[Michael A. Terrell]

Don wrote:

12au7 tubes cost less than $10, so don't replace with a 12ax7. A music store
or old-timer tv repairer shop may also have 12au7 tubes in stock. It also may
make sense to have a tv repair shop replace any electrolytic capacitors in the
unit, as these are the most likely components to fail. In fact, the tube may
have failed due to cap failure. Tubes also fail after many hours of use.

http://thetubestore.com/12au7types.html






Wassa matta? You've got something against fried screen resistors? ;-)

Now you've done it! You are never to speak of them again, lest the
audiophools surround your home, bash down your front door and make you
walk across thousands of broken tubes, barefoot! ;-)

 

[Baphomet]

music
store It
also may the
tube may of
use.

Now you've done it! You are never to speak of them again, lest the
audiophools surround your home, bash down your front door and make you
walk across thousands of broken tubes, barefoot! ;-)



Michael A. Terrell
Central Florida

 

[Luhan Monat]

That's cool! So I can use 12au7 in my phono preamp instead of 12ax7?
And 12ax7 in my HHScott tube tuner where 12at7 are used? Changing the µ doesn't
effect the gain. Hmmm, how about oscillation? Why did they make more than one
tube type?
-Don

Don,

Same reason they make more than one type of transistor today - the one
with the higher gain probably had to trade off some other
characteristic; price if noting else.

Tubes in audio circuits were not prone to parasitic oscillations unless
they ran at very high power.

I used to interchange these two tubes all of the time.

 

[Luhan Monat]

Michael A. Terrell wrote:

So, you've never seen a burnt cathode resistor? How about a smoked
plate resistor.

Ok, correction, tubes dont *usually* short out. 19 out of 20 times, its
just a burnt out fillament in the tube. Even then, burnt plate or
cathode resistors failed by slowly cooking do to being underrated for
wattage.

Back in the 60's, some TV repairmen were known as 'tube pullers' because
the only things they could fix were those with dead tubes and no other
problems. These guys got killed in the business when transistors came
along and they could not spot the bad ones just by looking at them.

 

[Michael A. Terrell]

Ok, correction, tubes dont *usually* short out. 19 out of 20 times, its
just a burnt out fillament in the tube. Even then, burnt plate or
cathode resistors failed by slowly cooking do to being underrated for
wattage.

Back in the 60's, some TV repairmen were known as 'tube pullers' because
the only things they could fix were those with dead tubes and no other
problems. These guys got killed in the business when transistors came
along and they could not spot the bad ones just by looking at them.

Luhan, I started working in a TV shop when I was 13 years old. I have
seen all kinds of "That doesn't happen!" things since. I have seen tubes
that had a dead short between elements that read under one ohm. These
definitely burnt the cathode and plate resistors.

As far as the bad TV repair people, the just moved on to cars, then
swapping boards in computers. They also sell used cars, aluminum siding,
and do shoddy home repairs at inflated prices. Every business was, and
still is open to shysters, idiots and lazy assholes looking for a quick
buck. Look at the internet with all the spammers, and crooks running
websites trying to sell you things you don't need, or want.

 

[Fred Abse]

Ok, correction, tubes dont *usually* short out. 19 out of 20 times, its
just a burnt out fillament in the tube. Even then, burnt plate or
cathode resistors failed by slowly cooking do to being underrated for
wattage.

Going back to the days when there was a lot of tube equipment about, ISTR
that the commonest problem was low emission, followed by interelectrode
shorts. Open circuit heaters rarely happened in the sort of equipment I
worked with back then. Heater to cathode shorts, certainly, plenty of
those, but then some of the gear was nudging the limit of permissible H-K
voltage.

 

[Baphomet]

Going back to the days when there was a lot of tube equipment about, ISTR
that the commonest problem was low emission, followed by interelectrode
shorts. Open circuit heaters rarely happened in the sort of equipment I
worked with back then. Heater to cathode shorts, certainly, plenty of
those, but then some of the gear was nudging the limit of permissible H-K
voltage.

--

Fred - Remember the old Sencore grid leak checker?

 

[tempus fugit]

I was assuming from the OP that the guy doesn't have several thousand
livestock and probably not even one million dollar race horse relying on the
accuracy of this meter.



" extra gain may literally translate to

 

[Don Bruder]

- Top-post moved to bottom, where it belongs. -

I was assuming from the OP that the guy doesn't have several thousand
livestock and probably not even one million dollar race horse relying on the
accuracy of this meter.



" extra gain may literally translate to


I was assuming from the OP that the guy doesn't have several thousand
livestock and probably not even one million dollar race horse relying on the
accuracy of this meter.

