I want to try different tubes for my tube mic-preamp. There are 4
tubes per channel: 2 12AX7 and 2 12AU7. What are the difference
between them and are 12AU7 simmilar to 12AT7. Can a 12AT7/ECC81 be
used in the place of a 12AU7?

"cvsound" wrote in message
om

I want to try different tubes for my tube mic-preamp.

Oh, boy.

There are 4 tubes per channel: 2 12AX7 and 2 12AU7. What are the
difference
between them and are 12AU7 similar to 12AT7.

These are all dual triodes, all have similar plate and filament voltages,
all have the same pin-outs.

They primarily differ in terms of gain or amplification factor.

The 12AU7 has the lowest gain of the bunch.
The 12AX7 has the highest gain of the bunch.
The 12AT7 is somewhere in-between.

Can a 12AT7/ECC81 be used in the place of a 12AU7?

With many pieces of tubed audio gear, you can mix and match any of the
three, and not instantly completely break the piece of equipment, or quickly
permanently damage the tube. ;-(

However, the people who designed the equipment, may have actually had
something in mind when they chose one over the other. ;-)

Listening tests that don't provide close comparisons to a standard reference
are often pretty non-critical. Tube swaps are almost never close comparisons
because there is only one piece of equipment in the test and it has to be
turned on and off to do the swap. There is often no standard reference,
because there is only one piece of equipment being tested.

People will no doubt say that a person has to be deaf to not hear the
difference something makes, but I've heard that story before and seen it go
"poof" before my very eyes.. I'm the guy who has done thousands of
level-matched, time-synched listening tests, and organized hundreds of
thousands more, and they are the people who have yet to do their first one,
no matter how easy they can be to do.

I've never done it, but I've often thought that I could probably mix and
match these three tube types in some audiophile preamps, and nobody would
ever reliably hear a difference, particularly if over-all gain was matched
after the swap. I've never even seen where someone did technical
measurements to see what technical changes swapping tubes makes.

I observe from over 50 years of experience with tubes, that while tube
swapping and tube wine-tasting sessions are popular, and people discuss the
results of them like they were important truths, I've never seen one done
using a listening test format that is worth the powder to blow it to...

San Diego or Baghdad.

Arny Krueger wrote:
"cvsound" wrote in message
om

I want to try different tubes for my tube mic-preamp.

Oh, boy.

There are 4 tubes per channel: 2 12AX7 and 2 12AU7. What are the
difference
between them and are 12AU7 similar to 12AT7.

These are all dual triodes, all have similar plate and filament voltages,
all have the same pin-outs.

They primarily differ in terms of gain or amplification factor.

The 12AU7 has the lowest gain of the bunch.
The 12AX7 has the highest gain of the bunch.
The 12AT7 is somewhere in-between.

I am reading a book on tube amplification design, which I have to say is
really good since an idiot like me can understand it. Kudos to the
author. Unfortunately, I am too stupid to remember his name, right
now.

Anyway, he has a section on tube model designation systems. The American
system is very logical. The first number refers to the filament voltage,
the next two letters to the application, the second number to the
number of elements and then the last letters to something else.

So a 12AX7WA is a dual triode (7 elements), with a 12 volt filament voltage.
The AX refers to its use and the WA to whatever that means.

(However, I remeber circuits where the filament voltage on the 12AX7s
is specified at 6.3 volts, so can anyone explain this?)

An "equivalent" to the 12AX7 is the 7025. Apparently this numbering system
was a designation that began at 5500 for some reason and each tube was
given a number in series as they were added to the system. This was an
industrial grade system which specified a better quality tube. So,
an old 7025 was actually superior to a 12AX7.

Rob R.

In article ,
cvsound wrote:
I want to try different tubes for my tube mic-preamp. There are 4
tubes per channel: 2 12AX7 and 2 12AU7. What are the difference
between them and are 12AU7 simmilar to 12AT7. Can a 12AT7/ECC81 be
used in the place of a 12AU7?

All of these are dual triodes with the same pinout, but with different
bias points and different gains. If you substitute one for the other,
it will not damage anything but it will also not sound very good. Same
goes for the 12BH7 which is the next tube in that series.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

On 22 Jul 2004 13:44:22 GMT, Rob Reedijk
wrote:

So a 12AX7WA is a dual triode (7 elements), with a 12 volt filament voltage.
The AX refers to its use and the WA to whatever that means.

"W" is usually ruggedized. "A" is a revision, often heater warmup
time controlled, sometimes greater plate dissipation, etc.


(However, I remeber circuits where the filament voltage on the 12AX7s
is specified at 6.3 volts, so can anyone explain this?)

The two filaments are brought out to three pins, so can be operated
series or parallel.


An "equivalent" to the 12AX7 is the 7025. Apparently this numbering system
was a designation that began at 5500 for some reason and each tube was
given a number in series as they were added to the system. This was an
industrial grade system which specified a better quality tube. So,
an old 7025 was actually superior to a 12AX7.

