good
new tube should test at. If, for example, the R.P. is 1000, the meter should
read somewhere in the 1500 range.
You have now tested for both leakage and mutual conductance. You can
perform addition tests for Grid voltage and Tube Life if you wish. Testing
varies somewhat for diodes and rectifier tubes and I am still not clear on
how to do the test for low-power Thyraton tubes. For now, I am hoping it
just never comes up.
Actually testing a tube is less intimidating than reading about it. There is a
lot of setup involved in testing any tube, though, so it makes a lot of sense
to test all of a given type of tube while you have the tester setup for it.
Between the tubes warming up enough to test and then cooling down
enough to remove, it is a fairly slow process. What becomes evident fairly
quickly after doing some actual testing is that, while the testing itself is
straightforward, figuring out what the results are telling you is not.
REALITY SETS IN
I have accumulated a pile of 12AX7 tubes, many of which had been
represented to me as new. I assumed that I would use the TT-1A to quickly
sort through them and find out exactly what I had. Well, things did not
proceed quite as directly as I had hoped.
First, none of the 12AX7 tubes I had were even reaching the R.P. point
(1040), much less going beyond it to the sort of reading that would be
expected from a good tube. Perhaps none of them were actually in good
shape? I also had some new and near-new tubes from my preamps that I
checked. These did not test much above the R.P. either. Were they all
these tubes bad? Was the tester working correctly? I was not sure.
I retested quite a few of the tubes and reread the Operational Manual several
times and my results stayed the same. I considered calling Tom Boffa but I
hesitated doing so for several reasons. First, I had only worked with one
type of tube and thought it might be useful to test some other tubes and
see what sort of results I got from them as well. Two, I know how Tom
fixed this tube tester. He started with page one of the Assembly Manual
and went through every single step in the manual. Operator error was a far
likelier possibility than a problem with the tester. Finally, even I get tired of
asking dumb questions. So I came up with an alternative strategy.
I went to eBay and bought eight new JJ 12AX7 tubes. I would test them
and see what sort of results I got, if these results were weird, then I would
call Tom.
This turned out to be a reasonably good idea.
The new JJ tubes, with one exception, tested in the sort of range that was
expected of a new 12AX7. Finally I was getting some results that made
sense. The new tubes were also testing consistently enough to leave me
feeling that I knew what I was doing.
Here are the results that I got. A 12AX7 is a dual triode tube so there are
two sections of the tube to test.
1. 1800/1700
2. 1400/1750
3. 1725/1750
4. 1800/1800
5. 1950/1750
6. 1750/1850
7. 1900/2050
8. 1700/1800
These results are fairly consistent, with the exception of tube 2, which has
one low side and tube 7, which tests somewhat high. With an R.P. of 1040,
good tubes should test at least 1600 on the meter. Overall, these results
look pretty good.
Even with these tubes, however, a very slight change in the Line Test
setting would make a large difference in the results. More confusing still, re-
testing a tube half an hour after I first tested it would give me a slightly
different result. This result would still be in the expected range but it was
disconcerting to see how much the value changed with even slight use on
the tube. Note: Buying tubes that have been burned-in is a good idea.
I ended up chatting with Tom anyway and he was surprisingly sympathetic
to my situation. It turns out tubes will test differently right out of the box
than they will with some usage on them. It turns out that two properly
calibrated tube testers will get different results. It turns out that testing
tubes is still more about comparing them to each other than to any sort of
absolute values. It turns out that new, good quality tubes can still test all
over the map.
It turns out tube testing is not an exact science. Damn.
I used tubes 1 and 5 in my Rogue 90 amp and used tubes 3, 4, and 8 in my
preamp.
Unfortunately, the one thing that a tube tester cannot tell you is whether a
tube is noisy. Tube 3 was crackling like an open fire within a minute of
turning the preamp on. I replaced it with tube 6. All of these tubes have
subsequently proven to be very quiet and there is a definite improvement in
the sound of my system with them in it.
I would strongly suggest that when buying new tubes that you buy more
than you need so that you can sort through them. I still have tubes 2 and
7 new in their boxes and I am not sure what I will do with them. I may put
7 in the Rogue amplifier with 5 and keep 1 as a spare for the preamp. In
fact, the more I look at it, the better I like that idea.
