Reduce AC Voltages
From kee–(at)–den.-x-this-out-.com Tue Apr 22 12:56:02 CDT 1997
From: kee–(at)–den.-x-this-out-.com (R.G. Keen)
Subject: “Vintage” AC Supply
Date: Tue, 22 Apr 1997 14:05:03 GMT
Xref: geraldo.cc.utexas.edu alt.guitar.amps:46531
Your vintage guitar amplifier is almost certainly not running on the
proper AC line voltage. Most of the older amps in use were designed at
a time when the AC power out of the socket was lower. 115VAC is a
common rating on power rating plates, some older ones may carry 110 or
112VAC ratings. Over the years, the nominal AC voltage rating has
crept up; 115 was “standard” for a while, then came 117, 120. Now many
cities have AC that is most commonly 125 or so. We certainly do here
This means that amps designed for 112VAC are runing at 10% high AC
voltages all the time. AC voltage can run even higher at times when
the average load on the power net drops off – like in the early
evening in spring and fall when all those air conditioners cycle off.
Obviously, most amps will survive life with higher input voltages –
they have and do – but it’s not necessarily good for them, either
>from the standpoint of repairs costs or tone.
The steady overvoltage can cause your power filter caps to wear out
sooner, make tubes die sooner from the higher filament voltage, in
general cook things from the extra heat the parts will dissipate and
possibly shift the tonal balance in the amp with the higher B+, as
well as changing the tone if the amp has been rebiased in a way that
unwittingly compensates for the higher B+.
Variacs are one way to correct this, but these things are huge, heavy,
and carry the temptation to twiddle the voltage just a – little – bit
more to see what it would sound like.
A handier and less potentially deleterious way to do this for your amp
is to put together an AC line voltage corrector. It’s not a panacea
for bad tone, but it may offer you one more way to get your sound the
way you like it in a quick, easy to set up way.
To do this, look at your amp’s power rating plate, usually on the back
of the chassis. This will tell you the full-power amperes that the amp
is expected to draw. Find a low voltage transformer that is rated at
at least that many amperes and a voltage that is the difference
between the nominal AC power voltage in your town and the rated
voltage on your amp, or at least close.
For example, if your wall sockets put out 125VAC, and your amp is
rated at 115VAC, 3 Amperes, pick a 10VAC, 3Ampere transformer
(8VAC 3Amperes would probably get you close enough, and is common).
Wire this transformer up so that the AC line connects to the primary,
and the secondary is connected with one end to the end of the primary
that is attached to the “hot” side of the AC line. The output of the
thing is taken from the “cold” side of the AC line and the free end of
the secondary. This makes an autotransformer that can add or subtract
the secondary voltage from the AC power line, depending on which way
the secondary is connected to the primary.
This corrects the AC power to your amp back to about what it was
designed to use.
Crude ASCII schematic:
“Hot” line –o/o—–o—–o
) || |
Primary ) || ( Secondary
120VAC ) || ( 5, 6, 8, 10 or 12 VAC
) || |
| o———|| AC Socket
“Neutral” line——–o—————-|| for amplifier
Safety ground ———-0———–== line cord
There are two ways to connect the secondary. One way, and the voltage
between the free end of the secondary and the “neutral” side of the AC
line will be the primary line voltage plus the transformer secondary
voltage. The other way round, the voltage will be the primary line
voltage minus the transformer secondary voltage, which is what you
want. Most of the wiring can be done with wire nuts, as in house
wiring. If you get a fuse holder and AC socket with pigtails attached,
it can all be done with wire nuts.
A suitable transformer can be bought at Mouser Electronics for $15 or
less. To this you will have to add the cost of a metal housing to put
it in, fuse holder, line cord, and AC socket, bringing the total up to
If you decide to make one of these, be very careful about the safety
aspects. You will be working with AC line voltages, which can kill
you. Do not try this unless you already know how to do primary AC
wiring safely. This note does not tell you how to do that. Do not
skimp on the housing, line cord, strain relief for the line cord, fuse
holder, or AC socket, and be certain that safety ground is connected
to the housing and to the secondary AC socket. Note that ALL of the
wiring in this power correction box are connected to the AC primary
power, and are all hazardous.
If you do this, be careful. If you don’t have any idea what the proper
precautions are but still want one, have a tech wire one up for you.
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