The Dynaco 70 is the most important tube amplifier ever
made. Sure, some may quibble at that
characterization, but I can’t think of any other amplifier – at least within
North America – that has turned on more audiophiles to the possibilities of
tube amplification. Like countless
others, it was the amplifier that turned me on to an alternative way of
listening to music.
Back in the late 1950s, the stereo wars were heating
up. Many audiophiles had invested in the
expensive monoblocks or stereo amplifiers from Marantz, McIntosh, and
Fisher. The smaller players – Heath,
Dynaco, EICO, and Pilot were nipping on the heels of their more upscale
brethren. David Hafler made the right
move at the right time by introducing a $99 stereo wonder – the Dynaco 70.
Sure, this little budget amplifier couldn’t compete with the
big boys at a performance level, but it was such a bargain that no one seemed
to mind. The sales numbers certainly
back this up. Yes, the power transformer
was woefully underrated for supplying the current to four EL34s, and yes, the
7199 tube is not an ideal driver/phase-splitter, but whatever failings the
Dynaco 70 has is ameliorated by the warm forgiving sound that throws a big wide
soundstage. This is an amplifier made
for playing music, and if paired with good tubes, the performance is damn well
good enough to have made me a tube convert for life.
I bought my first Dynaco 70 at the tender age of
nineteen. I was heading off to college
and needed a stereo system. At the time
I didn’t know anything about tube amplifiers but came enamored with them after
hearing a McIntosh 240 over at a friend’s house. I loved the black transformer and chrome
chassis look and lusted after the glowing tubes. When he asked if I wanted a tube amp, I
readily agreed. However, I will admit
that I had to hide my disappointment when he brought out the brown caged Dynaco
70. It certainly couldn’t compare to the
McIntosh in the looks department. But
little did I know how this little tube amplifier would change my life.
The amplifier started stock with Siemens EL34s, a Mullard
5AR4, and RCA 7199s. After a few weeks
of that, the same friend suggested running an outboard tubed regulator to
supply the front end. With the shortage
of 7199 tubes – this was before the Internet, mind you – I converted the PCB,
through the use of a RCA manual and my first soldering iron, to use
6GH8As. After that, it was modified to
run the EL34s in triode. After that, it
was sadly sold to another friend who began his own audio journey with
tubes. At that point I was running
modified Scott or EICO integrated amps, a small step before my DIY journey.
A few years ago, I exchanged emails with the fellow who
bought the Dynaco 70. He still had
it! We did a swap and I had my very
first amplifier back in my possession.
The original circuit board was long gone, the tube sockets were shot,
and the can cap had been replaced by a giant oil power supply capacitor. This old amplifier was due for a rebuild.
In order to stay true with the spirit of the Dynaco house
sound, my restoration was fairly limited in scope. The input circuitry was replaced with a PCB
from Triode Electronics that utilizes a pair of EF86 tubes and a single
12AU7. The power supply can-cap was
replaced with a SDS circuit board that fitted underneath one of the output
transformers. A bit of wire and solder,
some new output tube sockets, and the amplifier was ready to sing again.
For tube selection, I went with some budget Russian valves –
EH EL34s, Sovtek EF86s and a 5AR4. The
sole American tube was a late production Phillips 12AU7. The sound, as to be expected, was a little on
the harsh side. This was due to the
“solid-state in a bottle sound” that afflicts many of the lower-end Russian
tubes. So it was time for a spate of
rolling. In the end, the amplifier
finished with Matsushita EL34s, Dutch EF86s, a Hitachi 5AR4, and a 1950s-era
RCA blackplate 12AU7. This change made
for a much smoother and cohesive amplifier that was quite enjoyable to listen
to.
But, I’m not the sort of chap who wears rose-colored
glasses. The Dynaco, even with an
improved power supply and new input circuit, suffers from several flaws that
stop it from being a truly great amplifier.
First of all, the
power transformer is helpless underrated in the current department which will
cause the voltage to sag at higher output powers. Another issue is the use of a single 5AR4
rectifier to handle all four output tubes.
Sure, it’s within the specification of that tube, but still it is
hitting the top of the envelope. Of
course these two problems can be rectified (ha! – editor) by an aftermarket
power transformer and a different rectifier or even the use of solid-state
diodes (which have their own issues).
However, even with those changes, I believe the fatal flaw
of the Dynaco 70 rests with the A-470 output transformers. Like many things Dynaco – especially on their
budget gear – this output transformer was built to a price point. At higher output levels, the amplifier sounds
like the core is saturating, giving that pleasant “tube-like” compression. It sounds like the images are on springs,
causing the soundstage to shrink in size as large dynamic swings occur. Why do I pin the blame on the output
transformers? I’ve heard the same effect
on the Dynaco Mark IV, the monoblock version of the Dynaco 70 that have a much
beefier supply that isn’t shared between two channels.
So yes, the Dynaco 70 – and all the new kit versions now
available – is still a great introduction to the world of tubes, but it isn’t
the end game. There are much better -
and admittedly more expensive – amplifiers that really lift the veil over the
music and provide the sort of output that an audiophile wants to hear.
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