Oestex ©
GUITAR
AMPS PAGE
SECTION 9: TIPS AND TRICKS:
1. COMBO AMP/SPEAKER
The COMBO
amp/speaker unit is a popular form of construction, which dates back to at
least the 1940’s.
The
amplifier is mounted in the same cabinet as the speaker.
This
design offers a convenient way to amplify the guitar or keyboard, convenient to
handle and transport and reduces overall cost.
The
speaker connecting cable is often connected inside the cabinet, avoiding messy
cables and setup at gigs.
The
general practice is to instal small tube amplifiers upside down (inverted)
inside the same cavity as the loudspeaker.
This
method has some shortcomings:
1.1 Requires
the speaker magnet to be close to electronic circuitry, which can introduce
magnetic interference with amp operation, causing electronic instability.
Worse, the close proximity of the speaker cone can cause severe microphony
(audible rattling and/or mechanical feedback) in the tubes.
1.2. Poor ventilation. Tubes run very hot and
rely upon air convection through the cabinet for cooling. Heat damages tubes
and components – particularly electrolytic capacitors
1.3 Unconstrained tubes can loosen or fall out
of their sockets
1.4 Extreme vibration can cause tube failure in
“domestic/receiving” grade tubes –
premium or industrial versions should be used where practicable
I
would not recommend a combo amp of this style for amplifiers having a rated
power output of more than say 20 watts rms – e.g. one pair of 6V6G or
EL84/6BQ5.
The
use of a solid state rectifier instead of a tube rectifier reduces radiated
heat inside the cabinet.
Tubes
designed for high vibration/shock applications are preferable to standard
domestic grade tubes. The situation becomes more severe with high gain
amplifier/preamp stages. 4 digit military/industrial or SQ versions are readily
available. Also “W” suffix tubes usually mean “ruggedised”
A
COMBO cab can be modified to add an extra speaker socket to facilitate an
alternative or extension cabinet, however the effective loads must be matched
to the output transformer connections.
If
only one output socket is available a suitably rated matching transformer can
be used to couple an additional speaker(s).
Note: If a COMBO cabinet has a wholly or partially
open back the loudspeaker must be a low compliance type to prevent damage cone
over-excursion.
If a COMBO cabinet has a wholly sealed
cabinet the loudspeaker should ideally have medium to high compliance to
prevent restriction on cone excursion – i.e. over-damping.
2. SEPARATE HEAD AND SPEAKER
The
most popular setup is to mount the amplifier head on the top of the speaker
enclosure.
This
method offers versatility, insofaras the amplifier and speaker are not bonded
together as a pair.
This
feature permits the use of any cabinet with any amplifier – subject to load
impedance matching and power capability of the cabinet.
A separate amp head and cabinet
set is preferable for higher power situations. Separating the cabinet from the
amp head offers flexibility when using different cabs for different classes of
venue or different tonal properties. e.g. A single driver, twin driver, quad
driver or multiple cabinets may be used with a separate amp head.
It is
advisable to instal rubber mounting feet under the amplifier head to reduce
shock and vibration. Some stages can be very bouncy. A standard rubber door
stop may be suitable.
IEC
mains power cables are preferable to fixed cables because they allow the mains
cable to be stored separately, thus avoiding the need to wrap it inside the amp
head – avoiding damage to tubes.
3. CABINET MATERIAL
Loudspeaker cabinets can be
made from softwood plywood, hardwood plywood, particle board, MDF and
composite. Solid timber is not recommended except for bracing/ribbing because
of resonance.
Cabinets can be ribbed
(stiffened) or unribbed.
In my experience standard
particle board as used for kitchen and laundry cabinets and benchtops is not
suitable because it has too high a natural resonant frequency and is heavy. It
also swells or falls apart when wet.
If someone spills a beer all
over your cabinet dry it off as soon as you can to prevent it penetrating the
timber. Vynil covering over the cabinet helps to reduce the chances of liquid
damage.
MDF is strong but too rigid and
heavy and should not be used at all. It does not absorb sound well so reflects
sound within the cabinet and back out through the speaker cone.
