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REVIEW: Dynaco ST-70 KIT Dynaco ST-70 KIT Amplifier (Tube)

Model: Dynaco ST-70 KIT
Category: Amplifier (Tube)
Suggested Retail Price: $695
Description: An upgraded Dynaco ST-70 in KIT form
Manufacturer URL: Dynaco ST-70 KIT
Model Picture: View

Review by Wolf012558@aol.com on February 04, 2008 at 07:09:30
IP Address: 205.188.117.73
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After reading Louis Chochos, the owner of Omega Speakers, positive review of Bob's upgraded Dynaco ST-70 power amp on the Omega Speaker Forum I got in contact with Bob to get more information about his amp. I am familiar with the 70 since I already own an old Dynaco ST-70. It is an original amp and although it was still running and sounded OK, it really needed rebuilding. Rather than rebuild and after a few more conversations with Bob, I went with his amp kit. Although looking virtually identical to the stock amp, this amp has many upgrades over the stock amp AND every part was new, nothing was recycled from an old ST-70. I was really impressed with the quality of parts included. The power switch is rated at 16 amps! 16 amps ? My God, the fuse blows at 3 amps ! All seven tube sockets have cadmium plated pins for durability and better electrical contact. The beautiful stainless steel chassis made me want to throw rocks at my old 70 with it's hazy looking frame. The silk screening on the chassis is identical to the original Dynaco chassis. The three transformers, according to Bob, are USA made custom production transformers. The power transformer has about a 1/2 inch higher stack than my original power transformer. The USA made A-470 output transformers supplied are near exact copies of the originals with M6 grain laminations just like the originals but have 4 bolts holding the bells on instead of 2 bolts like the original output transformers. Bob says that winding technique on the output transformers is the same as the originals. The original amp had 90 Mfd of power storage on the quad cap and nothing on the driver board. On this amp the driver board and quad cap have a total of 258 Mfd of power storage. Also included are two triode/pentode switches which are installed in the holes where the original bias potentiometers used to be. The new amp has the bias potentiometers on the driver board so the holes are not needed for bias contols. All tubes are included with the amp. It may also be purchased without tubes if you already have the proper tubes - four EL34, three 12AT7's and 1 GZ34. You can also use KT77, KT88 or 6550 output tubes. The KT88 and 6550 tubes are +$60. I went with the standard EL34's that came with the kit, a new matched quad of ElectroHarmonix EL34's.

It was an easy kit to put together. The assembly manual was perfect with no confusing instructions. A nice pictorial with color coded transformer wires was included so that you could check each step against the pictorial as you went along. Best of all it fired right up and played perfect the first time I turned it on. After a few hours of listening I swapped in my old 70 and compared the two. There was no real comparison. The newer amp sounded so much smoother and more powerful with the upgraded power supply and better driver board. The bass was tighter, the top end was more extended and like many tube amps this one just sounded "right" not tubey but tonally very accurate. This amp was dead quiet with no hum even with your ear next to the speaker. The bias was easy to set on each output tube. Even after 3 or 4 hours on, it had barely drifted a fraction of a volt. Bias is .40 volts DC per each EL34 tube or .50 volts DC if you use KT88 or 6550 tubes.

The triode/pentode switches are a nice idea and can be switched from one mode to the other while the amp is running. I switched back and forth between the two and found I liked the triode setting over the pentode for most (but not all) music. I guess it's a matter of taste. Although advertised like the old 70 at 35 watts per channel, the specs show 40 watts per channel and a power bandwith of 17 Hz to 30,000 Hz in the normal pentode ultralinear configuration. According to Bob, about 22 watts per channel are available in triode. Even with just 22 watts per channel, there was enough power to drive my 90 dB speakers to a pretty loud volume in my 14 X 24 foot living room. This is a nice amp kit which IMHO is a bargain at $695 + shipping considering the quality of sound. It's an easy to build kit but if you're afraid of doing a kit, you can also get a wired and tested amp from Bob for about $100 more.

Charlie Wolf


Product Weakness: None really except this is a basic amp and you do need a preamp with this amp.
Product Strengths: Excellent quality of sound much better than a stock ST-70. Good value. High quality of parts. Excellent assembly manual.


Associated Equipment for this Review:

Amplifier: Dynaco ST-70
Preamplifier (or None if Integrated): Audio Research LS-16
Sources (CDP/Turntable): Classe CDP-1
Speakers: B&W 603
Cables/Interconnects: Blue Jeans cable
Music Used (Genre/Selections): Mostly jazz and some pop
Room Size (LxWxH): 24 x 14 x 8
Room Comments/Treatments: Wall to wall carpeting
Time Period/Length of Audition: 4 months
Type of Audition/Review: Product Owner




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Topic - REVIEW: Dynaco ST-70 KIT Dynaco ST-70 KIT Amplifier (Tube) - Wolf012558@aol.com 07:09:30 02/04/08 (3)

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