Manley TNT MICROPHONE PREAMPLIFIER - Manual (Page 11)

Tube Channel Continued
If the transformer is driving some heavy resistive load
like some piece of vintage gear still set up for 600
Ohms then you can expect some high frequency roll-
off and maybe a shift in the distortion and clipping
towards the tubes and away from the transformer.
Maybe the best approach is just to listen and compare,
pick the best sounding output for this track, and don’t
worry about trying to label it or defining hard and fast
rules for recording music (other than to impress your
clients, of course).
Tubes will need to be replaced occasionally. Sometimes
they last a few months, sometimes 30 or 40 years so
about the only thing we can tell you is replace them
when they get noisy, microphonic or the preamp stops
passing signal. Generally, there won’t be a major
advantage with new tubes, broken in tubes, or esoteric
and rare expensive tubes and in fact any of these might
be worse. Here is the thing – there are 2 trims for the
2 JFET-Tube stages that adjust the bias and thus set
the distortion null point. Inserting a fresh tube might
require adjusting both of those trims, and that really
should be done by a technician with a distortion
meter for the best results. And there are another two
trimmers that are “fine gain adjustments” for each
stage, and while less critical, should be tweaked with
fresh tubes. In other words, the performance of this
preamp is equally dependent on the tubes and the
trimmer tweaks, and the tweaks are there to get ideal
performance from a variety of tubes and compensate
for drift in a tube over decades. The JFETS used as
the first stage relax the requirements for a super-low-
noise, low-microphonic, expensive esoteric tube and
allow those pesky trimmers that should optimize for a
good variety of 12AT7A’s.
So the preceding paragraph was aimed at those who
like to buy $300 tubes on eBay, and that’s OK, but
most of us (and Manley Labs) tend to just use the $15
to $30 ones and get as good performance because we
follow the procedures. And those who just need to
change a 12AT7A and don’t have access to a distortion
meter or don’t have the time, generally, it will work
just fine and the difference between tweaked-out and
not will be negligible – You see, the circuits are also
set up to self-adjust to a large degree, so you can sweat
the details or not, and usually be OK. And changing
tubes is almost as easy as changing a light bulb, about
as easy as changing a 9 volt battery in stomp-box
and a whole lot easier than changing a transistor or
chip, especially if its surface-mount. Your grandfather
probably fixed the family TV any number of times.
Relax.
About the only other “tricks” we might add here are
more general and apply to most preamps and not just
the TNT. Avoid plugging in mics, cables, mic patches,
etc., when phantom power is turned on (and especially
if the monitors are up). What can and often does
happen is that one “leg” of the balanced line (Pin 2 or
Pin 3) connects first, which can put a spike of 48 volts
through a transformer and magnetize it. This has been
known to damage ribbon mics and there are usually
transformers in ribbon mics, dynamic mics, many
condenser mics and of course many preamps, the TNT
Tube side included. There are some engineers that like
to demagnetize input transformers on preamps before
big sessions. Probably a small, weak cassette tape-head
demagnetizer won’t be too effective because virtually
all mic pre input transformers are mu-metal shielded,
but a bigger pro head de-magnetizer, carefully and
slowly brought near and away from the transformer
is probably a good thing to use as yearly maintenance.
One might also feed in a strong (say +25 dB) low
frequency tone and slowly decrease its level to zero
once in a while and get similar benefits. As far as those
dynamic and ribbon mics and their transformers, best
not to try because they also need those permanent
magnets (in the capsules). It might be worth checking
with the manufacturer on transformer coupled
condenser mics, but probably they would rather you
not take a chance or risk breaking something. So lets
repeat, avoid plugging in mics, mic patches, etc., with
phantom on so that you never have to worry about it.
And lets also take the devil’s advocate point of view
to balance the issue. Originally phantom power was
called “phantom” for a reason and most consoles of
the 70’s and 80’s (before external mic pres) didn’t
have an on-off switch for phantom power, per channel
or even global – it was always on. For 99% of us
who were there, we always turned down monitors
when changing mics, and we didn’t think to de-mag
anything except tape heads and we occasionally made
great sounding records. Back then, we weren’t looking
for ‘air’, ‘warmth’, or ‘loud’, we were just having fun
capturing first takes, hopefully exciting performances,
and experimenting with mic choice and positions.
A real good trick with mic preamps, console channels,
etc., that isn’t mentioned nearly enough has to do with
the PHASE switch. It is relevant here because we
expect the TNT will often be a first choice for vocals
and many simple overdubs. In a single mic situation,
you in the control room probably won’t be able to hear
any difference with the phase switch in one position
or the other so you might write it off as insignificant.
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