Dynaco parts/mod sources

Bob Schneider, 5 November 1994

For the last few years, there has been increasing interest in the classic Dynaco tube amplifiers. There are many good reasons for the continuing popularity of vintage Dyna components:

However, most vintage Dynaco equipment is now over 30 years old. Any electronic device that old probably has parts that no longer meet specification or other problems which will keep it from performing as new. Also, due to improvements in component parts (capacitors, resistors, etc.), and circuit design over the years, Dyna gear can be made to work even better than new. Hence, before buying a vintage Dyna component, the audiophile should have some knowledge of his or her options regarding getting the unit to work in a modern audio system. This article will describe the common Dyna tube components, give sources for buying one, inspection tips, explain the owner's options for restoration and modification, and list sources for parts and modifications.


The original Dyna Company founded by David Hafler produced many different models over the years. The most common and desirable components include the following:

Stereo 70 Power amp--The most popular tube power amp ever built. Claimed 35 watts per channel (realistically closer to 25 watts RMS) from a pair of push pull 6CA7/EL34 output tubes per channel, operated in ultralinear mode. Standard Dyna one tube per channel driver circuit, using 7199 triode/pentode. All parts (except chassis parts) readily available, including new replacement power and output transformers, tubes (the 7199 is back in production in Russia), and multi section power supply capacitor. Many modification kits and articles available-more than for any other amp ever made. Clearly the best amp to buy for a beginner, if the lowish output power can be tolerated, or a pair 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 RMS) from a pair of 6550s or KT-88s in ultralinear mode. Driver circuit very similar to the Stereo 70 circuit, but uses 6AN8 triode/ pentode tube. Parts situation good, including tubes, although multi section power supply cap is now unavailable (there are workarounds involving adding caps under the chassis, which has plenty of room), and replacement power transformers would have to be tracked down used (try Angela Instruments, below). Some mod kits available, although not as many as for the Stereo 70. But most if not all Stereo 70 mods will also work in a Mark III, sometimes with minor modifications and/or custom circuit boards.

Mark IV power amp--a mono amp with identical circuitry to the Stereo 70. Sometimes called "half a Stereo 70." Output power slightly higher than for one channel of a Stereo 70, (40 watts claimed/30 or so RMS), due to less strain on the power supply. Some Dyna collectors consider this the most desirable Dyna power amp, due to monoblock construction, use of excellent sounding EL-34 output tube, and relative rarity compared with ST-70 or Mark III. Parts situation very good-the only hard one to find would be the power transformer.

PAS2 and PAS3 preamps--Full featured stereo preamps, using 4 12AX7/ECC83 tubes (two in the phono section and two in the line section), and a 12X4 rectifier. The only difference between the PAS2 and the PAS3 is cosmetic- the PAS2 uses a flat brass faceplate and plastic knobs, while the PAS3 uses the more modern looking sculpted anodized faceplate and metal knobs. Internally the preamps are identical. Parts situation good-the power transformer and chassis parts, and some of the control pots, are the only tough ones. A few mod kits available, and many mod articles have been published over the years.

PAS3x preamp--the same as the PAS3, except with a revised tone control circuit which takes the tone controls completely out of the circuit when set to flat. The special tone control pots used in the PAS3 are unavailable. Otherwise, the situation is the same as the PAS3.

FM-3 tuner--A tube stereo FM only tuner. Sounds OK but flawed stock, but can be modified for much better performance. Unique in that it can be aligned at home with no special equipment other than a multimeter. Made with both the PAS2 style faceplate and the PAS3 style faceplate, but both version called the FM-3. The prior mono Dyna tuner, the FM-1, is sometimes seen with the FMX3 multiplex board installed, and with that PCB is almost (but not quite) identical to the FM-3. The special tuning indicator tube on these tuners is now completely unavailable (except by scavenging one from another FM-3), but all the other tubes are available.

