Subject: Dynaco Products and Modification Overview
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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:

Increasing interest in tube audio equipment, and vintage audio gear;

Vintage Dynaco equipment is still relatively inexpensive, especially when
compared to modern tube equipment;

It is commonly available on the used market, or even at garage sales,
resale shops, etc., as it was very popular when it was new;

The sound quality was top notch by the standards of its day, and is still
very listenable by modern standards, even though stock Dyna amps don't have
the power or transparency of good modern amps;

Vintage Dyna gear was built to high standards, and is very reliable, so
that most old Dyna still works;

The Dyna equipment was all designed to be built from kits, and so has
simple circuit designs and easy to work on layouts, making it easy for the
hobbyist to restore or modify;

The strong point of the Dyna power amps is their output transformers,
which are at least as good as most modern designs. Their weak point is
their simplified driver circuits. Hence, they can be greatly improved with
modern driver circuits and/or power supply modifications;

There are many published modifications and improvements to the original
Dynaco circuits, and many sources for parts for both restoration and
modifications.

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.

COMMON DYNA EQUIPMENT
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.

SOURCES FOR VINTAGE DYNA COMPONENTS
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.

CHECKING OUT A COMPONENT
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.

RESTORATION AND MODIFICATION
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:

Replace the stock input and output jacks with good quality modern ones

Replace the coupling caps with good quality modern polystyrene or
polypropylene ones of the same voltage rating (or higher) and same
capacitance as the stock caps

Replace the power amp bias selenium diode with a modern 1N4007 from Radio
Shack (or better yet a fast recovery 1n4007 equivalent), and the stock bias
capacitors (the two typically brown paper cylinders under the chassis) with
modern low ESR electrolytics with the same voltage (or higher) and higher
capacitance. Be sure to observe polarity with both the diode and the caps.
The same mod can be made to the heater supplies in Dyna preamps and tuners.

Replace the stock carbon composition resistors with modern 1% metal film
ones of the same resistance and wattage rating.

Put Zenner diode clamps on the Bias supply (45V total for ST-70s and Mark
IVs, 60V for Mark IIIs).

Convert the output tubes to triode mode operation, by placing a 100 ohm
1/2 watt resistor between pins 3 and 4 of each output tube, and
disconnecting the ultralinear taps of the output transformers from pin 4 of
each tube socket. This cuts the output power in half, but gives a very
smooth clean sound.

Bypass the power supply caps with poly caps-1uF/630V Solens work well in
this application.

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.

SOURCES
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.

Information:
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).

Parts:
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).

Modifications:
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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