Stereo 70 Power amp--The most popular tube power amp ever built. = Claimed 35=20 watts per channel (realistically closer to 25 watts RMS) from a pair of = push=20 pull 6CA7/EL34 output tubes per channel, operated in ultralinear mode. =20 Standard Dyna one tube per channel driver circuit, using 7199 = triode/pentode. =20 All parts (except chassis parts) readily available, including new = replacement=20 power and output transformers, tubes (the 7199 is back in production in = Russia), and multi section power supply capacitor. Many modification = kits and=20 articles available-more than for any other amp ever made. Clearly the = best amp=20 to buy for a beginner, if the lowish output power can be tolerated, or = a pair=20 is used strapped, bridged, or in a biamped system.
Mark III Power amp--A mono power amp with a claimed 60 watts (more = like 50=20 RMS) from a pair of 6550s or KT-88s in ultralinear mode. Driver = circuit very=20 similar to the Stereo 70 circuit, but uses 6AN8 triode/ pentode tube. = Parts=20 situation good, including tubes, although multi section power supply = cap is now=20 unavailable (there are workarounds involving adding caps under the = chassis,=20 which has plenty of room), and replacement power transformers would = have to be=20 tracked down used (try Angela Instruments, below). Some mod kits = available,=20 although not as many as for the Stereo 70. But most if not all Stereo = 70 mods=20 will also work in a Mark III, sometimes with minor modifications = and/or custom=20 circuit boards.
Mark IV power amp--a mono amp with identical circuitry to the Stereo = 70. =20 Sometimes called "half a Stereo 70." Output power slightly higher than = for=20 one channel of a Stereo 70, (40 watts claimed/30 or so RMS), due to = less=20 strain on the power supply. Some Dyna collectors consider this the = most=20 desirable Dyna power amp, due to monoblock construction, use of = excellent=20 sounding EL-34 output tube, and relative rarity compared with ST-70 or = Mark=20 III. Parts situation very good-the only hard one to find would be the = power=20 transformer.
PAS2 and PAS3 preamps--Full featured stereo preamps, using 4 = 12AX7/ECC83=20 tubes (two in the phono section and two in the line section), and a = 12X4=20 rectifier. The only difference between the PAS2 and the PAS3 is = cosmetic- the=20 PAS2 uses a flat brass faceplate and plastic knobs, while the PAS3 uses = the=20 more modern looking sculpted anodized faceplate and metal knobs. = Internally the=20 preamps are identical. Parts situation good-the power transformer and = chassis=20 parts, and some of the control pots, are the only tough ones. A few = mod kits=20 available, and many mod articles have been published over the years.
PAS3x preamp--the same as the PAS3, except with a revised tone = control=20 circuit which takes the tone controls completely out of the circuit = when set=20 to flat. The special tone control pots used in the PAS3 are = unavailable. =20 Otherwise, the situation is the same as the PAS3.
FM-3 tuner--A tube stereo FM only tuner. Sounds OK but flawed stock, = but=20 can be modified for much better performance. Unique in that it can be = aligned=20 at home with no special equipment other than a multimeter. Made with = both the=20 PAS2 style faceplate and the PAS3 style faceplate, but both version = called the=20 FM-3. The prior mono Dyna tuner, the FM-1, is sometimes seen with the = FMX3=20 multiplex board installed, and with that PCB is almost (but not quite)=20 identical to the FM-3. The special tuning indicator tube on these = tuners is=20 now completely unavailable (except by scavenging one from another = FM-3), but=20 all the other tubes are available.
Other Dyna tube equipment--The following equipment is much less = common=20 and/or less desirable than the equipment listed above. Dyna made at = least=20 three other tube power amps-the Mark II (Dyna's first amp, similar to = the Mark=20 III except uses 6CA7/EL-34 output tubes for 50 watts claimed), the = ST-35 (a=20 very rare 17 watt/channel stereo amp), and the Mark VI (120 watt mono = tube amp=20 made in 1976, very rare, uses 8417 output tubes). There was the PAM-1 = mono=20 preamp, which was normally powered by a power output socket on the tube = Dyna=20 power amps. Dyna also made a tube integrated amp, the SCA-35, which = was a=20 simplified PAS preamp and ST-35 power amp on a single chassis. There = was also=20 the FM-1 mono tuner noted above.
