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Re: Dynaco ST-70: upgrading using mod kits or replacing existing parts?

208.243.96.98

Posted by Ross on July 25, 2002 at 12:22:49

In Reply to: Dynaco ST-70: upgrading using mod kits or replacing existing parts? posted by newbie on July 24, 2002 at 10:26:20:

The original Dynaco ST-70, as designed by David Hafler, was a landmark of balance. Truly an example of the whole being greater than the sum of the parts. Each part was chosen with regard to price and performance. Too expensive, no dice. Didn't sound good no dice.

To mod or replace the circuit is to alter the inherant balance and sound.

Right now, most ST-70 amps are at least 30yrs old, with most ~40yrs old. So the current sound, however good, is the result of 30-40yr old parts. Basicly a looonnngg burn in to translate into audiophile terms. I would be very surprised if most ST-70 amps can meet their original specifications for power, bandwith and extension. No problem with the design mind you, just a function of age. So even if everything is "working", you are not hearing the amp at its best.

To go ahead and modify the amp at this point will surely improve the sound. But this comparison is apples to oranges because you are comparing aged parts (and solder connections) to spiffy new ones.

Before you embark on changeing the basic design of the amp, which ALL of the mod kits do to a certain extent, how about listening for awhile to the amp as it was designed. You will learn a lot along the way, about the amp, electronics and the sound of your system.

So, what I advocate is a rebuild to stock specifications, with new close tolerance parts. I would not go for premium caps or resistors at this stage.

Take the time to match the resistors to at least 5% tolerance, better to try for 1-2%. Time consuming, but easy to do. Whether you opt for metal film or carbon comp is up to you. Metals tend to remain stable over time, but with a more SS sound. Carbons on the other hand, retain the vintage warm sound, but at the expense of instability over time. For circuit board caps, I would opt for Sprague 716P, but others like Illinois or Xicon. Definatly replace the selenium rectifier with at least a diode (1n4007 is fine), but others use Hexfeds. At this time, replace the bias supply circuit. Generic Rat Shack electrolyitics are fine here, same for the associated resistors. I would also replace the can cap at this time.
Pull all of the preamp socket wireing (except for bias pin !!) as well.

For tubes, Svetlana EL-34 quad definately. But I would go used for the 7199pr and 5ar4. With patience, you can find a pair of good used 7199 for $10-15 and good used 5ar4 for $25. Both are better sounding choices than current production tubes.

Button it up, test it, bias it and be prepared for a revolation !

To my ears, the difference in sound between old parts and new parts is an order of magnitude. A stock rebuild also gets you right to the point of diminshing returns in terms of value. I would stop here and listen for a few months. If you want to continue, fine, but at least you have a fair base to work from. The Triode or Curcio circuit kits, combined with the SDS Labs power supply seem to be the most popular choices. But be forwarned- the difference from a stock rebuld to one of the kits is small, and definately on the order of diminshing returns.

Best,

Ross

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