Other Dyna tube equipment--The following equipment is much less common and/or less desirable than the equipment listed above. Dyna made at least three other tube power amps-the Mark II (Dyna's first amp, similar to the Mark III except uses 6CA7/EL-34 output tubes for 50 watts claimed), the ST-35 (a very rare 17 watt/channel stereo amp), and the Mark VI (120 watt mono tube amp made in 1976, very rare, uses 8417 output tubes). There was the PAM-1 mono preamp, which was normally powered by a power output socket on the tube Dyna power amps. Dyna also made a tube integrated amp, the SCA-35, which was a simplified PAS preamp and ST-35 power amp on a single chassis. There was also the FM-1 mono tuner noted above.

Transistor equipment--Dyna made many transistor models, most of which were highly regarded in their day. However, as with almost all other 60's and 70's solid state gear, modern equipment surpasses these units in almost every area, so they have very limited collector's interest. Common Dyna solid state units include the PAT4, 5, and 5 Bi-Fet preamps, Stereo 80, 150, 400, and 416 power amps, SCA 50 and 80 integrated amps, and FM-5 tuner. One interesting vintage Dyna unit from this era is the QD-1 Quadaptor, which was the first commercial version of the famous Hafler passive surround sound system.

Modern Dyna equipment--the original Dyna Company founded by David Hafler went out of business in the late 1970s. At that time, Sound Values in Ohio bought up the entire parts inventory, and for several years continued to sell original Dynaco kits for products such as the Stereo 70 amp. These kits ran out a few years ago. Now Sound Values sells tube and transistor amps under their own name, some of which are very similar to the vintage Dyna designs such as the Stereo 70, Mark III, and Mark IV. Panor, Inc., an unrelated company, later bought the rights to use the Dynaco name, and introduced an updated Stereo 70 and PAS preamp. These have undergone some revisions in the years they have been on the market. While both the Sound Values and Panor components are relatively inexpensive by current high end standards, they still cost considerably more than good vintage units, and have little if any of the collector's appeal.


There are several potential sources for acquiring old Dyna equipment. The cheapest is likely to be at a garage sale, rummage sale, estate sale, or resale shop. However, while tube audio equipment often turns up in such locations, in the author's experience the more desirable Dyna units are rare from these sources, most likely because when it does turn up it goes quickly. Hence, this approach takes luck and time and perseverance. The equipment you get may also be in very poor condition. This approach can be worthwhile if there are a few different things you're looking for (the author has bought everything from vintage Altec speakers to old Scott amps to rare old RCA and Mercury LPs from such sources, but only 1 incomplete FM-3 among Dyna equipment), or you just enjoy the hunt, but it probably isn't worthwhile if you must have a pair of Mark IIIs in good condition anytime soon.

A more fruitful source is likely to be more audio specific markets. This can include anything from the classifieds of your local newspaper under "TV, Radio, & Hi Fi" or whatever, to the classifieds of audio magazines like Glass Audio and Audio Amateur (see below), and to a lesser extent Stereophile, to Audiomart magazine (only audio classifieds-PO Box 692, Crewe VA 23930), to the 'for sale' sections of audio BBSes like CEAudio on CompuServe and rec.audio.marketplace on the Internet. You can also try audio or electronic related flea markets, which are held in many locations once or twice a year. Most areas have ham radio clubs, which have annual flea markets where old audio equipment often turns up-ask around at ham or electronics supply stores for details for your area. In Chicago, there is an annual Vintage Audio Fair where much vintage Dynaco is bought and sold (write to 5308 W. Lawrence Ave, Chicago IL 60630 to get on the mailing list-the next fair will probably be in the summer of 1995). Record swap meets are another location where vintage Dynaco may appear. You can expect to pay more buying from sellers in these markets, since they'll probably have some idea of what the equipment is worth. But it should be a lot easier to find old Dyna from such sources. While there is still a chance you'll end up with a lemon, it's more likely in my experience that you'll get something decent from a knowledgeable seller, who may even be able to point out problems with and modifications to the unit. And except for purchases by mail, you'll also get to personally inspect the unit for problems, and should be able to power it up to confirm the tubes light up. However, it's unlikely that you'll get a decent chance to listen to it, and you certainly won't get any warranties.

