[Lord Valve]

flipper wrote:

Not a problem, per see. It's just that I never had occasion to observe
it for myself till this little guitar amp I built for the nephew. Did
a thanksgiving day field test at the family get together.

To keep costs down I'm using 12AT7s in the premp and first thing I
discovered on the test bench was being able to hear tapping in the
chassis in the speaker. Microphonics.

As luck would have it I just happened to have a low microphonic 6201
and, sure enough, no tapping sound with that one. Nice to see theory
play out in practice.

However, the source I normally use for tubes subs 12AT7WAs for 6201s.
Hmm. So I pulled out all my 12AT7s and a couple were WAs. One was
quiet (GE) but the Tungsol was microphonic as hell. And the 'plain'
Sylvania 12AT7 was as quiet as the GE WA.

One thing I learned, I can tell which will and which won't by simply
looking for the extra mica spacer. But the question is, how can I
order a 'quiet' one when 12AT7WAs don't necessarily have it but even
some 'plain' 12AT7s do?

Does anyone know if any of the 'current' production 12AT7s are low
microphonic? While the few I've got were distinguished by the extra
mica spacer when I look at photos at tube depot the Ei 12AT7 Elite
Gold claims low microphonics but doesn't have the extra spacer while
the 'cheap' Sino does (according to the pictures, the only one that
does). but make no microphonics claim.

It was kind of fun trying different dual triodes of the same basing.
While the microphonic 12AT7s were 'obvious' (if you tap the chassis),
they might be acceptable because it's not enough to oscillate but the
microphonic feedback screech with a 5965 ('computer' triode) was
stunning.

Microphonics is a crapshoot.

The brand, year of manufacture, etc., means *squat*. If
you're going to put a 12AT7 in a gain stage, you'll have
to test 'em yourself - in the circuit you're using them in.
Either that, or you'll need to get them from someone
who does. While the triple mica types will tend to
be less microphonic than some, the triple mica is
no guarantee. I've tested plenty of 'em that were
microphonic. And, as you found out, vaunted
badges from days gone by are no guarantee
either. As to the claim of low microphonics for
the Ei 12AT7, it's bull**** - if you go through
100 of 'em, you *might* get five or six that will
be OK. "Ei" (pronounced "AY!") is actually a
Yugoslavian word which means "shrieks like
a train whistle." Regardless of how fancy the box
is or what the pins may be plated with, you are *not*
going to get anything special in the microphonics
department if it says "Ei" on the glass. As for other
currently manufactured 12AT7s, give the EH a miss,
too. I recently had a special request for low-microphonics
12AT7s from Randall Aiken, and I went through the 80
or so EH 12AT7s I had on hand in the process of
selecting some from every brand I had; not
a single one passed. (In fairness to the EH, they
work great as phase inverters and reverb drivers.)
I don't have much experience with the JJ, as I don't
get much call for 'em. Besides, NOS JAN 12AT7WC
are cheap and ubiquitous - no real need for me to
stock lots of current ones. BTW, the Chinese
12AT7s from the Beijing factory are great tubes,
if you can find them. (Production ceased in the
early 90s, as I recall.) No experience with the
current Chinese 12AT7 from the Shuguang plant,
but if it's anything like their 12AX7, it'll be fairly
ho-hum.

Lord Valve


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[Patrick Turner]

Lord Valve wrote:

flipper wrote:

Not a problem, per see. It's just that I never had occasion to observe
it for myself till this little guitar amp I built for the nephew. Did
a thanksgiving day field test at the family get together.

To keep costs down I'm using 12AT7s in the premp and first thing I
discovered on the test bench was being able to hear tapping in the
chassis in the speaker. Microphonics.

As luck would have it I just happened to have a low microphonic 6201
and, sure enough, no tapping sound with that one. Nice to see theory
play out in practice.

However, the source I normally use for tubes subs 12AT7WAs for 6201s.
Hmm. So I pulled out all my 12AT7s and a couple were WAs. One was
quiet (GE) but the Tungsol was microphonic as hell. And the 'plain'
Sylvania 12AT7 was as quiet as the GE WA.

One thing I learned, I can tell which will and which won't by simply
looking for the extra mica spacer. But the question is, how can I
order a 'quiet' one when 12AT7WAs don't necessarily have it but even
some 'plain' 12AT7s do?

Does anyone know if any of the 'current' production 12AT7s are low
microphonic? While the few I've got were distinguished by the extra
mica spacer when I look at photos at tube depot the Ei 12AT7 Elite
Gold claims low microphonics but doesn't have the extra spacer while
the 'cheap' Sino does (according to the pictures, the only one that
does). but make no microphonics claim.

