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.

Project: The Command 1625 tube amplifier

Introduction:
In the world of single-ended amplifiers, the venerable triode still gets preference.  It's easy to see why since the 300B, 2A3, 845, and 211 - among many others - has a certain catchet and a sonic signature that is honestly very hard to beat.  The triode is quite the linear device with the downside of needing a bunch of voltage and current to make relatively low power.

The other option when running single-ended, minus the odd solid-state design, is to use a pentode/beam-tetrode in ultralinear or native operation.  This not only gives plenty more power, but these tubes also have the benefit of being inexpensive to buy.  Based on my experience with various SE amplifiers, most of the magic of single-ended is the circuit simplicity - no phase-splitter - and the Class A output stage.  Now this design I'm about to introduce was not meant to be an assault on the very best amplifiers, but instead offer a budget alternative to the sometimes stale and now very mundane 300B amplifier.  It's also an attempt to replicate 1930/1940s technology' but with modern components.  Much of the 'sonic footprint' of those pre-war years was not from inferior amplification, but the bandwidth limited source material. 

Of course running in pentode mode requires some negative feedback - a new fangled concept back then - in order to reduce the output impedance to something that modern speaker can use.  An alternative would be to experiment with Schade feedback - taking voltage from the plate of the output tube and tying it to the plate of the input tube. 

Tubes!
The first decision was tube choice.  If I wanted to be just a cheapskate, there are plenty of TV tubes that could be used.  Instead I went with a vintage classic that is still widely available - the 1625 output and 12J7 driver tube.  (Note that the 807 and 6J7 can be used - just don't use the voltage doubler for the filament supply.)  Expect to pay $5-$10 per tube, which is a bargain considering the quality of these vintage beam tetrodes.  Also note that the 1625 has a low screen voltage maximum, so running ultralinear isn't an option unless one can live with reduced voltage and the resulting lower power output.

The UX7 base top-plate cap 1625 is part of the famed 807/6L6GC family and was used in WW2 to provide radio communications - the AN/ARC-5 Command transmitter - for the majority of the planes in the United States Army and Navy.  This tube was made to military specifications with wonderful assembly and a high quality control - when it came to radio pilots lives were on the line. Since there was a war on, scads of them were made and every example I've seen is an impressive piece of engineering.  In tribute, I have given this project the name 'The Command 1625'.

The grid-cap 12J7 pentode is the 12V filament version of the 6J7 tube which is the octal version of RCA's earlier 6C6 tube, which in turn was their answer to Western Electric's famed 310A.  These tubes take the signal voltage and bring it through the cap on top of the tube - a nice way to keep the fragile signal away from noisy power supplies.  The 6J7 eventually begat the 6SJ7 which eventually became the smaller EF40/6AU6/EF86 types.  The 12J7 was selected for the fact that NOS versions are still cheap and plentiful in metal or glass versions.

(right click to open and see larger version)

A quick look at the signal schematic shows nothing too exciting, though using a pentode on the front end is considered a heresy these days of current sourced triodes and LED biasing schemes.  My idea was to keep this design as simple as possible - in the spirit of the vintage tubes selected - with minimal parts count and matching required.  Eagle-eyed reader will note that this same type of front end was used on my EL156 amplifier.  It's a good sounding front end so it was easy decision to use it again.

The feedback resistor for a 4-ohm output transformer is 6.8K.  For an 8-ohm secondary, increase the value to 8.2K.

(right click to open and see larger version)

For the power supply I stuck with tube rectification which has less switching noise than solid-state and better visual aesthetics.  The 5Z3 - a precursor to the 5U4G - was selected on looks and the decent prices, plus the ability to easily handle the current requirements for two output tubes.  Plenty of other options are available here.  Two Triad chokes and a smattering of generic capacitors provide the raw filtering for the output tube plate.  Of interest is the regulation for the screens of the 1625 and the input stage.  This stiff front end gives more control and separates it from the wranglings of the output stage.  For easy implementation, shunt regulation is accomplished with simple VR tubes: the OD3 and OC3 are put in series for 255 volts.  The small bypass cap here is just to clean up noise - going for a larger value will cause instability with these types of regulators.