The ideal material then is
softwood ply because it is soft and will absorb the sound waves faster and
therefore cabinet resonance or booming is reduced
It also makes the cabinet much
lighter to transport and handle
The softer the wood the better
the result.
The key to performance is in
the cabinet size and shape and whether open-back, closed back or vented
Cabinet components should
always be glued (and preferably screwed) to prevent air leaks - same applies to
open-backed cabinets.
4. CABINET LINING/DAMPING
As previously mentioned, to
reduce cabinet resonance it is desirable to line at least the back wall, and
preferably all internal surfaces, of the cabinet interior with cut pile wool or
80/20 wool/nylon carpet. Offcuts are readily available from carpet stores
Glue in with rubber based
adhesive such as vynil floor glue. Firmly instal with 3/4" tacks to hold
in place until the glue sets. The tacks may be left in place permanently.
This technique absorbs much of
the sound eminating/reflected from the rear of the speaker that would otherwise
travel through the paper cone and interfere with frontal sound radiation.
DAMPING reduces SPL.
DO NOT completely fill a
cabinet with padding such as polyester pillow or speaker enclosure padding
because back pressure on the cone is excessive. This rule applies also to
ported/vented enclosures.
The loudspeaker is an AIR PUMP
so has to breath.
5. SEALED ENCLOSURES
Sealed (closed box) enclosures
offer simplicity in construction, high acoustic damping to limit overshoot and
natural resonance.
The sealed enclosure can be
incorporated into an amp/speaker combo cabinet – often seen in low cost combo
amps.
A sealed enclosure will
increase the resonant frequency of the loudspeaker so high compliance speaker
suspensions are preferred. An indicator for compliance is to inspect the spider
– the yellow corrugated fabric suspension that centres the voice-coil.
Generally speaking, the smaller in diameter the spider the tighter the
suspension.
Fully closed cabinets require a
speaker having a low resonant natural frequency (20-50 Hz) and high compliance
suspension. Cone travel in and out is indicated by the number and depth of the
cone surround corrugations at the rim of the cone.
To prevent low frequency
attenuation (i.e. below 120 Hz) the cabinet should have at least one cubic
metre of internal volume. Small volume closed cabinets can be used but the low
frequency response is poor. Is a choice of size and weight v performance.
Although a high resonance
speaker will work in a closed cabinet a high compliance speaker will bottom out
through over-travel of the voice coil and self-destruct mechanically.
6. OPEN BACK CABINET OPENINGS
As to openings, this Jim Lill
video shows the effects - https://www.youtube.com/watch?v=-eeC1XyZxYs
One point worth mentioning is
that OEM partially open-backed combo cabinets from mainstream manufacturers
have been carefully engineered to suit the OEM speaker supplied with the combo.
If the speaker(s) is changed then
the cabinet should be re-calibrated for best effect
This Jim Lill video shows why -
https://www.youtube.com/watch?v=IC96P3icayc
Open-backed cabinets require a
speaker having a high resonant natural frequency (80-100 Hz) and stiff
suspension.
7. VENTED/PORTED/REFLEX CABINETS
A good compromise for some
situations, particularly bass, is the Vented
or Ported or Reflex enclosure. This design of cabinet needs to be tuned to
the speaker characteristics.
If the speaker(s) is changed
then the cabinet should be re-calibrated for best effect
One important advantage is that
the impedance of the speaker rise at low frequencies is reduced significantly
by splitting the system resonance into two peaks instead of one, with the
result that impedance change with frequency is reduced resulting in a more
linear sound level (flatter frequency response). This is very important with
solid state amplifiers, whose power output reduces proportionately to load
impedance rise.
Large vented cabinets for bass
may have the vent installed in the bottom/floor of the cabinet. Venting
is via a rectangular plywood tunnel positioned between the castors the cabinet
sits on. If the castors have 75 or 100 mm wheels then there is plenty of room
for this system, which reduces overall cabinet height by the height of the
castors. Port tuning is essential.
When fitting wheels or castors
use one pair of fixed units at one end and one pair of swivel units at the
other. This makes steering the cabinet easy. Using four swivel castors makes
the cabinet behave like a shopping trolley – i.e. a mind of its own.