It was kind of fun trying different dual triodes of the same basing.
While the microphonic 12AT7s were 'obvious' (if you tap the chassis),
they might be acceptable because it's not enough to oscillate but the
microphonic feedback screech with a 5965 ('computer' triode) was
stunning.

Microphonics is a crapshoot.

The brand, year of manufacture, etc., means *squat*. If
you're going to put a 12AT7 in a gain stage, you'll have
to test 'em yourself - in the circuit you're using them in.
Either that, or you'll need to get them from someone
who does. While the triple mica types will tend to
be less microphonic than some, the triple mica is
no guarantee. I've tested plenty of 'em that were
microphonic. And, as you found out, vaunted
badges from days gone by are no guarantee
either. As to the claim of low microphonics for
the Ei 12AT7, it's bull**** - if you go through
100 of 'em, you *might* get five or six that will
be OK. "Ei" (pronounced "AY!") is actually a
Yugoslavian word which means "shrieks like
a train whistle." Regardless of how fancy the box
is or what the pins may be plated with, you are *not*
going to get anything special in the microphonics
department if it says "Ei" on the glass. As for other
currently manufactured 12AT7s, give the EH a miss,
too. I recently had a special request for low-microphonics
12AT7s from Randall Aiken, and I went through the 80
or so EH 12AT7s I had on hand in the process of
selecting some from every brand I had; not
a single one passed. (In fairness to the EH, they
work great as phase inverters and reverb drivers.)
I don't have much experience with the JJ, as I don't
get much call for 'em. Besides, NOS JAN 12AT7WC
are cheap and ubiquitous - no real need for me to
stock lots of current ones. BTW, the Chinese
12AT7s from the Beijing factory are great tubes,
if you can find them. (Production ceased in the
early 90s, as I recall.) No experience with the
current Chinese 12AT7 from the Shuguang plant,
but if it's anything like their 12AX7, it'll be fairly
ho-hum.

Lord Valve

Well, you make interesting comments about 12AT7, one of the most
despised
twin triodes of all time with regard to audio by the cognescenti
knowa-ferkin-lotski.

I don't find so few that they generally could not be used in an phono
stage
and I am not so sure one would have to test large number before you got
some good ones.

But your'e right about many old famous brands of tubes testing like
****.
Just because one pays top dollar for supposed NOS brands, it don't mean
they
have not been used a bit, maybe 1,000 hrs, and then sold as NOS,
again......
preamp tubes can easily last 10,000 hrs. Some last 5 minutes,
but they are the minority....

Just never assume what the quality is until you have tested it.

I normally test a twin triode in common cathode test amp with
a 40k load and fully bypassed cathode and at about average Ia which is
usually
less than the data figure, and more like the real world figure. The
input grids are grounded via an RCA shorting plug to 0V.
The anode output is taken to a low noise amp with 20kHz of bandwidth and
at least gain = 1,000.

So let's say we have a 1/2 12AT7 giving a gain of about 32 into 40k
and let's assume there is an equivalent amount of GRID input noise =
2uV.
So there will be 64uV of noise at the AT7 anode.
This noise tends to swamp all the other noise mechanisms of the tube if
its OK.
64uV x 1,000 = 64,000 uV of noise at the output of the gain amp = 64mV.
64mV is easy to see on a CRO, and it can be watched for awhile to see if
there is a lot of sputtery
bursts of noise, and this signal can also be amped up further and
listened to on a speaker.
A constant hiss is what should be heard and without hum or any sputters.
After you have listened to a dozen tubes you will know what the good and
crook ones sound like.

If the tube is knocked lightly with a pencil it should not make a loud
long ring sound,
but just be a dull thud sound, and definately there should be no
acoustic
feedback from a nearby speaker.

Anyway, I find that plenty of the 12AT7 I have accumulated have less
than 1uV of
input noise and are not too microphonic.

When testing tubes this way you soon find out about tubes that are
gassy,
have low emission, become mechanically loose inside, have too much hum.
The noise will be over 5uv and if its 10uV I chuck em in the bin.
nearly all the common twin triodes will measure
between 0.6uV, very good, to about 3uV, beginning to be crook,
and some which have very low noise are microphonic, but
I put black textra pen ID with N = noisy, M = microphonic, NM = noisy +
microphonic,
and the noise figure for the worst of the two triodes within the twin
triode.

The ones that measure well just have their noise figure writ on them.

One can also test the gain for the tunes in the test circuit I have
but without the following gain amp.

If one hasn't got the type number for the tube under test
then measuring gain with Ia kept constant
for 2 different loads, say 20k and 40k will give two gain figures.
Since A = µ x RL / ( RL + Ra ) and Ra and µ don't vary,
the the µ and Ra can be worked out because one has two equations.

This all is difficult to test with a tube tester which I've never
bothered to acquire.

Patrick Turner.