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Thursday, February 7, 2013

The Most Important Amplifier
In the World: The Dynaco 70

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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Labels: 7199, A470, Dynaco 70, Dynakit 70, el34, history, overview, power supply, problems, replacement, review, transformers, tube amplifier, tubed, tubes, what's wrong

1 comment:

Rick said...

Enjoyable read!

Don't forget about Sherwood too in your list of tube
gear manufacturers.

February 13, 2013 at 1:59 PM

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