The easiest but most expensive approach is to buy from an audio dealer. There are specialized vintage audio dealers who should be able to sell you a vintage Stereo 70 or pair of Mark IIIs from inventory, or with a very short wait. They can also advise you what components will be appropriate for you, your room, your listening habits, and your other components. Two of the best known are Audio Classics in New York and Angela Instruments in Maryland (addresses below). The good vintage dealers will check out the component and make any necessary repairs before selling it to you, and will give you a short warranty (typically 90 days). If you want it =now= and/or want to make sure it works perfectly as soon as you get it, the premium these dealers charge over other sources can be well worth it.

Dealers who sell used equipment (trade-ins) as a sideline to new equipment sales are another possible source. These dealers will typically also charge top dollar, and will try to give the same services as the vintage dealers. But their knowledge of and competency to repair old tube equipment may not be as good as the specialized vintage dealers, simply because it's very different than the modern equipment they sell and service as their main business. But you should have the opportunity to personally inspect and listen to the unit you're buying, which will let you spot any gross problems. And sometimes such dealers will sell old Dyna they've taken as a trade in cheap, because they don't want to mess with old tubes. The author purchased his personal Mark IIIs from a well respected high end dealer for about half the going dealer rate, simply because that dealer carried only solid state equipment like Rotel and Mark Levinson.


Any 30 year old electronic component is likely to have some problems. That doesn't mean you shouldn't buy one, because most such problems can be easily fixed (some can't, and you should stay away from such units). But you should have a good idea of the condition of any unit you're considering, so you know what you'll have to do before you can use it. And considering that all tube audio components operate on several hundred volts, you need to be extra cautious before operating an unknown component, due to the shock and fire hazards from defects and malfunctions.

There are also a couple of specialized concerns with vintage audio equipment. Often the seller has had it stored for many years before putting it on sale. Tube audio equipment can deteriorate while not used, so that a component which worked fine when it was put in the attic in 1978 may have serious problems when next plugged in in 1994. Also, many vintage audio sales are conducted by mail, either with vintage dealers or between private parties. While vintage Dynaco gear is much more solidly built than most modern equipment, it is quite old, and may not be treated as well by the shipping company as you would like. This may cause parts to become loose or fail which 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 dealer.

The first step is a power off visual inspection. ALL COVER OFF VISUAL INSPECTION MUST BE DONE WITH THE POWER OFF AND UNIT UNPLUGGED FOR AT LEAST FIVE MINUTES!! UNTIL THE CAPACITORS DISCHARGE, THERE ARE LETHAL VOLTAGES ON BOTH THE TOP AND BOTTOM OF TUBE DYNA COMPONENTS. Look closely at the unit with the tube cage or top cover off. Generally, the cosmetic condition is a matter of taste (and the nickel plated chassis of Dyna power amps clean up quite nicely with automotive chrome cleaner). But beware of significant rust (small rust spots on power amp chassis are normal, though), or other signs of water damage. An amp which has been waterlogged in its life is a potential time bomb. Also look to make sure all tubes and other components are present, and not broken or burned (some of the larger resistors may show a little charring, which is normal). Inspect the power supply capacitor (the large silver can on the top of the chassis) for any signs of chemical leakage (which means it needs to be replaced).

Then remove the bottom cover and inspect the underside of the component. Look for the same things you did on the top. Closely inspect the bottom of the power supply capacitor for signs of leakage. Also look for burned wires (color fading is normal), other burned components (some charring of circuit boards under large resistors is normal in old Dyna gear), and any wires which aren't connected. With power amps, also check the line fuse. A blown line fuse is a sign of trouble with old tube amps. (Dyna preamps and tuners didn't have line fuses.)

