Amp Man,
For starters, let me say your column is the first piece I read
when
PG comes in the mail each month. It’s very informative
and easy to understand.
My question is about my Carvin Nomad, which I love
(even the Soak Channel, which some people knock). The only
thing that doesn’t totally please me is that when I dime the
amp to get some nice power-tube overdrive, the sound gets
a little spiky instead of turning big and round. I assume this
is because of the EL84s, and I’m wondering if larger power
tubes would solve my problem. I’ve done some reading on
tube amps and realize that this isn’t simply a matter of replacing
one type of power tube with another. The tube sockets are
different and I may need to upgrade the power transformer
and output transformer. Am I heading in the right direction?
If so, could I get my 50 watts from two 6L6s with my current
transformers? I’m also not sure if the amp’s wiring would
allow me to install octal sockets in place of the 9-pin EL84
sockets. I work in a sheet-metal fabrication shop, so I could
do any required changes to the chassis, and I’m confident I
could do the wiring too, while being safe. Any ideas?
—Nick
Houston, Texas
Hi Nick,
Thanks for reading—and for
writing. I applaud the fact that
you’re looking to improve the
performance of your Nomad, but
I’m not sure you realize what an
undertaking this would be. I’ll
discuss this and also offer a couple
of options that would be easier
to execute while still getting you
closer to where you need to be.
Let’s start with your question
about possible alternative output
tubes. Switching from the stock
EL84s to some other output tube
would certainly yield a sonically
different output stage, but I’m
not 100-percent sure it would
remedy your issue. That said,
if I were to suggest a different
tube type, I’d say the tube with
the closest match to the primary
impedance of the existing output
transformer would be a 6V6. The
problem here is that, as you said,
you would need to switch from
the 9-pin sockets to octal sockets.
On the surface, you being a sheet
metal guy, it may not be a problem
for you to punch larger holes
in the chassis and install the octal
sockets. The wrench in the works
here is that the 9-pin sockets are
mounted on a circuit board, and
you would need to start from
scratch installing all the appropriate
wiring and components necessary
for the new sockets and tubes.
Additionally, you would need to
modify the bias supply to get the
increased voltage required to bias
the 6V6s. That’s not terribly difficult
for a seasoned tech, but definitely
something to consider.
You mentioned 6L6s as an
alternative tube type. While the
primary impedance of the output
transformer in most amps
with four EL84s would be close
enough to work, it would not be
as close of a match as four 6V6s.
You also mentioned changing
transformers. The mains (power)
transformer should be fine, and
we’ll address the possibility of
replacing the output transformer
in a moment. In order to alleviate
all the chassis cutting and
rewiring required for the above
changes, let me suggest three
simpler (and cheaper) alternatives
you can try first.
1) Have the bias of the
output tubes checked and set.
[
Warning: This requires working
around high voltages, so the procedure
is not for a novice.] Carvin
recommends setting the bias for
a reading of 85 mA across the
Standby switch (in the off position).
This seems a bit low for
four EL84s, though. I’d suggest
setting it up closer to the 100
mA range. This will warm up the
output tubes a bit. To my ears,
EL84s always sound better when
they’re running a bit warm.
2) Experiment with different
brands of output tubes. Each
make has very different tonal and
distortion characteristics, so try
some Sovteks, some JJs—even
some new-old-stock tubes, if
you have a little extra money to
spend—or anything else you can
find. They will definitely make a
difference. Personally, I’ve always
liked standard Sovtek EL84s in
the amps I build. Always use a
matched set, though, and remember
you will need to reset the bias
when you change the tubes.
3) Install an upgraded
output transformer. An amp’s
output transformer makes a big
difference in the sound. The stock
transformer may do an adequate
job, but at higher output levels—
like those needed to push
the output tubes to the point of
global meltdown and fantastic
tone—the transformer may be
saturating. Replacing it with a
better transformer may make a
very noticeable difference.
If you choose option number
3, search around and see what
you can find that’s recommended
for using with four EL84s. A
Vox AC30-type transformer will
work, but you will be limited to
only 16 Ω and 8 Ω output taps
and you’ll need to cut a substantial
hole in the chassis, because
the transformer is a lay-down
type. You may also have limited
chassis real estate due to the size
of the circuit boards in the amp.
If you can’t find an appropriate
stand-up type transformer,
contact me and we’ll get you into
one of the transformers I use in
my 30-watt amps. That should
work well for your project.
There you have it. Hopefully
that’s all you’ll need to bring that
Nomad home!
Jeff Bober is one of
the godfathers of the
low-wattage amp revolution,
co-founded and was
the principal designer for
Budda Amplification. Jeff recently launched EAST
Amplification, and he can be reached at
pgampman@gmail.com.