The voltage doubler circuit is used to take the 6.3VAC filament tap and boost it up to ~12VDC required for the 1625 and 12J7 tubes.  Note, do not ground the center-tap.  If one wishes to use 807s, 6L6GCs, 5881s, or KT66s along with a a 6J7/6SJ7/EF86 input tube, the voltage doubler can be skipped.  In this case, do ground or lift the center tap of the 6.3VCT winding.

Parts:
Some of the passive-parts decisions were based on what I already had in stock.  For example, the PEC volume control, Wima coupling caps, and power supply electrolytics were chosen because I just happened to have them in my stash.  Builders can select their own goodies, provided they meet the electrical requirements of the circuit.  Other than that, I went with Kiwame and carbon-composition resistors, Nichicon cathode bypass capacitors, and a few Solens.

For the the output transformers I went with the Edcor CXSE model.  Alternatives are James, One Electron, or even Hammond.   For those on a tight budget, there are cheaper alternatives, but these will limit bass and treble response, not to mention that the core will be taxed more heavily.  I've found that overkill output transformers lead to a more relaxed, transparent sound.

Top aluminum panel is from the steadfast Front Panel Express.  Size was selected since I already had a leftover wooden chassis from an older project.  Layout was done to minimize power and signal circuitry interaction, while also giving a vintage "juke box" aesthetic.

Potential modifications:
Different Output tubes: With socket changes and 6.3VAC from the transformer instead of the voltage-doubler filament circuit, the 807 or any of the 6L6GC family can be used - 5881, KT66, 350B, etc.

Different input tubes: With wiring changes: the 12SJ7.  With filament and/or socket and wiring changes: 6C6, WE310A, 6J7, EF86, EF80. 6AU6, EF37, etc

Different rectifiers: 5U4/G/GB is the Octal version of the 5Z3.

Listening Results:
Turning on a new, freshly built amp is always an exciting moment.  With the test speakers - Pioneer SB-21s - place on the ground, I was immediately taken with the fast quality of this amplifier.  After checking over the power supply voltage and bias readings, I moved on to my main system consisting of Spica TC-50 speakers, a Quicksilver full-function preamplifier, and a VPI HW19 Mark III turntable with a Denon DL-110 cartridge.  This is a smaller, more budget orientated system than the setup at my old digs, so I'm still feeling my around the various changes.

First impressions - revealed an amplifier with a nice, neutral midrange.  This is not a romantic sounding unit but I would never mistake it for solid-state either.  Bass quality was very good: controlled and with no tubbiness.  This is hardly a surprise due to the heft of the Edcor output transformers.  The already mentioned speed was evident in the treble too - giving a slightly lean presentation which may be more a character of the Spica TC-50s than anything else.  Soundstaging, also strong point of these speakers, was wide and deep.  Texture and inner-detail was also top-notch.  With more dynamic music, the amplifier had no problem delivering the power while keeping everything in line.  This amplifier sounds much more powerful than its expected 10Ws, indicating good clipping characteristics common with Class A output stages.

Overall, I would still give the overall nod to the EICO HF-60s, but those monoblocks usually cost three or four times as much, and also sport much more exotic output iron plus increased power output.  Having said that - and it's always difficult to fairly judge your own creations - the Command 1625 amplifier offers great sonics for such a low price point.  It is also a great reminder that pentode outputs can compete in the world of hi-fi.

Parts list and Front Panel Express file are available upon request.  Shoot me an email.

Acrosound 20 amplifier update

 The job of updating the Acrosound 20 monoblock amplifier became much easier once I saw a PCB for sale on Ebay.  It's a modern iteration of the same circuit, but with all new resistors, capacitors, and tube sockets.  Replacement was an easy job and with all the leads soldered into position, the amplifier fired right up on the first try.  Now that's always good news.

