The Immortal Amp Mods, Pt. 3
Any time you combine electricity and coils of
wire, you have the potential to cause sharp-toothed electrical spikes that can puncture
insulation and cause shorts. Fortunately, we
have the ability to stop them cold.
conduct very well until the voltage across the
disc gets high enough, then it suddenly snaps
into conduction and can eat huge amounts of
electrical power. When it conducts, it eats the
excess voltage until things get back down to
normal, then it turns off.
conditions. However, in this position in the
amp, what it kills most often is itself.
Immortality Mod #3: Stop transient spikes
Our AC power lines get more polluted every
day with the leftovers of all the electronics
and mechanical equip-
ment that run on them.
There’s also the infre-
quent but real danger
of a nearby lightning
strike on the AC power
lines putting a big spike
into your amp.
For AC power line protection, use MOVs
rated for more than the line voltage. In the
You get output transformer spikes when
current in the transformer is suddenly inter-
rupted. It is unusual for the tubes themselves
to be able to turn the transformer off fast
enough to cause spikes, even in failing,
but a wire breaking or a speaker suddenly
unplugged can do it.
Here’s a quick cure to
make this a non-prob-lem in your amp. Buy
three “MOV” varistors
for around $0.50 each
and install one from
each AC power line to
the safety ground wire.
Put the third one between the
two AC power wires. Even if you
pay a tech to do this for you,
the cost is almost entirely for
the tech’s time to do it correctly
Some amps have a
string of diodes from
signal ground to the
plates of the output
tubes for just this reason. It’s a clever idea,
but it suffers from relying on the opposite side
of the output transformer to do the protection.
The leakage between
the two primary sections
is one of the places that
can generate spikes.
Warning! These mods are in the
highest voltages in the amplifier.
If you want to do this yourself,
you must be absolutely certain
before you start that you already
know how to do it safely. If you
have any doubts at all, take it to a tech that
can do it safely. Also, always unplug your amp
for at least several minutes before opening it!
If we place a MOV from one output tube plate to the other, across
the output transformer primary,
the problem is solved directly. One
MOV between each primary lead
and the center tap of the output
transformer clamps any transient
that is larger than B+ and eats the
excess energy. In normal operation, the MOV cannot conduct, so
normal operation is not changed.
This is what is inside most surge protection
power strips. Of course you could always
use a protected power strip, but you have to
remember it every time.
U.S., the nominal AC voltage is 120V, but I
often see it between 125 and 130VAC. There
are 130VAC-rated MOVs, but there is not
enough margin between the 130VAC rating
and the close-to-130VAC line – use MOVs
rated for at least 150VAC. As an example, at
the time of this writing, Mouser Electronics
lists the Littelfuse 14V150 MOV as 150VAC,
200VDC, and capable of clamping a peak current of 6000 amperes! It’s a tough little device
and it sells for $0.35 as of deadline.
The skeptic would ask me, “Well,
my vintage Fender-Marshall-Gibson is over
40 years old and still running. It doesn’t have
all that fancy stuff. How come we need that
now?” My answer is that there would not be
companies making replacement power and
output transformers if they were not needed;
having been lucky before does not guarantee
you’ll always be lucky.
Electrical spikes are so common that there
are special parts made to protect against
them. The simplest of these is the metal oxide
varistor, or MOV for short. A MOV is a chunk
of powdered metal oxides pressed together
and semi-melted into a solid disc, with leads
on each side of the disk. The oxides don’t
There’s another source of spikes in your amplifier – the output transformer. It’s an electrical
coil of wire, too. It can make insulation-punc-turing spikes very effectively under certain