Also, try to test all tubes with a good tube tester (not at the corner drugstore anymore, I know--I bought one), and replace any which don't test good before going to power up.

Assuming the unit passes visual inspection (or any problems found have been corrected), the next step is a power on test. Put the bottom cover back on, but leave the top cover off. With power amps, connect a disposable speaker to the speaker outputs (an old car speaker is ideal), a shorting plug to the input, and a multimeter to the bias test point. For tuners, just connect a cheap antenna. For preamps, don't make any connections. Ideally, you would power up through a Variac, which is a large transformer designed to let you slowly raise the voltage from 0 to 120VAC. A homemade substitute can be made 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 LETHAL VOLTAGE ON SOME CONNECTIONS, SO INSULATE IT CAREFULLY). This will reduce the voltage fed to the amp. Position the component so you can see the tubes while standing next to a power outlet at least 6 feet away. Turn the power switch to on, and then plug it into your Variac or lightbulb/extension cord assembly. Only then plug the Variac (set to 0) or extension cord into the wall outlet, but be sure to be able to unplug it immediately at the first sign of smoke, fire, sparks, explosion or hissing from the amp itself (a cap going), or tubes glowing red hot. (Do this by unplugging the amp, not by the power switch, since a defect may put lethal voltages on the chassis.) If you're using a Variac, slowly start turning it up, but be prepared to drop to 0 VAC at the first sign of trouble. With power amps, also watch the volt meter, and cut power if the bias voltage goes above 2.0 volts.

Assuming no problems (the first power up test is the scary part, but I haven't had any explosions yet), next make sure all the tubes are glowing. One or two out may mean a bad tube or socket, but all of them out may mean a bad power transformer (expensive and difficult to replace). With a power amp, next set the bias voltage by adjusting the screwdriver adjustment near the output tubes. All Dyna amps are supposed to be set to 1.56 Volts, but I usually use about 1.4 volts to preserve tube life. Listen for any sounds from the speaker. Some hum and/or crackling sounds can be easily fixed, but lots of hum may mean serious power supply problems (bad), or bias supply problems (easy to fix). With all components, use your multimeter to make sure there is no significant DC voltage at the output. If there's more than 50-100 mV, replace the output coupling caps in preamps and tuners. In power amps, DC on the outputs is very rare, but most likely means a bad output transformer (very 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 where the component needs improvement. Noisy controls can usually be fixed by spraying De-Ox-It (from electronic supply stores) or tuner cleaner from Radio Shack into the controls. Low hum usually means bad electrolytic capacitors in the power or bias supply. Hiss is usually caused by a noisy tube. Crackling noises are usually a bad resistor. And generally "blah" sound may be a weak tube, or may be coupling caps that need to be upgraded.


In using old Dyna equipment (or any other vintage audio equipment), the audiophile must first determine how much he or she wants to change the amplifier from its present condition. There are three basic approaches which can be taken. The most straightforward is simply to repair the unit. This would involve merely replacing nonfunctioning or out of spec parts with exact replacements (or the closest available equivalents). Under this approach the original circuit design and properly functioning parts would be retained. For example, worn out tubes, leaky capacitors, and out of tolerance resistors would be replaced, and the rest of the original parts retained. This approach will give the original sound of the unit, and will best preserve it's collector's value. It is also usually the simplest and cheapest to implement. Repairing the unit is the minimum work necessary to use the amplifier.

A more ambitious approach is to retain the original circuit design, but implement it with improved modern parts. Under this approach, the original parts are replaced with improved modern ones of the same values. Replacing the original carbon composition resistors with 1% metal film equivalents, and the original capacitors with audiophile grade polystyrene and polypropylene versions, can improve the sound quality while retaining its original character. This approach is somewhat more complex and expensive than a simple repair, but still should be within the capabilities of most audiophiles.