Some new power supply capacitors are still needed, but I'll wait until I have another matching amplifier before I make that move.

Listening in mono through a fairly efficient test speaker, it's a pretty good sound amplifier.  The Acro has some of that general "Class A" sonic footprint - an ease and naturalness to the music that makes tubes worthwhile.  It does run quite hot, and appears to be exceeding the maximum plate dissipation of the 6BQ5/EL84, but hey, this is a cheap little tube, so no worries there.  Buy 'em cheap and burn 'em up!

New Project: Acrosound Stereo 20 Monoblock

Stereo 20 monoblock?  Say what now?  Well, back in the early days of stereo, many audio nuts were still using mono systems or saving up their dollars to buy another amplifier.  A single monoblock was often cheaper than a stereo amplifier, giving a budget route to building up a true two-channel system.

 
In this case, the Acrosound 20 is exactly one-half of the (slightly) more popular 20/20 amplifier.  There was even a 20A model which has no power transformer but instead gets its B+ and filament voltages from the 20 Monoblock octal socket.  Very strange indeed.

 
I bought this little unit on Ebay.  It's a pretty funky, but pretty in that 1950s vintage audio way - a nice hammertone paint job with some bling gold transformers.  The circuity, except for the input caps, is DC coupled using a 12AX7 and a pair of 6BQ5 transformers.  The PCB is mounted on top, ala Dynaco style, but a small cage can be used to stop prying fingers from being electrocuted.  Output taps are 4-8-16-32, which should work with any crazy combination of speakers, provided just 18Ws of power is needed.

Of course I will have to be patient to find another monoblock - or even the more obscure 20A - but this should be a nice little project to tide my audio nervosa over for a few weeks.  No, I'm not expecting it to compete with the EICO HF-60s, but a pair of Acrosound 20 amplifiers would be a great basis for a second system.

Update: New PCB board installed.

Review: Eico HF-60 tube monoblocks

Introduction
EICO, unlike McIntosh, Marantz, or Fisher, isn't quite spoken with the reverent tones with discussing the best of the vintage amplifiers, but to do so would be a mistake.  From their budget units, up to the the flagship model, the HF-60, they represented a serious competitor to Heath and Dynaco, offering good value for the hobbyists of yore.  When I had the rare and unexpected chance to buy a pair, I went for it, knowing I would be hearing with one of the best EL34s amplifiers around.


The Amplifiers
So what is an EICO HF-60?  It's a monoblock amplifier - requiring two for stereo - featuring 5AR4 tube rectification, the classic Mullard 5-20 circuit with an EF86 and 6SN7 driving EL34 output tubes in Ultralinear, fixed bias, and the legendary Acrosound TO-330 output transformer.  There's nothing tricky going on here circuit-wise: current sinks, current sources, regulators, dc filaments, etc are non-existent, but component quality is still quite high with wirewound resistors, good tube sockets, and a really nice hunk of iron.  Biasing is through two pots: one to balance between output tubes while the other is raw bias.  These came as very basic kits with no circuit boards: the end-user was expected to be experienced enough to do point-to-point wiring.

Another thing to note is the high plate voltage and bias, making the HF-60 run steep Class A.  500VDC on the plates with 65mA means the EL34 output tubes are running 32.5Ws (plate + screen) of dissipation, right on the bleeding edge of their maximum rating.  If the lights are off, a faint red glow can be seen on the plates(!) which, according to the manual, is considered normal operation!  Back in the day, this was no big deal since you could pop down to the local store to pick up your new set of Mullard EL34s, but now with the high price of quality old-stock tubes, a user of this amplifier may want to dial back a few milliamps.

