Dynaco Stereo 70 II power amplifier
By Corey Greenberg   •   September, 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.