Rubber or soft polymer wheels
are best for normal sized cabinets – steel wheels make a great deal of noise
and can damage venue flooring.
The importance of fitting
wheels becomes evident when transporting the cabinet across brick paving or
tiled paths or across grass or rough surfaces.
Also assist in positioning on
stage.
Lockable wheels/castors are not
desirable because they look ugly on stage. If the stage floor slopes, as
sometimes happens, then place a chock or wedge under a couple of wheels to hold
it.
I have had good results with
vented BASS cabinets fitted with a single 15 inch driver and having a volume of
around 6 cubic feet. LF response and tone is acceptable but a larger cabinet is
still desirable for better deep tone.
8. CABINET DIMENSIONS
Ideally, cabinets should be
engineered to known performance criteria.
This Loudspeaker Cabinet Designer is accurate
and proven by real world experience.
9. CABINET POSITION
When playing live, for maximum
sound dispersal, it is desirable to lift the cabinet off the floor -
particularly carpeted floors, because much of the sound energy is transmitted
directly into the floor.
If the cabinet must be mounted
onto the floor then a piece of vynil flooring sheet or mat can be placed in
front of the cabinet to deflect the sound otherwise lost to the audience.
In a live audience situation
where dancers are located directly in front of the band, the speakers may be
angled upwards to direct the sound towards the ceiling and over the heads of
the audience, so that attenuation of sound level over distance is reduced -
i.e. people at the back of the audience will still hear the band
10. STANDING WAVES
Most venues are rectangular
shaped so sound is reflected from the back wall back towards the stage. This
effect can create standing waves that bounce back and forth between the stage
and back wall.
This phenomenon is frequency dependent
and is particularly true for low frequencies such as in lower register bass.
One effect is acoustic feedback
interference with the vocals from a PA that have significant levels of
delay/echo, caused by the vocal mikes picking up the standing waves and
amplifying them into a garbled mess of sound - or just simply annoying feedback
squeal or an “echo” effect. This effect can occur even with
"performer" unidirectional (cardioid) dynamic vocal
microphones.
One trick that works is to
locate the bass speaker cabinets as low as possible on the stage - i.e. on the
floor and laying on their side. This increases the physical distance between
the bass cabinets and the nearest microphone, the idea being to deliver the
bass energy below the level of the microphones so the mikes do not pick it up.
Another is to angle the
speaker(s) about 2 to 5 degrees towards the side walls of the room. This
increases the low energy gap between the radiated area in front of the speakers
and other players and microphones. It also helps to bypass people standing or
dancing in front to the stage whose bodies block the sound, allowing sound
energy to reach further back into the room.
Note that at high SPL levels
guitars can be acoustically stimulated by bass and/or drums energy.
The more echo is applied to
instruments and vocals the worse the situation becomes.
Although bass players may be
blamed the fault lies in the PA settings.
Graphic equalisers can also
reduce the effect when properly applied.
If the problem is serious there
are measuring devices that can identify frequencies causing feedback in the
acoustic sound system.
11. ELECTRICAL CONNECTIONS
The standard phono plug/jack
and socket has a current rating of 3A rms continuous.
This translates into 72 watts
rms at 8 Ohms and 36 watts rms at 4 Ohms and 18 watts at 2 Ohms. (average
continous).
The outer woven mesh shielding
in coaxial cable can handle this power level without distress however if the
inner fine gauge stranded conductor is too light, worn or damaged, an open
circuit may develop. This can cause damage to the power output stage in the
amplifier because there will be no load. Unlike solid state amplifiers, tube
amplifiers must have a load.
Although the external metal
connecting portions of plug and socket are capable of handling significantly
more current, the internal rod conductor and terminal are the weak link.
Temperature cycling responding
to music signal power can cause the internal connection to oxidise and lose
contact. Some brands of plug have staked (riveted) internal connection that
develop faulty contact during use. A staked internal terminal contact can be
soldered with care to prevent loss of conduction.
In the case of 2 Ohm loads the
temperature rise of the metal conductor will increase to 4 x the 8 Ohm load
conductor temperature rise.