The most elaborate approach is to change the actual circuitry for more sophisticated modern designs. This can range anywhere from increasing the capacitance of the power supply, to totally replacing the driver circuitry with an improved modern design and regulating the power supplies. This approach is the most costly and difficult, but can result in an amp which is equal to the best modern tube designs at a fraction of the cost. It will often totally eliminate the vintage character of the amp (which may or may not be an improvement to the user, depending upon taste), and may destroy the collector's value of the unit. I recommend that anyone taking this approach retain the original parts, and not do anything to the amp which would prevent it from being restored to stock condition. Even if you don't like the stock sound, a potential buyer may want that.

In practice, most Dynaco users follow some combination of these approaches. A typical power amp mod might involve replacing all worn out parts, installing modern audiophile coupling capacitors and RCA jacks, replacing the bias circuit with a modern 1N4007 diode (or fast recovery equivalent) and larger low ESR capacitors, and installing polypropylene bypass caps across the power supply capacitors. This would result in an amplifier with the basic characteristics of the stock amp, but with lower noise and improved transparency.

Simple modifications worth considering include:

Many more elaborate modifications exist, from making the bias of the output tubes separately adjustable for each tube to totally replacing the stock driver circuitry. Back issues of Audio Amateur and Glass Audio will give you many possibilities. For Power Amps, see GA 1:1989 (Joe Curcio's Stereo 70 with solid state power supply regulation) GA 2:1989 (Paul Becker's simple Mark IV mods), and GA 1:1992 (Norman Koren's Stereo 70 triode mode mod). For PAS preamps, see GA 2:1994 (Koren's Cathode Follower and other mods). For FM-3s, see GA 1:1991 (James Lin's FM-3 mods). Back issue ordering information for Glass Audio is below.


One of the reasons for the continuing popularity of vintage Dynaco components is the availability of parts for, information regarding, and modifications for these amps. A partial list of suppliers is below. While the author has done business with most of the companies on this list, I have not used all of the products listed, and cannot vouch for their quality, nor for the continuing existence or service from these suppliers. Addresses are the most recent I have as of November, 1994. All suppliers take credit cards, have minimum orders of $30 or less, and send free catalogs or price lists, unless otherwise noted. Of course, I have no connection with any of these companies except as a customer.


All owners of vintage Dyna gear should have copies of the original owner's manuals, as routine maintenance and repairs are very difficult without them.

Glass Audio magazine prints many articles on modifying vintage Dyna components. It's essential reading if you like tube audio and working with a soldering iron. Subscriptions are $28 for six issues per year (as of 1995-Canadian and overseas rates higher). Back issues are also available, as are back issues of its sister publication Audio Amateur (which had many Dyna projects before Glass Audio was spun off in 1989). They're at PO Box 176, Peterborough NH 03458, Phone 603 924 9464, Fax 603 924 9467.

There are no books specifically about vintage Dyna, and only two about 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 are available from Old Colony Sound Lab (an affiliate of Glass Audio), PO Box 243, Peterbrough NH 03458, Ph 603 924 6371, Fax 934 9467, (Catalog $3) or from Antique Electronic Supply (listed below under tubes).

Vintage Audio Dealers:

Audio Classics Ltd is at 34 Gardiner Place, US Post Office Bldg., POB 176, Walton NY 13856, Ph 607 865 7200, Fax 607 865 7222

Angela Instruments is at 10830 Gulford Road, Suite 309, Annapolis Junction MD 20701, Phone 301 725 0451, Fax 301 725 8823. Catalog $5 (although with any vintage dealer, the best thing to do if you're looking for something specific is to phone to see what they have available-Angela's catalog is still worth the $5 for its parts selection, opinionated advice and quirky humor, though).