Transistor equipment--Dyna made many transistor models, most of which = were=20 highly regarded in their day. However, as with almost all other 60's = and 70's=20 solid state gear, modern equipment surpasses these units in almost = every area,=20 so they have very limited collector's interest. Common Dyna solid = state units=20 include the PAT4, 5, and 5 Bi-Fet preamps, Stereo 80, 150, 400, and 416 = power=20 amps, SCA 50 and 80 integrated amps, and FM-5 tuner. One interesting = vintage=20 Dyna unit from this era is the QD-1 Quadaptor, which was the first = commercial=20 version of the famous Hafler passive surround sound system.
Modern Dyna equipment--the original Dyna Company founded by David = Hafler=20 went out of business in the late 1970s. At that time, Sound Values in = Ohio=20 bought up the entire parts inventory, and for several years continued = to sell=20 original Dynaco kits for products such as the Stereo 70 amp. These = kits ran=20 out a few years ago. Now Sound Values sells tube and transistor amps = under=20 their own name, some of which are very similar to the vintage Dyna = designs such=20 as the Stereo 70, Mark III, and Mark IV. Panor, Inc., an unrelated = company,=20 later bought the rights to use the Dynaco name, and introduced an = updated=20 Stereo 70 and PAS preamp. These have undergone some revisions in the = years=20 they have been on the market. While both the Sound Values and Panor = components=20 are relatively inexpensive by current high end standards, they still = cost=20 considerably more than good vintage units, and have little if any of = the=20 collector's appeal.
A more fruitful source is likely to be more audio specific markets. = This=20 can include anything from the classifieds of your local newspaper under = "TV,=20 Radio, & Hi Fi" or whatever, to the classifieds of audio magazines = like=20 Glass Audio and Audio Amateur (see below), and to a lesser extent = Stereophile,=20 to Audiomart magazine (only audio classifieds-PO Box 692, Crewe VA = 23930), to=20 the 'for sale' sections of audio BBSes like CEAudio on CompuServe and=20 rec.audio.marketplace on the Internet. You can also try audio or = electronic=20 related flea markets, which are held in many locations once or twice a = year. =20 Most areas have ham radio clubs, which have annual flea markets where = old audio=20 equipment often turns up-ask around at ham or electronics supply stores = for=20 details for your area. In Chicago, there is an annual Vintage Audio = Fair where=20 much vintage Dynaco is bought and sold (write to 5308 W. Lawrence Ave, = Chicago=20 IL 60630 to get on the mailing list-the next fair will probably be in = the=20 summer of 1995). Record swap meets are another location where vintage = Dynaco=20 may appear. You can expect to pay more buying from sellers in these = markets,=20 since they'll probably have some idea of what the equipment is worth. = But it=20 should be a lot easier to find old Dyna from such sources. While there = is=20 still a chance you'll end up with a lemon, it's more likely in my = experience=20 that you'll get something decent from a knowledgeable seller, who may = even be=20 able to point out problems with and modifications to the unit. And = except for=20 purchases by mail, you'll also get to personally inspect the unit for=20 problems, and should be able to power it up to confirm the tubes light = up. =20 However, it's unlikely that you'll get a decent chance to listen to it, = and=20 you certainly won't get any warranties.
The easiest but most expensive approach is to buy from an audio = dealer. =20 There are specialized vintage audio dealers who should be able to sell = you a=20 vintage Stereo 70 or pair of Mark IIIs from inventory, or with a very = short=20 wait. They can also advise you what components will be appropriate for = you,=20 your room, your listening habits, and your other components. Two of = the best=20 known are Audio Classics in New York and Angela Instruments in Maryland = (addresses below). The good vintage dealers will check out the = component and=20 make any necessary repairs before selling it to you, and will give you = a short=20 warranty (typically 90 days). If you want it =3Dnow=3D and/or want to = make sure it=20 works perfectly as soon as you get it, the premium these dealers charge = over=20 other sources can be well worth it.
