TECHVIEWS
The Immortal Amp Mods – Pt. 2
The first Immortal Amp Mod addressed a the safe side. And always remember to wait
weakness in the tubes. Our second install- a few minutes after unplugging your amp
ment will help prevent another way tubes before opening and tinkering with it!
can damage the amp.
Protect your transformers with fuses
I’ve heard this one a lot: “My amp has a
power fuse, but why doesn’t it keep my
power transformer from burning out when
my tubes short?”
The costs for this mod are mostly labor.
The present cost of a fuse holder is about a
dollar per fuse position. Since fuses fail so
infrequently, you might also consider using
leaded “pigtail” style fuses soldered in series
with the wires you put them in. Then cover
the fuse with heat shrink tubing.
For the primary winding fuses and the B+
fuse, it gets trickier, and some trial and error
may be needed. Start with 1A slow-blow
fuses and increase the fuse rating to the next
biggest size if playing at full power for a long
time makes the fuse open.
The short answer is – it can’t account for
something that was never an intended purpose. The AC power line fuse is intended to
keep it from starting fires. If it happens to
protect something inside your
amp, great, but that’s not its
calculated purpose.
To pick fuse values, add up the heater currents of your tubes and pick a slow-blow
fuse rated the next size larger than the total
Using several fuses can cause nuisance failures until you get the right values. So you
have to experiment when you know there are
no problems in your amp to protect against.
You may have to play your guitar for long
periods of time at full power to do this, but
what’s bad about that?
What you need is one fuse
per winding which sources
power. The drawing illustrates
a good protection setup for
your transformers. F1 and
F4 prevent shorts in a tube
filament from taking out your
power transformer. F2 and F3
are a bit of belt-and-suspend-ers protection if you have the rectifier diodes
from our first installment of the Immortal
Amp Mods [July ‘08], but they provide a lot
of cheap security; the incremental cost is
very small.
current. For example, a Fender ‘ 65 Deluxe
Reissue has inside it four 12AX7, two 12AT7,
one 5U4 and two 6V6 tubes (see The Fender
Field Guide, ampwares.com/ffg/). I looked up
the heater currents for each of these tubes in
one of the many web pages with tube data
( mif.pg.gda.pl/homepages/frank/index.html)
and found that the 12AX7 and 12AT7 each
are rated at 0.3A, the 6V6GT power tubes
need 0.45A each and the 5U4 requires 3.0A.
Why one fuse isn’t enough
The AC power fuse must let
current through to start the
amplifier; this can be three
or four times the maximum
power needed during regular
operation. That’s why they
must be “slow-blow” type
fuses. The delay allows the big
startup surge to pass and the
amplifier to start. A massive
internal short will pull high
currents for about a second or
two; then the AC fuse will open and prevent
the amp from starting a fire.
Immortal Amplifier Mod #2 - Fuses
Your output transformer is already pretty well
protected from shorts by the B+ fuse (those
two windings are not sourcing power, they
are accepting power, so you only need to
use one fuse) and the power limiting nature
of the output tubes. You may ponder this
predicament: What if my amp does not have
a B+ fuse? Well, put one in. The only tricky
part is setting the fuse values.
The total current for F4 will be four times the
0.3A for the 12AX7 ( 1.2A), two times 0.3A
for the 12AT7 (0.6A) and two times 0.45A for
the 6V6s (0.9A). The grand total of these currents is 2.7A ( 1. 2 + 0.6 + 0.9 = 2.7A). Both
3.0A and 3.15A are standard values for slow-blow fuses. The F1 rectifier fuse probably
needs a 3.15A rating, but the F2 could either
be 3.0A or 3.15A.
It is too much to depend on the same fuse
to protect against high-power AC faults
in the AC line, low-current/high-voltage
DC problems on the B+ winding and high-current/low-voltage faults on the heater
windings. The basic rule is: don’t count on
a fuse to decide which of these areas to
protect. And put a fuse where it can stop
one over-current.
The secondary of the output transformer is
likewise protected by the current-limiting
nature of the power tubes. So these four
fuses are about it.
Tubes and transformers will last long enough
for a fuse to open, as long as the fuse does
not have to meet conflicting protection
demands. You can’t merely count on the AC
power fuse to protect your amp, and what’s
immortality if you can’t count on it?
Warning! Just like our first mod, these mods
are in the highest voltages within the amplifier. If you want to do this yourself, you must
be absolutely certain before starting that you
can safely complete this repair. If you have
any doubts at all, take it to a tech to be on
The output tube type makes a big difference. People often swap for 6L6, 5881 and
EL34 tubes. The 6L6GC and 5881 tubes use
0.9A of heater current per tube, which is
twice as much as the 6V6, while EL34s use
1.5A per tube.
R.G. Keen
Chief Engineer
Visual Sound
visualsound.net
