acro

electron's

OPTIMISED ELECTRON STREAM © TECHNOLOGY

for
AUDIO and RF MODULATOR
TRIODE, TETRODE, PENTODE AND BEAM POWER TUBES

This website is dedicated to the protection and preservation of tube audio technology.

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CONSTRUCTION AND WIRING OF
TUBE AMPLIFIERS

This page presents examples of fastidious design, layout, wiring and construction in tube amplifiers.

There are many, many such examples available in the world and here is but a small selection to ponder upon.

If you have one to display please send me a link or details to the HOME PAGE email address set out below

First - "the rules":

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 ultra-sonics, RF, hum and noise, from adjacent circuitry and transformers.

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 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. When soldering components to prevent dry joints (imperfect metallurgical joint bond) 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.
always ground or earth the chassis (to prevent electrocution). Note: 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 electrocution or 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!!