Dealers who sell used equipment (trade-ins) as a sideline to new = equipment=20 sales are another possible source. These dealers will typically also = charge=20 top dollar, and will try to give the same services as the vintage = dealers. But=20 their knowledge of and competency to repair old tube equipment may not = be as=20 good as the specialized vintage dealers, simply because it's very = different=20 than the modern equipment they sell and service as their main business. = But=20 you should have the opportunity to personally inspect and listen to the = unit=20 you're buying, which will let you spot any gross problems. And = sometimes such=20 dealers will sell old Dyna they've taken as a trade in cheap, because = they=20 don't want to mess with old tubes. The author purchased his personal = Mark IIIs=20 from a well respected high end dealer for about half the going dealer = rate,=20 simply because that dealer carried only solid state equipment like = Rotel and=20 Mark Levinson.
There are also a couple of specialized concerns with vintage audio=20 equipment. Often the seller has had it stored for many years before = putting=20 it on sale. Tube audio equipment can deteriorate while not used, so = that a=20 component which worked fine when it was put in the attic in 1978 may = have=20 serious problems when next plugged in in 1994. Also, many vintage = audio sales=20 are conducted by mail, either with vintage dealers or between private = parties. =20 While vintage Dynaco gear is much more solidly built than most modern=20 equipment, it is quite old, and may not be treated as well by the = shipping=20 company as you would like. This may cause parts to become loose or = fail which=20 were fine when the unit was packed for shipping. Hence, the inspection = procedures below should be followed even when buying a unit from a = reputable=20 dealer.
The first step is a power off visual inspection. ALL COVER OFF = VISUAL=20 INSPECTION MUST BE DONE WITH THE POWER OFF AND UNIT UNPLUGGED FOR AT = LEAST=20 FIVE MINUTES!! UNTIL THE CAPACITORS DISCHARGE, THERE ARE LETHAL = VOLTAGES ON=20 BOTH THE TOP AND BOTTOM OF TUBE DYNA COMPONENTS. Look closely at the = unit with=20 the tube cage or top cover off. Generally, the cosmetic condition is a = matter=20 of taste (and the nickel plated chassis of Dyna power amps clean up = quite=20 nicely with automotive chrome cleaner). But beware of significant rust = (small=20 rust spots on power amp chassis are normal, though), or other signs of = water=20 damage. An amp which has been waterlogged in its life is a potential = time=20 bomb. Also look to make sure all tubes and other components are = present, and=20 not broken or burned (some of the larger resistors may show a little = charring,=20 which is normal). Inspect the power supply capacitor (the large silver = can on=20 the top of the chassis) for any signs of chemical leakage (which means = it needs=20 to be replaced).
Then remove the bottom cover and inspect the underside of the = component. =20 Look for the same things you did on the top. Closely inspect the = bottom of=20 the power supply capacitor for signs of leakage. Also look for burned = wires=20 (color fading is normal), other burned components (some charring of = circuit=20 boards under large resistors is normal in old Dyna gear), and any wires = which=20 aren't connected. With power amps, also check the line fuse. A blown = line=20 fuse is a sign of trouble with old tube amps. (Dyna preamps and tuners = didn't=20 have line fuses.)
Also, try to test all tubes with a good tube tester (not at the = corner=20 drugstore anymore, I know--I bought one), and replace any which don't = test=20 good before going to power up.
