"New" Chris Keller ST-70

knowledge base for the classic Dynaco ST70

"New" Chris Keller ST-70

Postby zonker92 » Thu Apr 14, 2011 11:50 am

OK, 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):

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.

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Last edited by zonker92 on Thu Apr 14, 2011 6:55 pm, edited 1 time in total.
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Postby Geek » Thu Apr 14, 2011 3:27 pm

VTA vs. DynaMutt shootout:

http://www.audiokarma.org/forums/showth ... light=st70

I wish someone did a comparison with the KTA HiFi board (or the diyTube)... they're nice too (and easier to build).

Cheers!
Last edited by Geek on Thu Apr 14, 2011 3:41 pm, edited 1 time in total.
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Postby mesherm » Thu Apr 14, 2011 3:32 pm

If it were a stock ST-70 with stock driver board then a driver board change to a VTA or Shannon's driver would be worthwhile but that amp has been pretty much tweeked to the max and I would leave it as it is.
The tubes and components are all first rate and the build quality is excellent. I myself would probably change the 4 coupling caps to Russian PIOs but thats only because I have a supply of them on hand.
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Postby Geek » Thu Apr 14, 2011 3:40 pm

mesherm wrote:If it were a stock ST-70 with stock driver board then a driver board change to a VTA or Shannon's driver would be worthwhile but that amp has been pretty much tweeked to the max and I would leave it as it is.


That's a stock driver.... compare:
http://cgi.ebay.ca/Dynaco-Dyna-ST70-ST- ... 5ad9ed4e7a

Cheers!
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Postby zonker92 » Thu Apr 14, 2011 4:05 pm

Thanks, all! Geek, I think mesherm meant an original ancient OEM driver board with original ancient OEM parts, as compared to a new replacement replica board with upgraded new parts.

I appreciate all the replies! It's so annoying to buy something, then do the research, then find out that I could have bought something different and allegedly better (in this case, a different driver board). But, and maybe it's all in my head, I think this amp sounds fantastic as it is. I really love it.
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Postby kheper » Mon Apr 18, 2011 8:24 am

zonker92 wrote: It's so annoying to buy something, then do the research, then find out that I could have bought something different and allegedly better (in this case, a different driver board). But, and maybe it's all in my head, I think this amp sounds fantastic as it is. I really love it.


The rap on the stock driver board is that it makes the bass response "muddy". If you like the sound of your amp, why do you care what others allege? Personally, I like the diytube board - because the bass response is sharp and precise, and the midrange and treble are "there". Another reason for the stock muddiness is that the original PA-60 is being taxed to its current limits.
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Postby Sal Brisindi » Tue Apr 19, 2011 5:55 am

I would have used a 525V or higher electrolytic capacitors instead of 500v. Whatever you do, don't install a solid state rectifier in place of the 5AR4 tube.

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Postby Geek » Tue Apr 19, 2011 6:26 am

Sal Brisindi wrote:I would have used a 525V or higher electrolytic capacitors instead of 500v. Whatever you do, don't install a solid state rectifier in place of the 5AR4 tube.

Sal


x2 this.

I'm glad my PS was rated for 700V, as when I did some experimenting with SS rectification, it shot up to +570V on powerup and down to 510V once warmed =:o

I did get a "ST-90" out of that experiment though (lol)

Cheers!
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Postby EWBrown » Tue Apr 19, 2011 12:16 pm

The front of those horns look a LOT like the Altec 811 800 Hz horns I have for my Karlsons, I wonder if the Valencias used the same size horns with different drivers in their design (???) 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.

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There was also a larger 500Hz horn, with a very similar construction.

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Postby burnedfingers » Wed Apr 20, 2011 5:52 am

There was also a larger 500Hz horn, with a very similar construction.


It was called a 511B
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