tubes 4 hifi
 "the path of least resistance is through a vacuum"
 

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 Board:
    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 Board:
    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 Board:
    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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