Oestex ©
GUITAR
AMPS PAGE
SECTION 8: CHASSIS AND COMPONENT LAYOUT AND WIRING
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
·
always 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)
·
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.
·
always twist Heater wiring together, to minimise the AC field around the
wires.
·
always use 105 deg C rated wire. 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
·
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.
·
always use speaker cables having as high a cross-sectional area of
conductor as possible (one Ohm of resistance in a 4 Ohm system - ie 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.
.
.
The
following tips for construction and wiring were provided in 1938 by the
Partridge Transformer Company UK.
.
It
seems most of that advice was ignored by commercial designers.
. .
Partridge Transformers Notes on Amplifier
Construction 1938.pdf (click to
view)
.
AN EXAMPLE:
Here
is a 100W tube guitar amplifier similar to a Marshall JCM800 Head.
It
has 4 x EL34/6CA7 power tubes and 4 x 12AX7 preamp tubes.
The
first picture is of the amp “as-built”. The amplifier was unstable and
self-oscillating at an ultrasonic frequency.
Top front view.
Top rear view:
Underchassis view showing
bundle of HV plate leads (red and blue) (white OT B+ centre-tap), and speaker
wiring (other colours) draped over preamp section and input circuitry at rear
of chassis – a recipe for self-oscillation.
Power supply strengthened – an extra 260 uf x 4 pieces – two
at power transformer end and two inside OT shielded box.
The purpose was to reduce hum, improve clarity of sound and
increase dynamic response.
Aluminium channel section shield installed to rear of chassis
and hv wiring installed inside the channel
All sockets on rear panel now shielded. The amplifier is now
stable.
The design of this amplifier follows
longstanding industry practice intended to balance the weight of the head unit when
the carry handle is centrally mounted in the top of the cabinet.
The effect of this choice is to cause
long leads to be installed from the power supply to the output transformer,
usually close to rear panel sockets as in this example.
The effect of this is to expose
effects loop circuitry to power tube, output transformer and speaker circuitry
– effectively placing sensitive inputs next to or touching output wiring and
components.
You will also see long shielded
cables running all the way across the PCB to the right side of the preamp PCB.
In this example, if the output
transformer was mounted adjacent to the power transformer (with 90 deg
orientation of the lamination stacks) the preamp section could be moved to the
opposite end of the chassis by at least 6 inches (152 mm), thereby completely
separating HV sections from preamp sections. A thin sheetmetal shield from
front to back of chassis to shield the preamp section would be beneficial.
The carry handle is then moved to the
centre of gravity near the transformers.
The four extra filter caps could
still be mounted between the transformers and the chassis.
Also the rear PCB could be moved to
the left away from the power supply and output stage section.
STEP 13: SWITCH THE
AMPLIFIER ON AND LISTEN TO THAT SWEET, CLEAR TUBE SOUND!!
These articles, courtesy of Electronics
Australia, June 1983 edition, may cause you to reconsider what you are actually
hearing!!
A NOTE OF INSPIRATION:
For those who want to be different and own
and use tube amplifiers for their hi-fi - notwithstanding their
shortcomings and limitations - good luck to you and enjoy what you have.
If you feel inspired to improve what you
already have then hopefully these pages will have helped in your quest.
There is no restriction or cost imposition
upon the home hobbyist constructor to using these concepts - the only
restriction is on commercial exploitation where copyright is applicable - so if
you do not like it do not do it.
If you want your hi-fi to improve its
performance at minimal cost to you then experiment. The concepts presented here
do work and cost very little to implement.
However to those who say that a product is
only as good as what you pay for it, then these concepts are of no value to
you because they are free. You would be wiser to spend a hundred grand on a
commercial system and feel better. While you are so doing, ask the manufacturer
to justify the circuit design parameters and component choices to you.
Thank you for reading and considering my tube
hi-fi ideas as expressed in these pages.
Please let me know if you can add to this
body of tube hi-fi knowledge and I will add it to this commentary, which is
intended to communicate the results of my personal research and experimentation
over about 50 years of my life.
HAPPY CONSTRUCTING!!
MAY YOUR PROJECT BE A
SUCCESS!!
SAFETY WARNING:
DO NOT attempt to design
and/or construct a vacuum tube audio amplifier unless you suitably skilled,
qualified and/or experienced.
The Author makes no claim
whatsoever as to the validity or accuracy or otherwise of any statement,
information or opinion contained in these pages and no liability will be
accepted for any error or omission of any kind whatsoever.
Proceed only at your own
risk!!
No warranty of any kind
is expressed or implied as to the workability or performance of designs,
concepts or equipment described herein.
