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.