This does not bother a tube amp
because the output transformer will have internal conductors designed to handle
the current, however direct output solid state amplifiers do not. In that case
the current through the power transistors with a 2 Ohms load will produce a 4 x
temperature rise within them compared to an 8 Ohms load.
The point is that a high
quality phono plug must be used for speaker connection (at both ends where
applicable) for either tube or solid state amplifiers.
Guitar flexible shielded cables
may be used for speaker cable but do so at your own risk. Speaker cable should
be flexible, insulated and an adequate cross sectional area (0.2 or 0.3 square
mm) for low loss.
Red/black figure 8 cable is
recommended because it is easier to identify the positive and negative ends.
“Speakon” connectors and
equivalent substitutes are rated for 10A rms.
Do the maths.
W = I²R (W = watts AC) I =
current (in Amperes) R = Load (Speaker impedance) - ignore speaker lead DC
resistance
I =√ WR
For very high power levels use
screwed terminals with the connector spade terminals crimped and soldered to
the cable. Alternatively just twist the cable end strands tightly together and
solder - pare or crush with pliers to fit into the screw terminal hole size.
Copper oxidises over time so
the cable ends will darken and increase contact resistance – to prevent this,
solder the cable when new and clean before use.
12. SPEAKER GROUNDING
Tube amplifiers and non-bridged
solid state amplifiers mostly have one speaker terminal grounded. (Class D amps
are bridged so do not ground one or other speaker lead.)
To reduce or eliminate unwanted
RF pickup from the cables feeding back into the amplifier, one side of the
speaker cable should be grounded. This will usually, but not always, occur with
phono sockets or colour coded screwed terminals.
Check first.
Next ground the FRAME of the
speaker to its grounded terminal - usually the negative. The speaker acts
as a large antenna and grounding it reduces interference with the
electro-magnetic guitar pickup frames which are usually "floating"
and not grounded. Steel or aluminium frames can be easily grounded by
installing a flat spade terminal under one of the frame fixing screw heads and
connecting it back to the speaker negative terminal via a short wire.
13. SPEAKER PHASING
When more than one speaker is
used - whether in the same cabinet or different cabinets - and driven from the
same source, the cones must be "phased".
That means both or all cones must move in and out simultaneously together -
i.e not one in and the other out.
Most speakers are marked with a
"plus" symbol or marked "positive" or marked
"red".
When the same model speaker is
used there is no problem matching but if different makes of speaker are used
together there is no guarantee the markings show the same polarity.
Use a 9VDc battery to determine
the plus and minus terminals of the speakers, then wire them to be in phase.
14. MIXING SPEAKER IMPEDANCES
Speakers having different
nominal impedance ratings - e.g. 4 Ohms and 8 Ohms can be safely used together
so long as the power distribution is calculated to ensure they share the load
within their ratings
The speakers/cabinets may be
series or parallel connected so long as their effective load is matched to the
amplifier
Use the formula set out above
for power.
The power distribution is
proportional to speaker (load) impedance.
Check the SPL ratings also.
e.g. A cabinet having two 4 Ohm
series connected speakers installed (=8 Ohms load) will deliver
approximately the same loudness as an 8 Ohm speaker cab of the same make and
model and/or SPL when the two cabinets are connected either in series or
parallel
Similarly, a cabinet having two
8 Ohm parallel connected speakers installed (=4 Ohms load) will deliver
approximately the same loudness as a 4 Ohm speaker cab of the same make and
model and/or SPL when the two cabinets are connected either in series or
parallel
Alternatively,
a suitably rated matching transformer can be used to couple an additional
speaker(s).
15. FREQUENCY RESPONSE
Typical guitar speaker drivers
have a useable frequency response of 80 to around 2500-3000 Hz
Typical twin-cone
guitar speaker drivers have a useable frequency response of 50 to around
12-14kHz
Typical bass sub-woofer speaker
drivers have a useable frequency response of 20 to around 200 Hz, with no
useful harmonics
Typical bass guitar speaker
drivers have a useable frequency response of 40 to around 1500-2000 Hz
16. SPEAKER FIXING
It is essential to prevent air
leakage around the rim of the speaker - i.e from front to back - because air
leakage shorts out the acoustic energy
Speaker frames should be
affixed with as many screws as practicable.