The guy with the meter doesn't have to. But there was no indication
whatsoever in the OP about whether he may or may not have been selling
his corn on to a feed mill, supplying neighbors, or otherwise "passing
along" his crop. All with the best of intentions, of course, and almost
certainly without even a hint of malice. But good intentions and lack of
malice don't prevent mold from growing, and since, as I mentioned, I've
got up-close-and-personal experience with a horse dying from this stuff,
I'm a little on the overcautious side about it anymore. It wasn't a
pretty death, to put it mildly. Assuming that substituting a similar
tube will restore *PROPER* function is a really big assumption in the
case of equipment whose use carries the very real potential for life and
death consequences if it isn't functioning precisely as expected.

 

[Luhan Monat]

Michael A. Terrell wrote:

Luhan, I started working in a TV shop when I was 13 years old. I have
seen all kinds of "That doesn't happen!" things since. I have seen tubes
that had a dead short between elements that read under one ohm. These
definitely burnt the cathode and plate resistors.

As far as the bad TV repair people, the just moved on to cars, then
swapping boards in computers. They also sell used cars, aluminum siding,
and do shoddy home repairs at inflated prices. Every business was, and
still is open to shysters, idiots and lazy assholes looking for a quick
buck. Look at the internet with all the spammers, and crooks running
websites trying to sell you things you don't need, or want.

Michael,

Near as I recall, I was also about 13 when I started fixing TV's and
stereos. I also had a ham radio licence (general class) and spent most
all of my free time studying anything I could find on electronics and
making many projects.

 

[Fred Abse]

Fred - Remember the old Sencore grid leak checker?

Never had one of those. In fact, I don't remember ever having used any
tube tester of any sort. Just tested 'em in circuit, it was usually fairly
obvious what was happening.

 

[Ryan]

It's likely only the tube.

Thank you for all of the replies to this thread. You guys are very
helpful.

Since the original post, I made a trip to see the failing unit. The
results left me confused.

I shipped a 12AU7A tube which I obtained new from a local radio supply
shop. The original tube was RCA, and the tube I got was a different
brand, but the same part number. The user of the moisture meter
installed this tube, and the unit still did not work.

He figured out that the on/off switch had actually stopped working, so
he bypassed it.

Here is how it should work:
You weigh out an exact amount of corn with a scale. You set the unit to
"calibrate" on the main rotary dial and then move a calibration dial to
set the needle straight up. I think the main dial is a variable
capacitor (semi circle plates inside other plates). You rotate the
calibration dial until the needle, which is your only electronic
readout, goes to the middle. It is now calibrated. You pour the corn
into the machine and the needle pegs to the right. You then rotate the
main dial CCW until the needle is again centered. When the needle is
straight up, the number printed on the plate which is attached to the
main dial indicates your moisture level. 19% for example.

When I arrived, the new tube was already installed and the unit would
not calibrate. The needle would just stay to the extreme left and
bounce a little.

I examined his old tube. It turns out that what the owner thought was a
burn out on the tube, was actually the getter on the side of the tube.
It looks fine, nice and silver.

We then put the original tube back into the unit and it works fine.

So the wrong diagnosis of his tube seems clear, but I'm bothered and
confused by the fact that the same part number did not allow the unit to
work. Calibration could not be obtained via the standard knob (fine
tuning) or with a screw on the front of the needle assembly that he said
was for big adjustments.

Is it most likely there is something that was overlooked on the
calibration aspect, or might the replacement tube vary too much from the
original?

Thank You.

 

[GPG]

The dark marks on the glass are metal lost from the cathode. This
indicates a tube that has seen a lot of service. The cathode coating
is a mixture of oxides
(electron emission) and metal (conductivity). loss of metal eventually
leads to low emission and possibly inter electrode leakage. On some
tubes that have holes in the plate the metal forms an image of the
hole. Tube "rejvenators"
over heat the cathode to redistibute remaining metal but this does not
usually last long.

 

[Fred Abse]

The dark marks on the glass are metal lost from the cathode.
No.

This
indicates a tube that has seen a lot of service. The cathode coating
is a mixture of oxides
(electron emission) and metal (conductivity). loss of metal eventually
leads to low emission and possibly inter electrode leakage. On some
tubes that have holes in the plate the metal forms an image of the
hole.

Those marks, which are usually brownish are believed to be due to X-ray
bombardment of the glass over a long period. The X-rays are generated by
electron collision with the plate, just as in an X-ray tube, but at much
lower energy. Many types of glass darken when subjected to X- or gamma
rays. They are mostly to be found in power tubes, or at least those having
high plate current and voltage. they are not an indication of lack of
serviceability, per se.

Tube "rejvenators"
over heat the cathode to redistibute remaining metal but this does not
usually last long.

The principle on which most, if not all work is to strip off some of the
coating, in the hope that there is something better underneath. Whether it
works depends on how generous the manufacturer was when the cathode was
coated.