Manley MAHI MONOBLOCK AMPLIFIER - Manual (Page 7)

While we're at it, and since this is a TUBE Amplifier, let's TAWK
about TUBES: As with all tubes, certain parameters degrade with age.
This is due to decreasing cathode emission, a natural process found in all
tubes. There are just so many electrons on that cathode and one day, they
will have all jumped off. An excessive increase in noise level or very
unstable output tube bias can indicate the need to replace a tube. The
electrolytic capacitors will probably eventually dry out or start leaking
and need to be replaced. Maybe in 15 to 20 or 30 years you will need
to do this but don't worry about this for now... didn't I tell you to stop
worrying?
But I do worry: How long will these tubes last? We can’t say for sure.
Nowadays, as in years past, bad tubes do emerge from the manufacturer’s
assembly lines. Some tubes die prematurely while others, especially
small ones like the 12AX7, AU, AT and so on can last for more than 30
years of continuous use! The chief determinants of tube life expectancy
are the stringency of the particular application, and the initial build
quality of the tube itself. The average for the tubes in the MAHI should
to be around 2000+ hours for the output tubes, hey, maybe lots more,
depending on usage, more for the input and driver tubes. We've seen
60,000+ hours on a set of tubes, but we're not trying to get your hopes
up as it is better for the life on your tubes to exceed what we tell you they
will do so you will feel better when they do and be all surprised.
How can I tell when I need to replace them? Most problems relating
to the output tubes will show up while performing the bias procedure (on
the next page). Tubes that cannot be adjusted within the specified range
or have a very unstable reading are should be replaced. If the tube’s plate
(the grey metal rectangular box-like part most visible from the outside)
is glowing cherry red or orange, then the tube is severely overheated.
Check its bias immediately; if unable to adjust, then turn off the amplifier
right away and replace the tube. The input and driver tubes can become
noisy (hiss) or the amplifier may exhibit audible distortion; substituting
known good tubes is the best way find the bad one. You got 2 channels
to play with! They all can't be bad at once.
All tubes are “microphonic” to some extent; that is, they will make
ringing noises through the speakers when tapped or vibrated. Here again,
substitution will determine which one is overly sensitive to mechanical
vibration. Obviously, any tube that is totally dark inside while powered
up or is cold to the touch (careful!) is defective, or not making good
contact with the heater contacts in the tube socket.
Most tubes have a silvery coating deposited on some area inside the glass
bottle. This coating is put there by the “getter”, and its job is to soak up
or “get” contaminants, such as air molecules left over inside the glass
envelope during the tube’s manufacture, and help keep the vacuum hard.
(Ooooh. You said "hard.") If the getter material has turned white
(compare to another tube), then the tube has lost vacuum (or gained air!)
and is definitely bad. Replace at once. Don’t turn the amp on. Throw the
tube out. Or shoot it.
Do I need to replace them all at once?
No, at least not with these amps, or unless all the tubes have clocked
some thousands of hours of use. Some tube amps do require that if one
tube has to be replaced that a complete matched set put in. All Manley
amps use individual bias trims for each output tube which allows a single
tube to be replaced. Absolute best performance is achieved when the
tubes are most similar, both in bias requirements and transconductance
characteristics. We batch them and label each tube so that in the event
of a replacement you can get one from Manley of similar characteristics
as the others in your amp. We need that hand written number on the top
of the tube (Output tubes only. We know what we need to give ya for
input or driver tubes).
Does the “sound” of the amp change as the tube ages?
Yes, but not too much, and even then, given good tubes, it is fairly
strongly related to the amp design. The tubes can be allowed to reach
their technical life limit, or they may be replaced more frequently
depending on the listener’s taste and accompanying equipment. In
general, the band edges will suffer first, with very gradual loss of the
deepest bass and ultrasonic treble. The MAHI is designed to meet
stated power bandwidth specifications at 80% of rated output power
at the end of useful tube service life. By way of contrary tube
application contrasts, it is not like big guitar amps where tubes are
replaced every 6 months for reasons of “tone”. The MAHI’s tubes are
strong, allowing a very long life and less change between old and new
tubes. This is where that figure of 2000 hours, or 4 to 5 years of use
comes from. You may notice an improvement between tubes this old
and new tubes depending on how critical you are. Keep in mind the
sound of new tubes changes most in the first weeks of use before they
can be considered “broken in”. At first they may sound a little “tight”
and “direct” (like some people we know).
Is it difficult to replace a tube? Yes, but only if you have trouble
replacing light bulbs. It is super easy. Turn off the power. Just let the
amp cool a few minutes so that you don’t burn your pinkies. It helps
to wiggle the tube gently rather than pulling it out straight. Use some
terry cloth material or an oven mitt if time is pressing.
Even if you don’t consider yourself “technical” you probably have
more technical ability than your parents and they used to fix the
family TV set by taking out the tubes and putting them through the
tube tester at the local pharmacy. It is almost as easy to re-insert a
tube. Just make sure it is correctly lined up with the socket and you
don’t bend a pin. Notice that the 9-pin miniature tubes have a gapped
set of pins, making a rotational installment error nearly impossible.
The larger 8-pin or octal-based tubes have no gap in their pin sets,
but instead have a larger keyed center pin formed from the inert
plastic plug material. The key makes insertion of the tube into the
socket nearly impossible unless the key lines up with the matching
hole in the amp’s octal socket. Notice that octal tubes which have
damaged or missing keyed center pins should NOT be used, since
some output tubes internal wiring can cause dramatic equipment
failure by short-circuit if incorrectly fitted to the amp’s sockets. But
you don't have any octal tubes in this amp so don't worry about that.
You can wiggle the tube when reinserting too. If you had a solid state
amp (heaven forbid!), transistor replacement would merely be
chapter 1 in the saga. You would have to open it up, diagnose the bad
transistors and burnt resistors, de-solder 'em, find replacements
(good luck on locating those germanium beasties) re-solder 'em, and
hold your breath as you turn it on. Best have your Platinum credit
card handy for ordering more. Or you could send it back, be without
music for a few weeks, pay for service by the hour and be ready to
administer nitroglycerine tablets under the tongue when it fries
again. (Sarcastic? We're not sarcastic!)
That's why TUBES RULE: If you need a tube, or set of tubes,
Manley Labs will be happy to sell you matched sets at a good price.
And if you prefer to send the unit back for repair or adjustment, our
warranty covers most everything except tubes (6 months only please)
and abuse (7 months only-- just kidding). Plus we handle ground
shipping back to you (unless you live overseas, cuz there's no ground
service!) Repairs usually take less than a week. Could we do more?
(That's a rhetorical question. It's late. Just keep reading...)
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