TECHVIEWS
The Immortal Amplifier Mod
A well-made tube amp can last a long
time, but there are a few things that can
make them uneconomical to fix, as well as
unreliable. If you were to make these weak
links stronger or easier to fix, the amplifier
would then last a long time – maybe even
longer than you, effectively making the
amplifier immortal.
(opposite) end of the diode, then slip a length
of heat shrink tubing over each transformer
lead, soldering each transformer lead to the
loop in one diode. When the solder cools, slip
the heat shrink down over the diode and heat
it. Don’t leave the wire leads uncovered – this
is important for safety!
diodes now take over the load. In fact, the
amp simply keeps playing if the rectifier tube
shorts. The high voltage supply actually rises
by maybe 50V, so the amp gets a touch louder. You can keep playing until the show is over
and you have time to put in a new rectifier.
Back up your tube rectifier so
it “soft fails”. Rectifier tubes fail, like all
tubes do. They usually fail by not conducting current at all, instead giving complete
silence, which is no big problem. As a smart
and well-prepared musician, you can just pop
in your spare
rectifier tube,
making good
use of the tube
sockets.
This is not an ideal mounting arrangement for
the diodes, as they are supporting the transformer leads. To take some strain off the diode
leads, use nylon cable ties to keep the transformer leads together, or, better yet, secure
them to nearby parts of the amp to keep the
leads from stressing the diodes mechanically.
You might ask, “Why are we doing this?” The
“immortal amplifier” idea originated in the
poem, “The Wonderful One-Hoss Shay” by
Oliver Wendell Holmes, and was furthered
along by stories of how Rolls-Royce made its
notoriously reliable cars. The basis of both of
these is that in any machine there are weak
points – places which will break first.
But sometimes
when the
rectifier tube
fails, it does
not open.
Sometimes
it shorts and
dumps the full,
high voltage AC power into the filter caps.
Polarized filter caps cannot stand AC, so
they usually fail shorted. Now there is a solid
short on the power transformer high voltage winding. The AC power line fuse might
blow in time to save your power transformer
from burning out, but it also might not, as
it’s deliberately made a time-delay (“slo-blo”)
type to allow the amp to get started. And
your power transformer is the single most
expensive part in your amp.
Immortal Amplifier Mod #1 - Rectifier Tube Fail Soft
Note that this mod requires working with
the highest voltages in the amplifier. If you
decide that you want to do this yourself, be
absolutely certain that you already know how
to do it safely before you start – unplug the
amp, let it sit for an hour, etc. If you have
any doubts, take it to a tech who knows how.
Amps can be valuable, but they’re not worth
dying for.
In the poem
by Holmes, the
horse-drawn
carriage was
constructed so
that every part
was as strong as
the rest, so it all
failed at once
– like a bubble
bursting – after
100 years. In
the case of Rolls
Royce, they did
abusive testing on entire cars, found what
parts broke, then made those parts stronger
so they did not break.
Tube amplifiers are already pretty reliable.
If we were to find the few weak points and
add some reinforcements there, they could
last even longer, suitable for leaving to your
grandkids. The fact that they don’t fail you
on stage or are cheap to repair when they do
manage to fail is icing on the cake.
Fortunately, the fix is simple: add – or have
your tech add – two solid-state diodes, one
in series with each anode of the rectifier
tube. The diodes must be rated for more
than twice the B+ voltage of the amp and
at least the full current. One-amp (1A) rated
diodes are sufficient for nearly all tube amps.
In most amps, the power transformer leads go
directly to the rectifier tube socket. The simplest way to wire in the diodes is to unsolder
the wire from the rectifier tube socket, then
solder the cathode (bar) end of the diode to
the rectifier tube. Bend a loop in the anode
The diodes have to be able to withstand at
least twice the B+ voltage in the amplifier.
For amps up to about 450V in normal operation, you can use the cheap and widely available 1N4007, which is rated for 1000V and 1
amp. For amps with B+ supplies over 450V
up to about 550V, use the ST Semiconductor
STTH112U, which is rated for 1A and 1200V,
available from Mouser Electronics.
After the mod, the B+ will be 0.7V smaller
– an insignificant amount. If the rectifier tube
just wears out and quits conducting, the amp
still quits working; that’s what replacement
tubes are for. But if it shorts, the solid-state
This mod is intended to strengthen a weak
spot in tube amps. After doing these mods
to my amps, they may not really be immortal,
but they should outlast me!
R.G. Keen
Chief Engineer
Visual Sound
visualsound.net