For tubes, the two most reasonably priced suppliers are New Sensor in Manhattan (133 5th Ave NY NY 10003, Ph 212 529 0466, Fax 212 529 0486) and Antique Electronic Supply in Arizona (6221 S. Maple, Tempe AZ 85283, Ph 602 820 5411, Fax 602 820 4643). They both carry many obscure types (Antique Electronic lists more in their catalog, but New Sensor can get many they don't list), and their prices at least 80% off most "audiophile" tube suppliers. New Sensor has a $50 minimum order, and doesn't take credit cards-Antique Electronic takes credit cards, and has a smaller minimum order. Antique Electronic also carries many useful parts and books for old Dyna (although their main emphasis is old radios), including the best prices I've seen on multisection capacitors.

For general "audiophile approved" parts, sources include The Parts Connection and Welborne Labs (both listed below under modifications), as well as Audio Electronic Supply (111A Woodwinds Industrial Court, Cary NC 27511, Ph 919 460 6461, Fax 919 460 3828) and Michael Percy (PO Box 526, Inverness CA 94937, Ph 415 669 7181, Fax 415 669 7558--apparently no credit cards). For good quality non audiophile (i.e., reasonably priced) parts, try Digikey (1-800-DIGIKEY for a catalog), Mouser (1-800-346-6873 for a catalog), or Newark (check your local phone book for the nearest branch). Radio Shack parts are generally the cheapest possible quality, and overpriced-they're only good for emergencies (although they sell good reasonably priced silver solder).


Joe Curcio has modifications for the Stereo 70 and Mark III power amps, and PAS preamps. These are really complete revisions of the amps-all that's really retained are the chassis and transformers. They convert the Dynas into modern tube amps. (The author uses Curcio's amp and preamp in his main system.) His mods are available directly from Curcio Audio Engineering (PO Box 643, Trexlertown PA 18087) as blank PCBs, completed and tested PCBs, or installed in your Dyna chassis or theirs. Curcio's circuits are also available from The Parts Connection (below) as kits (Blank PCBs and all necessary components). His Stereo 70 circuit only is also available from Old Colony Sound Lab (above) as a kit.

The Parts Connection has Curcio's circuits as kits. It also has rebuild kits for most tube Dyna amps and preamps. These retain the original circuits, but upgrade most caps and resistors to modern audiophile equivilents. They also have a wide selection of audiophile parts, including Alps volume and balance control pots, good quality tube sockets, and RCA jacks. They're at 2790 Brighton Road, Oakville Ontario, Canada L6H 5T4, Phone 905 829 5858 (or 800 769 0747 for orders), Fax 905 829 5388. Catalog is $5, but includes a $10 discount coupon for your first order over $100.

Welborne Labs has Alan Kimmel's Mu Stage modifications for the Stereo 70 and Mark III. They also have many other parts, and kits for SE triode amps of their own design, and distribute Erno Borbley's excellent solid state amp kits. They're at PO Box 260198, Littleton CO 80126, Ph 303 470 6585, Fax 303 791 5783. Their catalog is $12, but well worth it- it contains much information on tube and solid state audio, including schematics for all their designs.

Audio by Van Alstine is well known for their Dynaco modifications. Their PAS modification spent several years on Stereophile's Recommended Components list. They're at 2202 River Hills Dr, Burnsville MN 55337, Ph 612 890 3517. They also sell custom selector switches and gold jack sets for PAS preamps, which are very high quality and useful to any PAS modifier.

Triode Electronics in Chicago sells a "Williamson" type driver board for Stereo 70s and Mark IIIs. They also sell many other useful parts for building and restoring tube amps, and carry a wide selection of tubes. They're at Box 578751, Chicago IL 60657, Phone 312 871 7459, Fax 312 871 7938.

Entire contents © 1994 by Robert Schneider. All rights reserved. Permission is hereby granted for this article to be posted on public electronic computer bulletin boards, to be reprinted in a publication of any bona fide not for profit audio society, or for one copy to be made by any individual for personal use, provided that the article is reproduced in full, including this copyright notice, and no special charge is made for access to this article. All other uses, including any commercial use, are strictly prohibited without payment of a whopping big royalty to the author. If you're interested, let's talk.

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