The results that I got with these 12AX7s left me feeling that I understood
what I was doing and that the tube tester was working properly. So I went
back to look at some used tubes that I have.
I have two sets of used 12AT7s from my VTL amp and wanted to select the
best four of them to use in that amp. Again, I tested and these are the
results:
1. 1800/1500
2. 1750/2300
3. 1450/1350
4. 1450/1600
5. 1450/2000
6. 1700/2750
7. 1750/>3000 (off the scale!)
8. 1775/1800
The R.P. for a 12AT7 is 1190, so a good tube should test in the 1800 range.
In looking at these results, the question becomes just which of these tubes
are good, if any? Tube 3 tests low on both sides, so it is probably fairly
worn. But is it worn out? Tube 4 also tests low but the 1600 side is
probably very usable. Tube 5 tests somewhat low on one side but like new
on the other. What is the deal with tube 7? It pegged the meter!
These are the sort of results that I have gotten from used tubes. There is
not a lot of consistency and the test values fall into a gray area that is
neither clearly good nor obviously bad. The low testing tubes are not
necessarily unusable but I would not want to put them in my equipment,
especially not knowing which side of the tube is being used in the circuit.
I ended up putting tubes 1, 2, 6, and 8 in the VTL. I paired them as 1/6
and 2/8. They sound fine but I am already thinking about investing in some
new tubes.
HERE IS WHAT I FEEL THAT I MAY HAVE POSSIBLY LEARNED FROM ALL OF
THIS
Assuming you own, or have access to, a working, properly calibrated tube
tester:
Invest in some new tubes of the types you use a lot. Get more than you
actually need. Test these tubes and let them determine what constitutes a
good test result. Keep notes. Use those results to evaluate used/unknown
tubes. It is not a bad idea to set aside a tube that tests close to ideal as a
standard for future testing. These “standard” tubes will also let you know if
there is any change in the function of the tester itself.
Having some tubes test in the expected range helped put the performance
of the other tubes I had in perspective but even then it is a judgment call as
to what level separates good from fair or poor condition. This is further
complicated by not knowing exactly how the tube is being used in a piece of
equipment. A tube might test somewhat weak but will be perfectly fine in
some applications and inadequate in others.
Relative results are more important than absolute results. If you test four
new tubes and they test very close to each other in value, they are probably
all very good tubes, even if none of them tests in precisely the range
predicted by the Reject Point.
If you are thinking of investing in a tube tester, look around for one that is
being offered as restored or rebuilt with, hopefully, some sort of guarantee.
If you get one for free, that’s great but you will probably need to find
someone who can, at the least, calibrate it, or more likely go through it and
get it working properly. Like a lot of tube audio equipment itself, most of
these testers are getting old and you should assume that they are not
going to be accurate if they are completely original.
No tube tester is of any value if you do not have the data sheets for setting
it. Thanks to eBay finding a lot of this information is fairly easy. In the
course of locating an Assembly Manual for the TT-1A, I found Vintage
Manuals, Inc. of Batesville, Indiana. I cannot recommend them highly
enough. They are on the web at
www.w7fg.com. Check with them first if you are in need of manuals, data
sheets, and so on for any type of older equipment.
IN SUMMARY
This is not the article that I thought I would be writing when I got my TT-1A
back this summer. I thought I would be telling you how I plug in a tube, run
the test, and, bingo, here is the result. It does not work quite that way.
Very good tubes are obvious; very bad tubes are obvious (and fun to test
just for the novelty of seeing the meter do something different) but the
80% of the rest of the tubes are largely judgment calls. Should I have just
stuck with the Lafayette tester? No, even though there is less
“definitiveness” involved in testing tubes with the TT-1A than I had
expected, it is light years more informative than the Lafayette was.
I hope this article does not dissuade anyone from getting a tube tester.
They are useful, informative, and kind of fun to work with. It is just that a
tube is not a light bulb. It is not obviously good or bad. Determining where
a tube falls on the scale between new and unusable is much trickier than it
first appears; but it is well worth spending some time to try to understand.
I feel much better knowing something about the performance of the tubes I
am using in my equipment and that, in turn, has led to an improvement in
the sound I am getting. And that’s the whole point.
Kent Johnson
October 25, 2007