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Tube
Power Amp Reviews

Dynaco Stereo 70 II power amplifier

By Corey
Greenberg • Posted: Aug 31, 2004  • Published: Sep 1, 1992

I dig tube amps. When all's said and done, good
tube amps seem to sound more like real life than most solid-state gear; even
after listening to and enjoying the hell out of musical solid-state designs like
the Audio Research D-240 II and the Muse Model One Hundred, once I hook up the
big VTL Deluxe 225s again it's just like going home. I could go on about timbral
accuracy and clearer midrange textures, but the bottom line is, music just plain
sounds better when you shoot it through good tubes, and once most people
experience that magic, they're hooked.

Only problem is, that ol' valve magic don't usually come cheap.
Stereophile readers sifting through the various reviewers' reference
systems will find Audio Research's $10,000/pair Classic 150s and $3500 Classic
60 pumping out the Vagrants and Vaughan Williams in the homes of J's E and
A, respectively; the sumptuous vibrato'd stylings of Lesley Olsher wafting
through DO's $6000 Air Tight ATM-2; and VTL's $5000/pair Deluxe 225s teleporting
Steve Morse into Bob Harley's listening room and Jimi Hendrix into mine.

I mean, ten grand to play the tube game?! No wonder most
real-world audiophiles say, "Make mine silicon!" Most of the tube amps mags like
Stereophile cover are megabuck rigs only the rich and plump oppressors of
the working class can afford. High-dollar tube rigs are cool man cool, but what
I'm after are the affordable stars on the tube scene, amps I can safely
recommend to anyone, young or old, looking to get some magic into their homes
without putting too much hurt on their Pulse card. In upcoming issues, I'll
tackle kilobuck offerings from the best and brightest tube names—Music
Reference, VTL, and others. For now, let's see what happens when two companies
try re-tooling a classic '50s tube amp for Modern Lovers.

An important message
I'd like to take this opportunity to hereby
denounce Albert Goldman's new Elvis: The Last 24 Hours (St. Martin's
Press, $3.95). While Goldman's earlier Elvis was an interesting,
thought-provoking read, this 189-page paperback is nothing but sensationalistic
tabloid-style nonsense. A sample:

"There stood Elvis, looking as though he were playing a scene in a movie. He
was wearing a black jumpsuit with a Drug Enforcement Agency patch. Around his
waist was one of his gladiator belts, inside which he had stuffed .45s. In his
hand he held a Thompson submachine gun, which he had obtained through his
friends in the Chicago Mob. David could see the cotton balls soaked with cocaine
inside Elvis's nostrils.

"'David!' he shouted. 'Get up! We're goin' headhuntin'!"

I urge you to spend your money instead on I Am Elvis (Pocket Books
$8.95), the definitive guide to Elvis impersonators. In addition to features on
"Black Elvis" Clearence Giddens and six-year-old "Little Elvis" Miguel Quintana,
I Am Elvis lists phone numbers and addresses for each entertainer for
wedding, bar mitzvah, and audiophile club bookings. Highly recommended!

Dynaco Stereo 70 II: $995-$1095
The original Dynaco Stereo 70 was
without a doubt the most popular tube amp of all time, with many tens of
thousands of working models still out there in the field. One of the reasons for
its popularity has been the wide array of modifications offered for the 70;
while stock Dynas sound very good, the substitution of modern passive parts like
high-quality film caps and low-noise metal-film resistors can literally
transform these vintage gems. Go a bit further—improved power supplies and
better input/driver stages—and a modified Stereo 70 can embarrass most of what's
out there in the sub-thou solid-state world.

Panor Corp. has acquired the rights to the Dynaco name and reintroduced the
venerable line of tube gear, including the classic PAS-3 II tube preamp and
"Dynaquad" QD-1 II 5-channel home theater system. Their Stereo 70 II reissue
looks similar to a vintage Dyna, but contains several circuit additions claimed
to improve the original design's performance. For starters, the old Dyna
"Bias-set" output tube biasing circuit has been replaced with a pair of red LEDs
per channel; instead of having to measure the bias directly with a voltmeter and
adjust accordingly, all you have to do is twiddle a front-panel trimpot until
both LEDs are equally bright. The power supply, long considered one of the
original 70's weaker points, has been substantially revamped; the power
transformer is now larger and better-regulated, the smoothing capacitance has
been increased from 90 to 336µF, and the hard-to-find 5AR4 rectifier tube has
been replaced with four 1N4006 diodes configured as a full-wave bridge.

The original 7199 input tube, long since out of production, has been swapped
out for the much-easier-to-source 6GH8A. BEWARE!! Vintage Dyna owners should be
aware that the 6GH8A is not, I repeat, NOT a drop-in replacement for the
old 7199—if you stick some in an old 70 and burn down your whole trailer park,
consider my butt litigation-free!

In the biggest deviation from the original design, Panor has added
bandwidth-limiting filters to the 70 II's inputs, simple 6dB/octave series/shunt
RC networks that both AC-couple the inputs and filter out high-frequency
signals. The filters, placed directly after the input RCA jacks, consist of a
series 0.018µF cap/shunt 470k resistor for the high-pass section, and a series
10k resistor/680pF shunt cap for the low-pass section, for -3dB corner
frequencies of 19Hz and 23kHz, respectively.

This idea of adding such filters to the original Dyna circuit first appeared,
I believe, in modifier/manufacturer Frank Van Alstine's Audio Basics
newsletter as part of a series of his own Stereo 70 modifications. Based upon my
phone conversations with both parties, you can expect some hot words about just
who stole what idea from whom in both Frank's expected "Letter to the Editor"
and Panor's "Manufacturer's Comments"; I'll let these geezers sort it out in the
funny pages. In any event, Panor claims that these bandwidth-limiting filters
keep signals that are too high or low for the amp to properly handle out of the
circuit, with better sound as the result.

NEXT:
Page 2 »

COMPANY INFO

Dynaco

125 Cabot Court

Hauppage, NY 11788

(631) 434-1200

http://www.dynaco.com/

ARTICLE CONTENTS

Page
1

Page 2

Specifications

Review system

Measurements

Manufacturer's Comment

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