The standard for speakers to 8
inch is usually four screws but if eight holes are provided use them.
Allen head screws are easier to
work with because the screwdriver sits firmly in the head of the screw. Always
hold the screwdriver handle firmly with one hand and the shaft firmly in the
other as guide to avoid it slipping and penetrating or tearing the cone - is a
common unintended situation. Just holding the handle with one hand is risky.
Speakers are expensive.
Do not overtighten the screws
such that they unevenly compress the sealing gasket or distort the frame.
The speaker may be installed
inside the cabinet or outside the cabinet. Some people prefer internal whereas
others prefer external.
Some claim outside fixing
delivers a more dynamic sound response.
Either way, a gasket must be
installed between the speaker frame and the baffle board to which it is
affixed.
Do not glue the built-in
speaker front gasket (usually cork or cardboard) to the baffle or you will
destroy the cone if you try to remove the speaker.
If the speaker is mounted to
the front of the baffle – i.e. from the outside of the cabinet, felt covering
over the cabinet usually provides sufficient air sealing. In the case of vynil
cabinet covering it is wise to use either cork or felt strip cut from sheet.
If a gasket is not supplied or
available use felt strip cut from sheet
For bass speakers it is
recommended that a seal of either black silicon rubber or black mastic material
be installed around the join between the outer rim edge and the baffle. This
can be easily cut away if the speaker requires removal.
17. SPEAKER REPAIR
Sometimes someone will tear or
rip the cone – usually punctured by a screwdriver.
Do not despair.
A puncture can be repaired by cutting a small round
(where possible) piece of cone material from a dud speaker, or a piece of
polyester/cotton cloth such as business shirt material, or piece of linen.
If the puncture is across the
outer cone suspension just contour the fabric up and down the corrugations.
Is a rubber suspension just
wrap the fabric around it.
Deposit a thin film of CONTACT
adhesive onto the cone and fabric, then quickly place the repair in position,
folding as required to eliminate creases and air bubbles.
If the cone itself has
concentric corrugated rings on its face try to push the fabric into the hollows
so that it contacts the entire cone surface under it. Is not hard to do while
the adhesive is wet.
Contact adhesive is temperature
sensitive and will set very quickly in hot summer conditions. Try to do the
repair in the coldest room you can find.
In winter the adhesive may take
while to set so be patient.
A tear from the voice coil to
the rim can be properly repaired by this method
To colour the repair black it
can be dyed with black spray paint or liquid shoe polish. Do not apply directly
but spray into a container until a small quantity of liquid paint forms. Then
using a very small brush or cotton bud, dip the paint and carefully paint it
onto the repair. Apply only enough to do the job. The idea is not to change the
cone weight and stiffness if possible.
18. SPL - SOUND PRESSURE LEVEL
The SPL (Sound Pressure Level)
rating of a speaker is an indicator of how loud it will be at a predetermined
input of 1 watt at 1 metre.
SPL is expressed in decibels - a
unit of loudness
Decibels (db) is expressed by a
logarithmic scale.
A 3db increase in SPL requires
twice the electrical power into the speaker.
A 6db increase in SPL requires
four times the electrical power into the speaker.
A 10db increase in SPL requires
ten times the electrical power into the speaker.
A 20 db increase in SPL
requires one hundred times the electrical power into the speaker.
A typical live performance will
be set at about 100 db
It follows that an amp driving
100db efficient speakers will be required to deliver only one watt of electrical power to produce a 100db sound level.
It follows that an amp driving
90db efficient speakers will be required to deliver ten watts of electrical power to produce the same sound level.
It follows that an amp driving
80db efficient speakers will be required to deliver 100 watts of electrical power to produce the same sound level.
The SPL is frequency dependent,
so SPL at every frequency across the guitar scale is subject to these
efficiency ratios on an individual frequency basis. In other words if the
speaker is 10 db down at 4000 Hz it will require 10 times the power at that
frequency to meet the same loudness level as at the average across the range
Bass speakers suffer from SPL
roll-off below 120 Hz so combined speaker and cabinet SPL efficiency is
extremely important when designing or selecting your system
Always check the speaker
specifications before buying.