Assuming the unit passes visual inspection (or any problems found = have been=20 corrected), the next step is a power on test. Put the bottom cover = back on,=20 but leave the top cover off. With power amps, connect a disposable = speaker to=20 the speaker outputs (an old car speaker is ideal), a shorting plug to = the=20 input, and a multimeter to the bias test point. For tuners, just = connect a=20 cheap antenna. For preamps, don't make any connections. Ideally, you = would=20 power up through a Variac, which is a large transformer designed to let = you=20 slowly raise the voltage from 0 to 120VAC. A homemade substitute can = be made=20 by putting an ordinary incandescent light bulb socket in series with an = extension cord, and using a 40 watt or so bulb in the socket (THIS ALSO = HAS=20 LETHAL VOLTAGE ON SOME CONNECTIONS, SO INSULATE IT CAREFULLY). This = will=20 reduce the voltage fed to the amp. Position the component so you can = see the=20 tubes while standing next to a power outlet at least 6 feet away. Turn = the=20 power switch to on, and then plug it into your Variac or = lightbulb/extension=20 cord assembly. Only then plug the Variac (set to 0) or extension cord = into the=20 wall outlet, but be sure to be able to unplug it immediately at the = first sign=20 of smoke, fire, sparks, explosion or hissing from the amp itself (a cap = going),=20 or tubes glowing red hot. (Do this by unplugging the amp, not by the = power=20 switch, since a defect may put lethal voltages on the chassis.) If = you're=20 using a Variac, slowly start turning it up, but be prepared to drop to = 0 VAC at=20 the first sign of trouble. With power amps, also watch the volt meter, = and cut=20 power if the bias voltage goes above 2.0 volts.
Assuming no problems (the first power up test is the scary part, but = I=20 haven't had any explosions yet), next make sure all the tubes are = glowing. =20 One or two out may mean a bad tube or socket, but all of them out may = mean a=20 bad power transformer (expensive and difficult to replace). With a = power amp,=20 next set the bias voltage by adjusting the screwdriver adjustment near = the=20 output tubes. All Dyna amps are supposed to be set to 1.56 Volts, but = I=20 usually use about 1.4 volts to preserve tube life. Listen for any = sounds from=20 the speaker. Some hum and/or crackling sounds can be easily fixed, but = lots of=20 hum may mean serious power supply problems (bad), or bias supply = problems (easy=20 to fix). With all components, use your multimeter to make sure there = is no=20 significant DC voltage at the output. If there's more than 50-100 mV, = replace=20 the output coupling caps in preamps and tuners. In power amps, DC on = the=20 outputs is very rare, but most likely means a bad output transformer = (very=20 bad).
Assuming you've gotten this far, now you're ready to hook up your new = component and listen to some music. You'll also need to listen for = areas=20 where the component needs improvement. Noisy controls can usually be = fixed by=20 spraying De-Ox-It (from electronic supply stores) or tuner cleaner from = Radio=20 Shack into the controls. Low hum usually means bad electrolytic = capacitors in=20 the power or bias supply. Hiss is usually caused by a noisy tube. = Crackling=20 noises are usually a bad resistor. And generally "blah" sound may be a = weak=20 tube, or may be coupling caps that need to be upgraded.
A more ambitious approach is to retain the original circuit design, = but=20 implement it with improved modern parts. Under this approach, the = original=20 parts are replaced with improved modern ones of the same values. = Replacing=20 the original carbon composition resistors with 1% metal film = equivalents, and=20 the original capacitors with audiophile grade polystyrene and = polypropylene=20 versions, can improve the sound quality while retaining its original = character.=20 This approach is somewhat more complex and expensive than a simple = repair, but=20 still should be within the capabilities of most audiophiles.
The most elaborate approach is to change the actual circuitry for = more=20 sophisticated modern designs. This can range anywhere from increasing = the=20 capacitance of the power supply, to totally replacing the driver = circuitry=20 with an improved modern design and regulating the power supplies. This=20 approach is the most costly and difficult, but can result in an amp = which is=20 equal to the best modern tube designs at a fraction of the cost. It = will often=20 totally eliminate the vintage character of the amp (which may or may = not be an=20 improvement to the user, depending upon taste), and may destroy the = collector's=20 value of the unit. I recommend that anyone taking this approach retain = the=20 original parts, and not do anything to the amp which would prevent it = from=20 being restored to stock condition. Even if you don't like the stock = sound, a=20 potential buyer may want that.
In practice, most Dynaco users follow some combination of these = approaches. =20 A typical power amp mod might involve replacing all worn out parts, = installing=20 modern audiophile coupling capacitors and RCA jacks, replacing the bias = circuit=20 with a modern 1N4007 diode (or fast recovery equivalent) and larger low = ESR=20 capacitors, and installing polypropylene bypass caps across the power = supply=20 capacitors. This would result in an amplifier with the basic = characteristics=20 of the stock amp, but with lower noise and improved transparency.