Not Quite Stock
Before I go and describe the sound of these amplifiers, I would like to mention that mine aren't exactly stock.  Of course these days it's hard to find a vintage tube amplifier that sports its original power supply capacitors, but my set of EICO HF-60s came with some additional modifications that will effect the sound.  First off was the addition of a small choke in the power supply.  The original just used 40uF of electrolytic to clean up the voltage.  Instead, I have a 25uF electrolytic bucket, followed by a 1.5H choke, and then a large 40uF Suzuki polypropolene capacitor.  This change gives a clean and fast power supply.  The second modification is to the EF86 input tube which now uses a battery to supply the bias.  This was done by the local "tube guru" and is purported to clean up the treble and whatnot.  Original coupling caps have also been replaced with MIT RTXs and some Jantzen Z-Superiors.

Tubes used for this review: Genalex CV4085, GE 6SN7GTBs, Mullard XF2 EL34s, and Hitachi 5AR4s.


First Impression
Expectations were high when I dropped the needle on Side 2 of Supertramp's Crime of the Century.  I've often been disappointed by audio purchases since the hype doesn't often live up to the reality.  In this case I had nothing to fear - within seconds I recognized that the EICO HF-60s are something special.  They really gripped the UREI 813A speakers hard, delivering power in an effortless way, all while casting a big, deep and detailed soundstage.  It thoroughly demolished the budget wunderkind Yaqin MC-10T, and my memories of other tube amplifiers - the Dynaco 70, Dynaco Mark IV, Dynaco Mark III, Harman Kardon Citation V, Heath UA-1, various Scott, Heath, and EICO integrateds, and anything else that I missed over the years of this crazy hobby.  The EICO also beat out my now departed Threshold S/500 or any other solid-state amplifier I've owned (which hasn't been many).


Listening Notes
Okay, enough gushing already, eh?  In order to give some further insight into these amplifiers, I invited a fellow longtime audiophile friend over to listen, hoping a second set of ears would keep my effusive praise somewhat in check.  After spending a chunk of cash it's easy to get carried away with new gear, missing the warts and blemishes of music reproduction.  We listened to a number of records and this is what we heard:

Steely Dan - Aja:  Recorded by Roger Nichols, Aja's technical prowess - both musically and sonically - is laid out perfectly with the UREI speakers.  The bouncy dynamics are engaging and fun, with plenty of little details to keep the brain locked into the music.  At no time do the amplifiers run out of power - delivering the power effortlessly. 

Grateful Dead - Terrapin Station:  Side 2 of this album is a sorta short rock opera with bits of Renaissance-inspired flourishes and backup vocals.  Yeah, it's a little cheesy at times, but still well-recorded with big dynamic shifts that sound diminished with lesser speakers and amplifiers.  Needless to say, the EICOs passed with flying colors, keeping everything locked into place without ever sounding strained. 

Faces - Long Player (German pressing):  Not exactly an audiophile recording, the Faces good-time boozy musical party record sounds better than I ever heard it.  Every instrument is audible, standing by itself in the soundstage without getting lost in the thick mix.  

Supertramp - Crime of the Century (British pressing): before I got the UREI speakers, I always considered Supertramp to be lightweight rock 'n' roll music.  But with the right system, the darkness of the music becomes more apparent.  Once again the dynamics and bass impact was amazing, as was the midrange and treble detail.  Soundstaging was deeeeeep, making my listening space sound like a larger than the confines of the walls.

Fleetwood Mac - s/t: Back when I was a kid, I pretty much loathed this 1975 album whenever my parents would play it.  But changing tastes and Stevie Nick's wonderful voice on Rhiannon changed my mind.  On this system, the high production values were evident, along with various multi-tracks used to create this sonic gem.

 
Sonic Scorecard
Bass: For the longest time, the king of bass reproduction was my Threshold amplifier.  250WPC with a huge amount of available current and the high damping factor helped to give some of the most fluid and detailed bass response I had ever heard.  A known limiting factor of tube amplifiers has always been - at least in my experience - the slightly sloggy and underdamped bass.  But somehow through the Acrosound TO-330 output transformer, those normal limitations are no more.  Low frequencies are extremely accurate, controlled, and easily equal that of the massive solid-state Threshold.  Amazing for a tube amplifier.