19. CHASSIS AND COMPONENT LAYOUT AND WIRING
Review
of past design practice shows us that many designers had no idea of the
principles essential to optimising chassis layout.
In
a wide range amplifier, the wiring and componentry act as antennae, to pick up
stray and induced signals, such as ultrasonics, RF, hum and noise, from
adjacent circuitry.
The
basic rules for component layout and wiring are:
·
do not mount
electrolytic capacitors close to tubes - the caps will dry out and fail
·
do not mount
electrolytic capacitors close to transformers - the caps may induce hum and
interference into the B+ bus
·
do not mount hot
tubes near transformer windings - the windings will burn or dry out
·
do not mount driver
stage tubes near transformers or mains wiring - to avoid induced interference
or hum
·
do not mount
transformers close together - eg side by side. If this is not practicable mount
the transformer cores at right angles to each other to minimise magnetic
interaction
·
do not mount power
transformers near signal leads or unenclosed components such as a magnetic
pickup cartridge - in the case of pre-amplifiers this may mean a physical
separation of a couple of feet - ie half a metre. Generally speaking, the
larger the transformer the further the magnetic field will extend.
·
do not mount
exposed driver tubes near output tubes or rectifier tubes - they may pick up
induced signals such as RF, hum and even audio, and trigger parasitic
oscillation in the circuit
·
do not mount tubes
near the edge of the chassis - they can burn the enclosure or cabinet when the
chassis is installed and operating
·
do not mount input
wiring near output wiring either within a stage or across stages - to avoid
stray signal induction or oscillation
·
do not mount input
terminals near speaker terminals or mains supply leads or plugs - to prevent
instability and/or interference from induced signals
·
do not mount tubes
upside down without restraining devices - they can fall out partially, with
loss of one or more contact pins, or completely fall out and break
·
do not mount tubes
horizontally unless their grid wires are installed in the vertical plane - to
prevent grid wire sag when hot and thus uneven electron flow, or internal
shorts
·
always use shielded
wiring for signal pathways in the early stages - to prevent stray hum and noise
pickup and induced feedback signals
·
never ground
potentiometer wires to the chassis at the potentiometer - always ground
directly to the relevant cathode (through a shielded co-ax lead)
·
always shield input
tubes with discrete shields or a grounded metal enclosure (there are currently
more than 100 million mobile telephones in the world, thousands of radio and
television transmitters and significant numbers of high-powered industrial and
military transmitters, and they do generate a significant amount of RF energy
for your hi-fi amp to pick-up)
·
always mount tubes
as close to each other as space will permit - to minimise wiring length and
minimising exposure to induced signals or interference (but always leave at
least a 10 mm gap between bottles for natural air cooling - more for larger
tubes having more than 25 watts plate dissipation)
·
always use short
leads between connection points - to minimise wiring length and minimising
exposure to induced signals or interference
·
never connect pins
on tube sockets directly together with straight or solid or heavy wire unless
there is a loop to enable each socket pin to locate properly about its tube pin
- tube pins do not bend well to match socket misalignment, so some movement in
the socket terminals is essential
·
if practicable, mount
tube sockets such that the heater pins are aligned with the direction of wiring
- to minimise heater lead length
·
always twist heater
wires together - to neutralise AC radiation to nearby wiring and components
(cancel-out hum signals)
·
always ground grid
and cathode resistors (and bypass caps where applicable) to the cathode pin, if
grounded, or to the grounded end of the cathode resistor. One very effective
technique is to instal the resistors vertically, straight up from the socket
pins, then simply join the ends to be grounded together (neatly). Connect them
via a wire to chassis ground. This bundle of components does not need further
support as they are a strong structure mounted directly from the tube socket.
Some miniature sockets have a centre tube spiggot for shielding the pins from
each other - this should be also grounded. The tube shield provides a useful
structural support
·
where practicable,
instal interstage capacitors with the outer layer connected to the previous
stage plate - to minimise hum pickup Vintage caps often had a band printed on
one end to indicate the outer layer
·
try to layout the
chassis following the circuit diagram (schematic) - this method actually
works!!
