Tube Replacement - DYNACO Stereo 80 Owner's Manual

Tube Replacement

CAUTION: Before replacing tubes on your Stereo 80 ensure the AC power cord is un- plugged from
its AC (wall) outlet (or that the AC power cord is unplugged from the back of the unit) and
wait for three minutes to allow the high voltage power supply to discharge. Gloves may be
worn when changing tubes. Never touch live circuit elements in vacuum tube equipment
because lethal voltages are present when these are turned on.
Note: There are no user serviceable parts inside the Stereo 80 chassis. If a problem arises other than
normal periodic tube replacement, refer to your dealer or a qualified technician.
The most common type of problem in tube equipment is excessive noise caused by a worn or faulty tube.
The characteristics of this noise can vary a great deal, such as: "excessive hiss," "sputtering," "rustling
noises," etc. If a noise problem occurs with your Stereo 80, 99% of the time it will be due to a faulty
tube. Vacuum tube equipment owners become familiar with tube noises and usually become less alarmed
by them as time goes on.
Microphonics (various "pinging" or "howling" sounds) is due to a tube being overly sensitive to physical
vibrations. To avoid this, do not operate vacuum tube equipment on top of vibrating surfaces (such as
loudspeakers). But, if a tube is so sensitive that just having the amplifier turned on causes the
microphonic sounds, the tube will have to be replaced. It can happen that a tube becomes microphonic
with age or that it is only microphonic before it warms up (or only after it warms up).
Because it is possible for oxidation to build up on vacuum tube pins, it is a good idea to remove and clean
them with a small (brass bristle) wire brush once a year. At this time it is also recommended to clean the
tube pins with contact cleaner. Contact cleaner leaves a thin coating on the pins which helps prevent
oxidation. The contact cleaner can either be sprayed directly onto the tube pins (don't get any on the
glass part of the tube) or can be sprayed on a clean cloth which is then used to wipe the tube pins.
Notes: When removing a tube, grasp the tube socket with one hand and the tube with the other hand.
Move the tube (slightly) from side to side while pulling upward.
An easy way to determine if a tube in any position is causing undesirable
noise is to swap the suspected tube with its counterpart in the opposite channel.
If the noise jumps to the other channel, the tube you suspected is indeed faulty.
The most certain method for confirming a faulty tube is by substitution of a known
good tube.
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