Reprinted from The Tube Amp Book
Survival Tips for Tube Amps
by R. Aspen Pittman
Tube amps are simple, and so they are easy to keep
running smoothly. However, if you neglect to follow a few simple rules, you can buy
yourself some expensive trouble. What follows are some suggestions you can try that
will put your amp in top condition and keep it there.
TIP #1
Speaker Impedance
The proper matching of the impedance between your tube
amp and speaker is extremely important. Improper matching will cause severe tube
wear and is a common cause of early tube failure. Some amplifiers are more sensitive
to this than others . Among the most sensitive are Marshall amps. Pay
attention that the Marshall's impedance selector is on 16 ohms when your running a common
16 ohm Marshall cabinet, and reduce it to 8 ohms when adding a second identical
cabinet. Always check your cabinets by measuring with a volt meter on the ohms scale
(these meters read low, i.e.: an 8 ohm cabinet might read 6 ohms while a 4 ohm cabinet
could read 3 ohms). Another way to determine the impedance of your cabinet is to
read the individual speaker impedance and note how they are wired. If there are two
8 ohm speakers wired in parallel (+ to both +'s and - to both -'s) then the cabinet will
be a 4 ohm load. If the two speakers are wired in series (+ to spk #1 +, #1 - to #2 +, #2
- to -) then the cabinet will have a 16 ohm load. In other words, parallel wiring
halves the impedance of the speakers while series wiring will double it.
Find out the specified output impedance of your amp by
asking a service station or perhaps your local dealer. The common amps are:
Marshall, variable 4, 8, and 16 ohms; Fender Deluxe and Princeton, 8 ohms;
Fender Twins and Dual Showman, 4 ohms; Fender Super/Reverbs and 4-10 amps, 2
ohms.
Beware the dangers of using a power attenuator with your
Marshall as most power attenuators do not match impedance closely enough for these
amps. Using a power attenuator might let your Marshall distort at lower levels, but
at the expense of much more rapid output tube wear -- premature failure of the output
tubes is common in Marshalls used with power attenuators. Fender amps are not as
sensitive to power attenuators as Marshalls, because of differences in design in the
output section. However, since the tubes are putting out full power into the
attenuator, they will wear out quicker than if they were just coasting at a moderate
output level. If you like the sound you get with the attenuator, be prepared to
spend a little more on power tubes.
TIP #2
Power Tube Replacement
The regular replacement of power tubes is normal in amps
with regular use. Just when to change them can vary with the type of use the amp
gets and how often it's used. Most players should change their tubes once a year if
they play moderately loud and fairly often. As the output tubes wear out, both the
bass and treble responses of the amp will begin to suffer. This power loss from worn
out tubes isn't always noticeable because it occurs gradually over time, and because
power level differences aren't easily noticed. It takes twice the power for the ear
to hear just 3 dB more, and that's just barely audible! Worn tubes will
usually have poor, mushy bass response. Regular power tube replacement will
guarantee consistent and reliable performance. It's cheaper in the long run.
TIP #3
Drive Tube Replacement
The driver tube operates in conjunction with the power
tubes to form the power-amplifier section of the amp. The best power tubes will
sound bad with a weak driver tube, as this is the tube that controls the output
tubes -- if it can not control the output tubes, the amp can't sound its best.
This will show up particularly at higher power playing, or when playing the amp
distorted. REPLACE THE DRIVE TUBE WHENEVER REPLACING THE OUTPUT TUBES! In most
amps, the driver is the smaller tube (12AT7, 12AX7, 7025, 12AU7 or similar), which is
adjacent to the output tubes.
TIP #4
Re-Tensioning Tube Sockets
NOTE: Because the tube sockets are connected to the
very highest voltages in the amplifier, we suggest that the following work be done only be
those having the proper knowledge of electrical safety.
When tubes are changed again and again over time, the
sockets female parts begin to stretch and not make good tight contact with the tube
pins. This can lead to arcing and intermittent connections between the tube and the
amp. This condition can be aggravated by the vibration from your speakers and so may
occur on certain notes on your guitar or keyboard. You can correct this by replacing
the socket (last resort) or by re-tensioning the socket with a large safety pin, jeweler's
screwdriver, or small ice pick. Use a tool with an insulated handle if at all
possible.
First: disconnect the amp from the AC outlet and
allow the amp to drain off any voltage by leaving your speakers hooked up to the amp with
the standby "ON". This takes just a few minutes and could save an awful
experience later. Now remove the tubes and notice the contacts located inside each
pin hole of the socket. These contacts spread the pin hole -- do not push the
contacts in so far that the tube will not re-insert. After you've re-tensioned all
the contacts, replace the tubes and notice how much tighter the tubes are held.
You may also find corrosion on the contacts. Try
spraying a little contact cleaner or WD-40 on a tube and inserting it into the socket a
few times. This will improve the connection to the tube and prevent future
corrosion.
TIP #5
Capacitors and Resistors
The most common problem we see in tube amps (other than
tubes) is worn out capacitors and bad resistors. What follows are some common
symptoms of bad resistors and capacitors, why they can go bad, and how to locate and fix
the problem.
NOTE: Tube amplifiers contain high voltages which
may be lethal, even if the amp has been off for some time. We do
not recommend that you open your amp, or try to perform any repair operations unless you
are properly trained in electronic servicing. Again, there are large voltages
present in your amplifier that can kill, even with your amp unplugged from the wall.
Having said all that, you may now read on.
A common result of cheap tubes failing is that they will
take out a screen grid resistor with them (usually located across the inside of the tube
socket, or near by). These take the heat when the tube shorts and can fall out of
specification easily. This will cause improper function of any power tube you place
in the faulty socket -- if the resistor is open, the tube may as well not be in the
socket! In any case, the amp will not be reliable until the screen grid resistor(s)
have been replaced. Fender amps usually have a 1 watt 470 ohm screen grid resistor,
while Marshalls generally use a 5 watt 1000 ohm resistor for this purpose. The
screen grid resistors can be checked using an ohm-meter to measure their resistance.
The measurement should be within 10% of it's marked value.
Another common source of poor sound quality would be
worn-out filter capacitors in the output or supply stage of the amp. This is
especially common in amps over ten years old. These are fairly large components and
are often mistaken for "metal tubes" at first glance. The filter caps
"filter out" the 60 cycle hum from the power source and through the years they
dry out and filter less and less. As the 60 cycle hum is now present in your audio
output, it will create an odd harmonic that will seem to follow your notes up and down the
scale. It's almost like having somebody singing off-key all the time. In
addition, since the amp is now producing sub-harmonic notes, the power is sapped and the
overall response of the amp will become weak and sound mushy.
Inspection of filter caps can usually determine if they
are bad. These large metal cylinders are easy to spot. Fender amps have them on the
under side of the chassis, between the transformers, covered by a 4" X 6" metal
pan. It is therefore not usually necessary to remove the amp chassis from the wood
cabinet. Remove the pan and "drain" the capacitor by touching a
screwdriver from the hot side of the caps to ground. Now inspect the top site (or
positive) of the part, looking for a broken or swollen seal. This can look like a
little bubble about to pop, or it could have already burst and have powder coming
out. Capacitors have this relief seal to expose when they go faulty. Be sure
to replace them with the same value (or greater value) and make sure they are placed with
the proper polarity.
Marshalls have their filter capacitors placed upright on
the chassis held at the base with a clamp. The chassis must be removed from the wood
cabinet to inspect the filter caps. Observe the same procedure for inspection of the
capacitors. It should be mentioned that if you replace your filter caps, you should
connect your amp to a variac and power the amp up very slowly to allow the caps to charge
and form properly.
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