Midrange: They actually reminds me of an DIY amplifier I built a few years ago - a SE 300B design with a hefty SV83 pentode driver.  The same kind of speed and snappiness, along with a natural reproduction of vocals, guitars, and everything else that dominates the most important part of the frequency range.  No fine sand or loss of detail, but just a natural but uncolored presentation.

Treble: At my age, my hearing of highest frequencies isn't what is used to be, but still there was no sign of harshness or glassiness.  I may niggle here and there and say that I've heard better - the above mentioned SE 300B design comes to mind or my SE EL156 amplifiers - but this is very fine hairsplitting indeed.  Cymbals have a nice shimmer with natural decay while synthesizers snap and howl like the real thing.

Soundstaging: Deep, wide, and layered with everything sitting properly in it's place.  At no point did the sound ever collapse or shorten like I've heard with several other amplifiers with undersized power supplies and output transformers.

Detail: I'm no hyper-detail freak, but the EICO HF-60s certainly didn't fog over the sound with a "tubey" mush or give fake information by cranking the upper-mids on up.  Also different recordings sounded, well, different instead of congealing into the same tonality.  I've heard plenty of tube amplifiers that suffer from "too much character", making every record sound as if it was recorded in the same studio.

I'll also note that I did some brief listening on the Magnepan 1.6/QR speakers and got great results, but the maximum volume was limited.  Those speakers love to suck current and even 60Ws of tube power wasn't enough unless you enjoy baroque/folk/light rock music at moderate levels.


Conclusion
Okay, I hate to enthuse too much about just an amplifier since getting here has been a long journey.  That's to say that an amplifier is only part of the chain of electronic reproduction.  It took several other dead-ends and paths to finally come to this point.  First of all, it helps to have a solid front end.  Though not the best in the world, the VPI HW19 is still a solid performer, as is the Rega arm coupled to the Denon DL-103R and Cinemag transformers.  The Quicksilver is really a nice bit of kit, and none of what I'm hearing would be possible without the amazing extension, speed, and explosive dynamics of the UREI 813A speaker.  This same pair of monoblocks, especially stock, may not be quite so amazing with a lesser combination of gear.  Back in the past, I used to be amp crazy, thinking "just the right amplifier" would catapult my system from mediocre to greatness.  I've since learned that a solid front-end along with a good pair of speakers should be your first step before embarking on a path of expensive amplification upgrades.

Even though the EICO HF-60s are just a part of an overall, what I did hear from these amplifiers was a revelation.  The most amazing part was the way they delivered power.  I've owned some heavy-hitter gear in the past, but it's almost like the HF-60s "know" beforehand what wattage to deliver to the speakers.  This sense of ease in reproduction just makes you forget about the amplifiers and instead just revel in the music experience.  Yes, I've owned amplifiers that can deliver much more power, but they could never do with the finesse of this electronic antique.  Much of this has to do with the Class A biasing, but also the Acrosound TO-330 output transformers.  There is definitely something special about these hunks of iron.  It's too bad that their design blueprint has been lost in the mists of time, though I'm sure some of the best offerings from Tamura, Magnequest, Tango, etc could match their performance.

It's been a long journey but I'm happy to say that I've finally found a pair of "keep for life" amplifiers.  Until I hear something better at his price point - which may be possible - I will instead turn my attention to improving my turntable, stock up on my system's tube needs, and keep buying records.


Main System:
VPI HW19 Mark III with SDS Power Supply
aluminum rebodied Denon DL-103R
Rega RB300 with Cardas wiring
Cinemag CMQEE-3440A in custom aluminum box
Cardas Cross 1M interconnects
Quicksilver preamplifier with Mullard short-plate12AX7s, RCA 12FQ7s, and a Raytheon black-plate 5814
Cardas Quadlink 5C 1M interconnects
EICO HF-60 monoblocks with Mullard XF2 EL34s, GE 6SN7GTBs, Genalex CV4085s
Cardas Hexlink 2M speaker cables
UREI 813A speakers
VTI BL503 equipment rack