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發表於 2013-11-12 13:39:26
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本帖最後由 auraheart 於 2013-11-13 08:41 編輯
回復 柒喜 #1 的帖子
The Dynaco ST 70 is a pretty decent amp to begin with. While the unmodified version is not as good as the Marantz 8B, still nice nevertheless. It took me a long time to get it done because I had to buy good, new parts at low prices. Shopped at a electronic flea market for parts was the best and only choice. Very often I see military grade parts at dirt cheap prices. How much improvement is needed? No such thing as much as possible...Modifiers do certain things differently may have different results.
I did my things a section at a time and actually witness the improved performance over time. Stereo 70 sounds better with tube 整流..no need to replace it with a solid state device. I replaced a better, 1H power choke and added three 220uf filtering caps which greatly improved the voltage regulation. Result was much tighter bass and more dynamic headroom across the spectrum. I don't know why the reviewer here did not do anything about the power supply. It is a must.
Replacing the coupling caps was next. I never like old antique parts. Unlike the author here, I used brand new military grade 5% or better tolerance (they hard to find, dynaco used 20%). Damn, it was like a true hi end amp..lol...and I was only half way done. However, I finished the major part of upgrade in one day. For what I wanted, the rest were minor...(was major to the author here, but we look for different things). I enjoyed the amp for a couple weeks, really like it and more parts needed from the flea market. Was not in a hurry cuz did the major improvements already.
I did not want to change the "Q" point. I needed every watt there is. Class A/B is fine with me, having 270 degree amplification or 360 degrees (class A). Not too important in my case. Class A is very inefficient and draw lotta power, not good for this power transformer. If other people wanna take a chance of burning the power transformer, let them do it.. . I did remove the 高頻抑制電容 knowingly that has been the weakness of this amp and crossed my fingers not to have oscillations. Over the next weekend, I replaced parts I can replace lol...with better tolerance stuff. Usually 1% resistors and 5% caps. I know Dynaco used cheap parts. Just costed a few dollars from flea market anyway. feedback 電阻1K, changed to 2k, guess about 6db more gain..more dynamic...like he man..I have THE POWER ! lol... . Less risky than author here, gotta keep harmonic distortion under control lol...and connected that to the 8 ohm tap of the out put transformer, sounds better that way instead of the 16 ohm tap.
I would leave the bias adjustable thingy alone, never wanted have fixed bias cuz what if tubes got old and Dynaco does allow to use few other power tubes like KT66 via bias adjustment...lol..not good to fixed the voltage for just a very minor improvement (major to some). For the ones who must have fixed bias, you better have hella good matching pair of EL34/KT77/6CA7. Any American made 6CA7 "Fat Boy" is good for daily use if you don't wanna spend upto $500 USD for match premium quards. 4X "Fat Boy" should be about $100 USD. They are almost as good as Mullards if made in US. Please do not be influenced by advertisements. I would not not buy those NOS tubes in general.(could be $1000 USD for Quad easy) Picture of "Fat Boys" below: (Those are 500 Bucks...hehe)
How it sounds at least partly depends on how the tech dude wants that to sound, other than some must thingy...rest depends on our personal taste. For all the parts I have changed, it was a major operation. Yes, I practically overhaul this amp plus modification. It was better than new by far.
It was like a new birth of a high end amp. The sound quality report will be on a new review if Mr 7up doesn't complaint about the long boring crap so far. I kept this amp until I bought the Bruce Moore custom design/build 40 Watts KT88 based pure class A mono tube amps. Now, my brother in law still got it lol...Total cost of this amp was about $45.00 USD... That 40 watts amp costed me $2,300 USD in 1981. 45 Vs 2300, talking about minor sound improvements...haha. With the Chinese tubes, my modified ST 70 should sound better than the Marantz 8b. They are very much on a par after my modification.
補充內容 (2013-11-12 17:09):
In sum, as a hi fi fan and tech hybrid. I knew what to look for to make this amp sound better. ST 70 wasn't very good at both extreme hi and low, plus lack of dynamic headroom/range. By juicing up the power supply lows and headroom were improved. Removing the caps enable better hi frequency response. Reducing the amount of negative feedback added dynamic range/headroom. It was just fun  |
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Fat Boy
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樓主: 柒喜
在線雷達
發表於 2013-11-12 11:32:41
DYNACO ST-70想當年既一代經典
仲有書刊雜志教??
