Heathkit IT 17 Testing 12ax7 and 12au7 bad?

Linehand

Active Member
I picked up this tester (emissions, shorts) to go through some tubes I found in an abandoned organ- mainly 12au7 and 12ax7. This checker lists two sets of testing parameters for this tube and all tubes will pass the test one way, but most fail the test in the second (only 2 out of 20 pass both tests).

I can physically see that both elements at the top under the getter are glowing in the tubes that pass, but all others that fail only show one side glowing under the test.

If these tubes are placed in circuit in my amplifier, both sides will glow.

I'm assuming its an issue with the tester but find it strange that a couple tubes pass the test. I did replace some out of tolerance resistors and successfully tested several output tubes, one of which was known bad, and the tester indicated a short.

Anyway, I'm really just experimenting and learning as I go - what should I be checking? The signal tubes are RCA and Sylvania 12A_7's from the mid 50's

thanks
 
both are dual triodes and require a different (tester) setting to be able to test them
separately (usually by the tester using two sets of three pins (plate, grid, cathode))
and usually most testers light both filaments at one time,

in my (bad) memory - the 12xx7s have a 12v filament using 2 pins but I recall pin 9
being a center tap and with it and one of the other two pins can be powered by 6v.
(may be a bad path to run down and is one "bad" explanation of why one side lights)

check the tester for the filament setting, usually a voltage and the pins it applies
to. some thing is wrong, applying 6v to a possible filament configuration may
allow proper testing. but you'd need to check the tester's schematics to see
how this is done. warning, testers are complex due to the vast number of
possible configurations for all the tubes it supports.

for the most part, testers are trust measurements for gas (think tacos) and
shorts. they are rarely useful for amp use because they do not measure
tube specs at operational voltages, etc.

on my tester, there's a green meter zone for emissions, no markings, no
numbers. like a 1960's car's idiot lights.

the easiest explanation is that you have a contact problem in the 9-pin
socket used for testing 12xx7 tubes, the filament selector circuit,
the filament power supply, etc
 
I picked up this tester (emissions, shorts) to go through some tubes I found in an abandoned organ- mainly 12au7 and 12ax7. This checker lists two sets of testing parameters for this tube and all tubes will pass the test one way, but most fail the test in the second (only 2 out of 20 pass both tests).

I can physically see that both elements at the top under the getter are glowing in the tubes that pass, but all others that fail only show one side glowing under the test.

If these tubes are placed in circuit in my amplifier, both sides will glow.

I'm assuming its an issue with the tester but find it strange that a couple tubes pass the test. I did replace some out of tolerance resistors and successfully tested several output tubes, one of which was known bad, and the tester indicated a short.

Anyway, I'm really just experimenting and learning as I go - what should I be checking? The signal tubes are RCA and Sylvania 12A_7's from the mid 50's

thanks
Ignore the tester. Sell it if you like.
Best place to test preamp tubes is in an amp, there other stuff like microphonics and hum will be detected.
 
both are dual triodes and require a different (tester) setting to be able to test them
separately (usually by the tester using two sets of three pins (plate, grid, cathode))
and usually most testers light both filaments at one time,

in my (bad) memory - the 12xx7s have a 12v filament using 2 pins but I recall pin 9
being a center tap and with it and one of the other two pins can be powered by 6v.
(may be a bad path to run down and is one "bad" explanation of why one side lights)

check the tester for the filament setting, usually a voltage and the pins it applies
to. some thing is wrong, applying 6v to a possible filament configuration may
allow proper testing. but you'd need to check the tester's schematics to see
how this is done. warning, testers are complex due to the vast number of
possible configurations for all the tubes it supports.

for the most part, testers are trust measurements for gas (think tacos) and
shorts. they are rarely useful for amp use because they do not measure
tube specs at operational voltages, etc.

on my tester, there's a green meter zone for emissions, no markings, no
numbers. like a 1960's car's idiot lights.

the easiest explanation is that you have a contact problem in the 9-pin
socket used for testing 12xx7 tubes, the filament selector circuit,
the filament power supply, etc

Thanks Bob, you hit the nail on the head- intermittent bad contact...
 
there was a set of 12xx7s a while back that had just slightly thinner pins and would require
tweaking of the socket. I recall they were a European manufacturer.

good, now enjoy the music!
 
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