Simple modifications worth considering include:
Glass Audio magazine prints many articles on modifying vintage Dyna=20 components. It's essential reading if you like tube audio and working = with a=20 soldering iron. Subscriptions are $28 for six issues per year (as of=20 1995-Canadian and overseas rates higher). Back issues are also = available, as=20 are back issues of its sister publication Audio Amateur (which had many = Dyna=20 projects before Glass Audio was spun off in 1989). They're at PO Box = 176,=20 Peterborough NH 03458, Phone 603 924 9464, Fax 603 924 9467.
There are no books specifically about vintage Dyna, and only two = about=20 vintage audio in general--the Vintage Hi Fi Spotter's guides by Charles = Kittleson. These and many other books about vacuum tube circuits in = general=20 are available from Old Colony Sound Lab (an affiliate of Glass Audio), = PO Box=20 243, Peterbrough NH 03458, Ph 603 924 6371, Fax 934 9467, (Catalog $3) = or from=20 Antique Electronic Supply (listed below under tubes).
Angela Instruments is at 10830 Gulford Road, Suite 309, Annapolis = Junction=20 MD 20701, Phone 301 725 0451, Fax 301 725 8823. Catalog $5 (although = with any=20 vintage dealer, the best thing to do if you're looking for something = specific=20 is to phone to see what they have available-Angela's catalog is still = worth the=20 $5 for its parts selection, opinionated advice and quirky humor, = though).
For general "audiophile approved" parts, sources include The Parts=20 Connection and Welborne Labs (both listed below under modifications), = as well=20 as Audio Electronic Supply (111A Woodwinds Industrial Court, Cary NC = 27511, Ph=20 919 460 6461, Fax 919 460 3828) and Michael Percy (PO Box 526, = Inverness CA=20 94937, Ph 415 669 7181, Fax 415 669 7558--apparently no credit cards). = For=20 good quality non audiophile (i.e., reasonably priced) parts, try = Digikey=20 (1-800-DIGIKEY for a catalog), Mouser (1-800-346-6873 for a catalog), = or Newark=20 (check your local phone book for the nearest branch). Radio Shack = parts are=20 generally the cheapest possible quality, and overpriced-they're only = good for=20 emergencies (although they sell good reasonably priced silver solder).
The Parts Connection has Curcio's circuits as kits. It also has = rebuild=20 kits for most tube Dyna amps and preamps. These retain the original = circuits,=20 but upgrade most caps and resistors to modern audiophile equivilents. = They=20 also have a wide selection of audiophile parts, including Alps volume = and=20 balance control pots, good quality tube sockets, and RCA jacks. = They're at=20 2790 Brighton Road, Oakville Ontario, Canada L6H 5T4, Phone 905 829 = 5858 (or=20 800 769 0747 for orders), Fax 905 829 5388. Catalog is $5, but = includes a $10=20 discount coupon for your first order over $100.
Welborne Labs has Alan Kimmel's Mu Stage modifications for the Stereo = 70 and=20 Mark III. They also have many other parts, and kits for SE triode amps = of=20 their own design, and distribute Erno Borbley's excellent solid state = amp kits.=20 They're at PO Box 260198, Littleton CO 80126, Ph 303 470 6585, Fax = 303 791=20 5783. Their catalog is $12, but well worth it- it contains much = information on=20 tube and solid state audio, including schematics for all their designs.
Audio by Van Alstine is well known for their Dynaco modifications. = Their=20 PAS modification spent several years on Stereophile's Recommended = Components=20 list. They're at 2202 River Hills Dr, Burnsville MN 55337, Ph 612 890 = 3517. =20 They also sell custom selector switches and gold jack sets for PAS = preamps,=20 which are very high quality and useful to any PAS modifier.
Triode Electronics in Chicago sells a "Williamson" type driver board = for=20 Stereo 70s and Mark IIIs. They also sell many other useful parts for = building=20 and restoring tube amps, and carry a wide selection of tubes. They're = at Box=20 578751, Chicago IL 60657, Phone 312 871 7459, Fax 312 871 7938.
Back to = the Dynaco=20 page.=20