"New" Chris Keller ST-70
Posted: Thu Apr 14, 2011 11:50 amOK, I'm a tube noob, and here's my first post: I'm just learning about these things, but here's a restored amp I just bought on eBay. I'm running it through some Valencia 846Bs that I've upgraded to Model 19 specs (GPA 902 horn drivers, 416-8B woofers and Zilch's Z19 crossovers, with Darst horn foams). I'm running a Sonos ZP-90 into it with no preamp; just using the gain control on the Sonos, playing mostly lossless music files.
Here's what Chris said about the amp (I'm blending his eBay posting and his notes to me):
Per Chris's instructions, I've biased the channels at about 1.45 volts, and I've been running it for an hour or two at a time, to break it in.
I'm tempted to upgrade the driver board to a VTA (I'm not sure this cap can would work with the VTA, and I'd have to add two bias test points), but man, does it sound great already. Just not sure it's necessary. Any thoughts on this? This thread makes me wonder.
In any event, I'm delighted and just wanted to share.
Here's what Chris said about the amp (I'm blending his eBay posting and his notes to me):
Here is a gorgeous example of a Dyna ST-70 power amp. The ST-70 is one of the most popular amps of all time, with over 300,000 of them made. This one has been totally restored to better than new spec. Chassis totally stripped and painted; new power supply capacitors, doubling the original capacitance; new bias supply; new FR-4 driver board outfitted with top grade Solens and precision resistors; new cooler running ceramic tube sockets; new bias pots; new RCA's and speaker binding posts; heavy duty power cord. Tubes are a quad of Svetlana EL34's, RCA 7199 driver tubes and a fat bottle rectifier. This has the original cloth lead Dyna trannies of course, which are among the best ever made.
The amp runs dead quiet and sounds absolutely superb. The upgrades really bring the ST-70 to a whole new dimension. Much better low and midrange response. Wonderful soundstage and imaging. This amplifier will outperform most amplifiers out there costing five times as much.
Cosmetically, it looks stunning in the stealth black. Wife approval no problem.
I restore these as a hobby, when I have the time. I've done about 100 of them over the past few years. As for biasing, it's very easy, and I send a power up and bias sheet with the amp. The two 7199's are NOS. The output tubes are newer Svets, but I have new Electro Harmonix if you would prefer those instead. The rectifier is a vintage type.
Everything was stripped off the amp and replaced with newer and better parts: driver board, sockets, jacks, pots, power supply filtering beefed up, which keeps it running nice and quiet and provides a better bass response. It is completely re-wired and soldered with Cardas Eutectic solder. Trannies of course are the original cloth lead PA060 and A-470's. They are in excellent shape.
Per Chris's instructions, I've biased the channels at about 1.45 volts, and I've been running it for an hour or two at a time, to break it in.
I'm tempted to upgrade the driver board to a VTA (I'm not sure this cap can would work with the VTA, and I'd have to add two bias test points), but man, does it sound great already. Just not sure it's necessary. Any thoughts on this? This thread makes me wonder.
In any event, I'm delighted and just wanted to share.
My drivers are the 806-8A, 800Hz, 8 ohms, and I use the original Altec 800 Hz "dividing network", with better quality (5-way) input binding posts which I added later.
