Update 2002 - Test of Dynaco ST-70 tube amplifier modifications
by Roy Mottram
NOTE: we hope to test, evaluate, and listen to each of these within the next few months,
but for now we are relying on input from several other sources.
The Dynaco driver board is the weakest part of the ST70 design, and after 20 or 30
years it is due to be tossed out and replaced. The resistors and capacitors were not
too high-quality to begin with and usually are way out of tolerance.
The 7199 tube was considered a good tube 35 years ago, but is now known as being
non-linear, being noisy, and prone to distortion. Also, these tubes were out
of production until recently.
Here is a list of some of the more popular modifications, ranging from slight
to extreme, available for the ST-70 and the more popular opinions of these
modifications, as voiced on rec.audio.tubes.
- stock replacement board:
These are advertised on Ebay.
The driver board uses the original
driver-stage circuitry. Unlike the cheap original PC board, this one is of
much higher quality and very reasonable, too. In addition, you can also buy a
pre-packaged set of high-quality/premium parts for stuffing the board. If you
want to have a stock amp and don't want the hassles of replacing parts on the
original board (where traces are very prone to coming off), this is the
replacement to get. You get the stock sound, as intended by the designer, with
high quality parts.
- Sound Valves ST-70 Driver
Essentially, this mod replaces the original driver board
with newer, high-tolerance parts and replaces the 7199 driver tubes with
6GH8s. The rest of the circuitry is unchanged. Sound Values correctly
describes this as being more of a restoration than a modification. They claim
this board reduces hum, noise and distortion. Their ST-70 based amp is very
popular, and this is a very inexpensive mod that probably offers good value
for the money.
- Triode Electronics
Originally meant as an experimenter's board, Ned Carlson's
replacement driver board is widely acclaimed as being one of the
better-sounding mods out there as well. It consists of a Williamson
implementation, using a EF-86 driver stage along with a 12AU7 splitter. The
overall design is very close to the original, but results in a wider
bandwidth, lower noise and less distortion. This board has also won an award
from The Sensible Sound as being the best ST-70 replacement board out there.
- Kennedy Tube Audio
Kevin Kennedy sells a high-quality replacement board for the
Dynaco ST-70, as well as the Mk-IIIs. This uses 12AX7A and 12AU7A tubes in a
self- balancing cascade differential amplifier driver circuit. There have been
several very favorable reviews of this board on rec.audio.tubes.
- Curcio Audio :
Joe now has a choice of two boards, one a drop in replacement using 3 tubes,
and the other one his complete hi-end redesign which essentially builds a new amp
using the Dynaco transformers. It replaces both the driver stage and
the power supply. This is the mod we reviewed in 1990. Joe Curcio is an
acclaimed and highly regarded designer of tube audio products and has been
featured several times in magazines like Glass Audio. His designs offer
excellent performance and very good value for the money. You can buy just the
boards, or a kit with the boards and parts, or assembled ready to install.
- Welborne Labs :
Ron Welborne's products are to the DIY world what
Rolls Royce is to cars: expensive but top of the line parts and commensurate
performance. I have yet to hear a single person have anything bad to say about
his various kits - the Laurel, the Moondog, etc. His ST-70 mod is no exception
and will certainly give you excellent sound. For those of you on a tight
budget, Welborne Labs sells the unstuffed circuit boards at a very inexpensive
price - use components of your choice and in your price range and you can
still end up with a high quality design.
- Audio By Van Alstine :
A moderately-priced, high-quality rebuild, AVA finally did it right and quit
selling his remake of the original board, and came up with a new design.
The driver board replaces the 7199 with 6GH8A tubes and other improvements.
He also uses his "bandwidth limiting" to keep the circuit and output transformers
within their useful frequency limits without distortion.
- Vacuum Tube Audio:
We have to put in an ad for our own PCB, which uses a proven circuit with a long-tailed
pair, using 3 mil-spec 6201 tubes (12AT7) for the driver. See more info under the
PCB section of our website.
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