Never forget Murphy's Law: IF SOMETHING CAN GO WRONG IT WILL !!!!
·
VACUUM TUBES AND
CIRCUIT COMPONENTS OPERATE AT HIGH VOLTAGES OF BOTH ALTERNATING CURRENT AND
DIRECT CURRENT THAT CAN BE FATAL.
- VACUUM TUBES OPERATE AT HIGH VOLTAGES – UP TO
1,000VDC - AND TEMPERATURES THAT CAN CAUSE PERMANENT INJURY,
DYSFUNCTION OR DISABILITY - TO YOU OR SOMEONE ELSE.
- DO NOT ATTEMPT TO CONSTRUCT, TEST, WORK ON, OR
COME INTO PHYSICAL CONTACT WITH, LIVE ELECTRONIC EQUIPMENT UNLESS YOU ARE
TRAINED OR OTHERWISE QUALIFIED TO DO SO.
- NEVER REMOVE A PROTECTIVE COVER FROM ELECTRONIC
EQUIPMENT UNLESS YOU ARE TRAINED OR OTHERWISE QUALIFIED TO DO SO, BECAUSE
LETHAL MAINS AND HIGHER VOLTAGES ARE PRESENT INSIDE. ANY PERSONAL CONTACT THAT WILL CREATE A
CIRCUIT IS SUFFICIENT FOR ELECTRICITY TO CONDUCT. DEATH IS PERMANENT.
- Extra care should be taken with circuit designs
using the B+ voltages listed in this guide.
- Safety is of extreme importance not only to
yourself the constructor, but also to other persons who may come into
contact with your creation - particularly little children.
- Live wiring should be fully insulated to
prevent electrocution of yourself and/or users.
- Tubes for use at B+ voltages over 450v DC
require sockets made from materials that can withstand high voltages.
Inferior materials may cause flashover or arcing, leading to burning,
short-circuits and/or tube damage.
- Many tube types require a top cap plate
connection and such tubes must be carefully mounted to minimise wire
length, proximity to other components and risk of accidental contact.
Insulated caps are recommended. Do not solder directly to tube caps. Do
not touch cap leads or terminals.
·
Most vacuum tubes
are glass encased devices that have been evacuated of air – i.e. they
contain a vacuum atmosphere. If dropped or impacted they are likely to implode,
shattering glass in any direction. Handle with care and respect. Do not drop or
impact. Always pack and store in padded wrapping or the manufacturer's carton.
·
When inserting
into a tube into its socket always take care to align all the tube pins
correctly with the matching socket. Incorrectly aligned tubes may result in high
voltages being applied to low voltage circuit components or systems, and may
result in short-circuit fault currents being applied with resultant expensive
damage to tubes and equipment.
·
DO NOT grasp a hot vacuum tube –
particularly power tube and rectifiers
·
To avoid serious
and permanent tube damage when constructing an amplifier, always ensure the
grid bias voltage is present at grids 1, 2 and 3 before applying B+ high
voltages to the circuit. In this regard, cathode bias and back bias configurations
are safer and more reliable designs than fixed bias.
·
Unloaded
electrolytic capacitors can hold their full charge for a week or more. Always
ensure large HV electrolytic capacitors are fitted with suitable bleeder
resistors to automatically discharge them after the amplifier is switched off -
otherwise you may expose yourself to lethal current if you accidentally touch a
live conductor. Persons familiar with
assembling and servicing personal computers and suchlike - all of which feature
fully insulated construction - must appreciate that tube amplifiers and
ancillary equipment operate at high-voltages and, unlike computers, have
unprotected open construction that is exposed to the touch - particularly
components connected to the AC mains supply.
·
Amplifiers that
incorporate a “standby” switch still have full mains voltage and high voltages
present in the system
·
ALWAYS ensure
your amplifier and guitar are grounded via the mains earth. Making contact with
another amplifier or mains connected device that are separately grounded, such
as microphone can result in electric shock.
Now that you have been suitably warned, let us proceed together to explore the
world of vacuum tube audio.
. © NOTICE: INTELLECTUAL PROPERTY COPYRIGHT © D.R.GRIMWOOD 2002 -
ALL RIGHTS RESERVED.
Copyright in all quoted works
remains with their original owner, author and publisher, as applicable.
Please note that no warranty is
expressed or implied - see footnote notice.
Intellectual property in the
original applied engineering concepts expressed in this paper remains
exclusively with their author Dennis R. Grimwood.
The whole or part thereof of
this paper and/or the designs and design concepts expressed therein may be
reproduced for personal use - but not for commercial gain or reward without the
express written permission of the author and copyright owner.
All rights reserved.
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/8_gawiring.html
This page last modified 12 July 2023