7025's had noise performance specified.

Chris Hornbeck
"Vote or Die" - P. Diddy

On 22 Jul 2004 03:16:59 -0700, (cvsound)
wrote:

I want to try different tubes for my tube mic-preamp. There are 4
tubes per channel: 2 12AX7 and 2 12AU7. What are the difference
between them and are 12AU7 simmilar to 12AT7. Can a 12AT7/ECC81 be
used in the place of a 12AU7?

You might also want to try the 12AY7/6072, a nice bottle but
out of fashion. And if you want a half dozen 5965's email me
an address.

Chris Hornbeck
"Vote or Die" - P. Diddy

In article ,
Chris Hornbeck wrote:
On 22 Jul 2004 03:16:59 -0700, (cvsound)
wrote:

I want to try different tubes for my tube mic-preamp. There are 4
tubes per channel: 2 12AX7 and 2 12AU7. What are the difference
between them and are 12AU7 simmilar to 12AT7. Can a 12AT7/ECC81 be
used in the place of a 12AU7?

You might also want to try the 12AY7/6072, a nice bottle but
out of fashion. And if you want a half dozen 5965's email me
an address.

The problem is that there are a LOT of tubes on the market being sold
as 12AY7s or 6072s, which don't come close to meeting the original
specifications.

I could actually use some 5965s, since I have a pulse generator that
requires them and won't work reliably with tubes that aren't specified
with the special grid.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

"Arny Krueger" wrote in message
...
"cvsound" wrote in message
om

I've never done it, but I've often thought that I could probably mix and
match these three tube types in some audiophile preamps, and nobody would
ever reliably hear a difference, particularly if over-all gain was matched
after the swap. I've never even seen where someone did technical
measurements to see what technical changes swapping tubes makes.

I've done them, although not published the results. In circuits with little
feedback, changing from one tube type to another, keeping the associated
components the same, changes the amount of distortion, typically in the
direction of up, and changes the spectrum of the (harmonic) distortion,
typically in a direction of uglier (more high harmonics). In other words,
about what you'd expect from using Tube A in a circuit designed and
optimized for Tube B. When there's more feedback around the circuit, the
effects are less prominent. Also what you'd expect.

Peace,
Paul

"Rob Reedijk" wrote in message
...

So a 12AX7WA is a dual triode (7 elements), with a 12 volt filament
voltage.
The AX refers to its use and the WA to whatever that means.

(However, I remeber circuits where the filament voltage on the 12AX7s
is specified at 6.3 volts, so can anyone explain this?)

The 12a*7 tubes all had 12.6V filaments that ran between pins 4 & 5; their
center-point (these being dual triodes) was connected to pin 9. So you could
run them from 12.6 volts, leaving pin 9 unconnected, or you could run the
two halves in parallel, joining pins 4&5 and hooking the 6.3V supply to the
joined pair and to pin 9. Depended what you had available.

An "equivalent" to the 12AX7 is the 7025. Apparently this numbering
system
was a designation that began at 5500 for some reason and each tube was
given a number in series as they were added to the system. This was an
industrial grade system which specified a better quality tube. So,
an old 7025 was actually superior to a 12AX7.

Better-quality tube in some respect, but not necessarily one which is useful
in a particular application. Some, for example, were ruggedized versions of
standard tubes, so they could survive being knocked about in airplanes,
boats and missiles. Others, like the 7025, were less microphonic, something
which might be useful to us. Others had controlled filament warm-up time,
which is really useful only in series-string applications, which nobody uses
anymore.

Peace,
Paul

In article writes:

I want to try different tubes for my tube mic-preamp. There are 4
tubes per channel: 2 12AX7 and 2 12AU7. What are the difference
between them and are 12AU7 simmilar to 12AT7. Can a 12AT7/ECC81 be
used in the place of a 12AU7?

The most apparent difference is in the gain of the tube. Simple tube
amplifier circuits such as are likely to be typical of your mic preamp
aren't like op-amps. In an op-amp circuit, the gain from input to
output is determined almost entirely by the feedback around the gain
stage. In a tube preamp, there's usually a little feedback, but the
gain is more dependent on the gain (mu, or amplification factor) of
the tube.

There are other differences, but all of that series has the same
pinout, so you probably won't blow anything up by substituting one for
the other. It's not likely that you'll actually like what the "wrong"
tube does, though.

--
I'm really Mike Rivers )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me he double-m-eleven-double-zero at yahoo

On 22 Jul 2004 11:47:41 -0400, (Scott Dorsey) wrote:

I could actually use some 5965s, since I have a pulse generator that
requires them and won't work reliably with tubes that aren't specified
with the special grid.

I'll send you a dozen each GE's and GL-5965's marked GE but look
like Sylvania construction. Should have your address somewhere here.
I'd better check 'em first; there're from 1956 and pulls.

Chris Hornbeck
"Vote or Die" - P. Diddy