·
ensure the
amplifier is well ventilated - to ensure adequate cooling and to facilitate
long-term component life
·
always insulate and
isolate high voltage and mains power circuitry - death is permanent!!
·
always use an
isolating transformer between the mains power source and the rectifier and
heater/filament system (to prevent electrocution)
·
always insulate and
isolate high voltage capacitors - particularly large capacitance units - they
can store electrical energy for several days. An amplifier switched off may
still be hazardous or even lethal
·
always use an
enclosed METAL chassis system -
if necessary fit a removable lid/cover for component access. 20 gauge
galvanised steel sheet (1 mm) thick is ideal but aluminium is more suitable for
large chassis to reduce weight. Note that it is not possible to solder
components to aluminium using tin/lead solder so all chassis joints must be
tightly screwed together.
·
never use 50/50
tin/lead solder - that is for plumbers. Use only a high quality 60 tin/40 lead
solder having a cored flux. To prevent dry joints (imperfect metallurgical
joint bond) when soldering components always allow time for the joint to
thoroughly heat before removing the soldering iron. Printed circuit board
conducting strip tends to separate from the bakelite/fibreglass base board if
overheated - best to avoid them. Take care not to overheat plastic covered
components.
·
always ground
or earth the chassis (to prevent
electrocution).
·
When sharing a mains supply try
to use a common supply for all equipment. Shared power supplies on stage may
experience a reversal of mains supply polarity between equipment resulting in
guitars or microphones being live
·
the earth side
(outer shielded cover) of the input shielded lead must be grounded to the
amplifier chassis to prevent hum pickup, however modern television receivers do
not have an isolating power transformer and use a "hot" chassis
system. This means that if you connect yourself - or someone else - between the
TV chassis and ground there is a high likelihood of death. If connecting audio
signal leads to a TV receiver or similar device ensure there is an isolating
capacitor in series with each lead - ie both live and earthed input leads, to
prevent direct connection between the amplifier and the hot (ungrounded)
source. If you are not qualified to do this then take your system to a
qualified technician - you only have one life!!
·
always use wire
having a cross-sectional area adequate for the current to be carried. To
minimise voltage drop and wire heating effects, it is desirable that Heater
wiring should be arranged so that each heater is wired directly to the source
transformer, rather than sharing a common pair of wires for all tubes.avoid heavy
solid wire when connecting to tube sockets – the socket pins should be able to
flex to accommodate variations in pin alignment
·
always twist Heater
wiring together, to minimise the AC field around the wires.
·
always use 105 deg
C rated wire insulation. This produces a better looking result because the wire
will not melt when soldering the conductor ends.
·
always try to use
common wire colours for discrete circuit functions – red for DC positive and black
for DC negative. Discarded audio equipment and computers are a good source of
insulated coloured wire
·
when using filter
capacitors in series, always bypass with equal value bleed/bypass/voltage
equalising resistors. If resistors are not installed the capacitors will
self-balance, which may result in one being subjected to excessive voltage
leading to failure.
·
where one capacitor
in a series connected pair is above ground – e.g. voltage doubler rectifier system
or a series pair for high voltage B+ supplies – ensure the can is insulated
from ground. If a chassis mounted can type with clamp insert a couple of layers
of PVC insulating tape between the clamp and the can – do not allow the can to
touch ground.
·
always use chassis
mounted can type capacitors for the rectifier/B+ supply for security. Large pigtail
capacitors can vibrate and snap off the fixing leads. If used pigtail caps can
be secured with a cable tie.
·
always use speaker
cables having as high a cross-sectional area of conductor as possible (one Ohm
of resistance in a 4 Ohm system – i.e. 0.5 Ohm in each conductor - produces a
25% loss of voltage and a 25% loss of power at the loudspeaker terminals)
Note:
The above rules are not listed in any particular order.
To be continued
REMEMBER - ALWAYS TAKE CARE WHEN WORKING WITH
HIGH-VOLTAGE - DEATH IS PERMANENT!!
Contact:
"electron"
Email: contact
This page is located at http://www.oestex.com/tubes/guitaramps/9_tipsandtricks.html
This page last modified 15 February 2024