ASK AMP MAN
Tubes for Tweeds
I used to get Musician’s Hotline Magazine
delivered and always headed straight for
the “Ask Budda” column. About 98 percent of the stuff was over my head, but I
soaked it up anyway. I always thought you
handled the explanations well, given the
wide range of audience understanding. I
mean, even I could understand some of it!
So I thought, given your ability to explain,
I’d put a question to you.
My question concerns the
compatibility of different
input tubes. In the late
eighties I was fortunate
enough to be given an old
Fender wide panel Deluxe
(5B3) that has quite a story
behind it. It was surviving in
a barn when the barn owner
offered it (and another
old tweed amp) to me in
exchange for some manual
labor. To make a long story
short, the big amp didn’t
work but the “Deluxe” did
(though it was noisy). So
one day I took it in and had
it fixed up, and I’ve been
using it ever since.
I have acquired a small collection of new and NOS
fifties-era tubes of various
makes for the rectifier and
power section, but all I can
find for the preamp section
are the little metal 6SC7s. I’ve seen glass
6SC7GTY tubes out there and am wondering if they are a direct replacement.
I’m not unhappy with the sound from the
current setup, but I like to try new things
too. A Marshall-owning friend said he
thinks changing preamp tubes can have a
big impact on tone and he likes to experiment. I don’t, however, wish to damage
my little amp with a foolish experiment
myself. I’ve found information online
regarding tube spec sheets, but don’t
know enough about electronics to draw a
conclusion. The spec sheets mention the
low gain of the 6SC7s (GE brand in this
case) and also talk about a higher gain of
the GTs (Tung Sol brand). Can you help me
avoid a costly mistake or allay my fears
about attempting this substitution?
Thanks for your long-term commitment to the
publication (both current and past) and to the
column as well. I’m sorry if some of the explanations were over your head, but let me see if
I can make the answer 100 percent useful.
The metal envelope tubes, such as the 6SC7s
in your amplifier, were the style of tubes
used up until the early fifties. Since your amp
is more than likely circa 1952-53, the tubes
might actually be the originals. Nice find!
It’s the ultimate example of the phrase “all
original.” This amp soon became the 5C3
version and, according to the available information, more than likely transitioned over to
the “newer” glass envelope 6SC7 tubes at
that point. Since both tubes were produced
to the same specs, they are interchangeable.
The “GT” suffix was simply added to indicate
the glass envelope. There are no changes in
operation or pin configuration between the
6SC7 and the 6SC7GTY, and in my experience
the glass tubes are generally less microphonic
than the metal envelope tubes. That’s not
to say, however, that the glass tube versions
that you come across will not be microphonic.
There are plenty of new tubes produced even
today that are microphonic straight from the
factory, but chances are that the glass bottles
will be an improvement over the metal.
Since the 6SC7 is a dual-triode tube, you may
want to also try some “modern day” 9-pin
equivalents. There is a converter manufactured by Groove Tubes called a Substi-Tube
that will plug into the 8-pin
preamp tube sockets on the
amp and give you the ability to
try some of the more common
9-pin preamp tubes available
today. The closest of the most
common tubes today in terms
of amplification factor is the
12AT7, so you may want to start
there. If you’d like more gain,
you can move up to a 12AX7.
Less gain would be a 12AY7.
Remember also that there are
two 6SC7s in the amp. One is
the preamp for the inputs and
the other is the phase inverter
that drives the output tubes.
Different substitutions in each
of those two locations will yield
different results, so try as many
combinations as you wish.
One more consideration: if you
are planning on playing the
amp regularly at high volumes
or with distorted tones, you
might want to consider replacing the original
speaker while it is still in good working order.
Be sure to keep the original, as it will be
an important factor in the value of the amp
should you decide to sell it.
Enjoy your Barn Beauty.
Co-Founder and Senior Design Engineer
firstname.lastname@example.org or www.budda.com
©2008 Jeff Bober