I bought a stock-looking used Dynaco ST-70 that was dirty and had one bad tube. I selected the amp because I wanted to try tubes per the advice of this board and a friend of mine had restored about 8 Dynaco ST70's and said he would do one for me for the cost of the parts...he even did the "elbow greese" work and only charged me for the cleaning materials. I will return the favor which is appreciated.

Other than cleaning, checking/fixing the existing wiring he replaced the following parts and set the bias (once after restoration and again after letting an audiophile friend burn the amp in).

I'll be using it connected to my Outlaw 1050 but I assume I'll want a modern tube pre-amp to get the best out of my 2-channel system. I saw Kelly's list of tube pre-amps in the $600-$800 range. Is that the short list or are there other recommendations --not more expensive ones? I need phono cabability and actually will be exploring a TT upgrade after the electronics.

Here's the new parts list:
4 EL34 Eiectro-Harmonix tubes by Sovtek
1 GZ-34 Rectifier
4 Hovland Caps
2 .047uF/630V
2 47uF/160V
1 JJ 20/20/20/40 Filter Cap (4 section electrolytic is mounted under the chassis, and the original has been left in place))
2 470 ohm 1/4W Res 1%
1 1N4007RT 5A diode
1 16GA H.D. Line cord
4 STD Rubber Feet
4 Gold RCA Jacks
8 Gold Plated 4-40 screws

I read several articles on modifications and restoration. This seemed like an inexpensive way to start by using the basic original design but with some important upgrades in signal caps/matching new output tubes/and gold plated connectors. Those that have heard the box feel it's a desirable representation of the ST70. I'm hoping it comparable to other ST-70's short of the modern redesigned mods that basically just use the chasis & transformers.

I'll be sharing sound impressions and bringing it to the D/FW Klipsch Forum for comparison to some other amps and for other opinions. My big question is will this extra effort make a notable improvement over my Outlaw SS (using the Outlaw for a preamp and then using a moder tube-preamp)?

Looks pretty nice and once you get used to the sound of it. You might want to try replacing that driver board with Curcio or Triode's front end board. The difference should be pleasing !

Craig

No one bothers to replace the two feedback and driver compensation caps? Why is that?
mdeneen

I looked at the circuit design mods from Curcio, Triode, and VanAlstyne. I was originally going to buy the Triode board kit since it's the most economical kit where the VanA appears to be a totally different circuit that basically just uses the chasis and transformers. Rather than make that investment I thought I would start with a restoration/parts upgrade and not change the basic circuit design.

I was thinking the next upgrade would be to a different amp. The heavily modified ST70's are basically different amps that would have to compete with other brands for my next purchase.
Unless you think the ST70 modified amps are the best amps for under $1k, I was thinking I would sell this one and try something totally different.

If I go the mod kit route I think I'll shop for a different ST-70. It seems that all I would really need is an attractive chasis and transformers. I'd be replacing most everything else so I would sell the one I have and apply the money towards the next purchase.

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On 1/19/2003 3:09:48 PM mdeneen wrote:

No one bothers to replace the two feedback and driver compensation caps? Why is that?
mdeneen
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We did replace them, but not with modified values. Are you suggesting something different?

I was looking at the picture, and from what I can see they looked original. If you replaced them - good.

FWIW, the st70 with the pentode input is pretty limited in performance - - the pentode is never going to be smooth sounding, and they are inherently noisy. Any of the mods which change out the pentode to a triode are well worth the money.

The thing is with ST70, you have real good iron and of course EL34's can be fine, but you are driving it with a super-cheap budget driver. Pentode inputs like 7189 are rough sounding, and the splitter is inacurate in terms of AC balance. Then there is no bias balance native to the circuit, so you have all this nice potential, but for the lack of a few bucks that potential remains locked up.

Nice job!
mdeneen

Thanks for the info...sounds like an important upgrade...I was tempted to go with the Triode board from the beginning since it wasn't as expensive as the other mods. Given that we checked all the components and swapped many it probably wouldn't have been much extra cost and effort to simply start with a Triode board and stuff all new parts, but since my friend was doing the work I deferred to his preference.


For my next upgrade I'm wondering if doing a mod. to my amp would be better than selling it and buying something else...such as one of the more popular amps on this board. The ST-70 seems to be selling in the $280-$350 range and the mod. kits range from $100 to $250 plus I would have to install them. Would there be more highly recommended solutions in the $600+/- range? I also will be needing a pre-amp solution (I'm selling my PAS3) so before doing a mod on the ST70 and buying a pre I'm wondering where I should go after using my current ST-70 with my OUTLAW. Maybe an integrated SCOTT or EICO plus restoration service would be a better route. Integrated amp, a nice TT and my Corns...maybe that's all I would need.

The ST70 is my first experimentation in tubes. I'd like my next upgrade to be more long term.

Craig

NOS, thanks for the note. So it looks like my upgrade path may stay with the St70 chasis. I've always intended to get a tube pre-amp but based on feedback from this board I'm selling the old PAS3 and now I'm searching for a tube pre-amp. I'll pull Kelly's $600-$800 list of pres from a few weeks ago on this board. Any other suggestions?

Do you think the Van Alstine mod would be the way to go? I think the Triode is about $100 for the parts and the VA is $250 for the parts. I've heard a VanA w/Klipschorns. I'll do some more reading.

In a general sense, the separates have more potential than the integrateds. Of course, I am heavily biased in favor of separetes. You have more flexability, more potential performance, and a bit more cost. The VA mod used to get raves, he does a rather unique thing which is filtering the input, I copied his mod on this once and found it DID have a benefit with certain speakers. This is all taste, of course and I never try to predict what others would like.

If you are into vinyl, I'd suggst you look into the revival period preamps rather than vintage. The R.P. units from Paragon, ARC, CJ, others circa 1975 have VERY much improved phono circuits over the vintage stuff. These generally go for $500 or so. I mention these in particular, because it is a group that doesn't get too much discussion here.

mdeneen

Check this interesting Mod package. Not much for details on the site I wrote the guy to see what the mod accomplishes .

http://www.bobsradio.com/dynaco.html

Check out http://www.curcioaudio.com/ also they have a great reputation at Dynaco mods.

I'm now listening to my ST-70 connected to my Outlaw pre-amp...not ideal but until I find a tube preamp it's all I have.

So far so good! The amp works great, no noise/hum or any other problems.

I need to do some more testing but I think there is a reduction in harshness. Is it possible that the imaging is better? Maybe I've psyched myself into liking the heavy iron box but I seem to enjoy my records and CDs better with the new toy. I've certainly dramatically improved my sound from the days when I had the Hitachi SS receiver driving them.

I'll spend some time with it and then disconnect it and return to the complete Outlaw and see if I miss it.

I now wonder how much better I'll like the sound with all the future upgrades I've been thinking about: tube pre-amp, new TT/cart, Curcio mod. I've hit a comfort zone so maybe my listening needs to get more critical before really becoming hungry for the upgrades.

I'm listening to replica HORNS from 1795 on my original HORNS from 1977.

Great you like the amp. Let it settle in and get used to it a bit before making too many changes. It should sound VERY different from you solid state amps just in general. Putting on your first good tube amp is almost like a shock, at least it was for me (this either guitar or audio).

To be honest, I wouldnt keep dumping ducats into that Dynaco ST-70. Keep it or sell it. But if moving up, dont go back into the whole thing. There are better options out there and with the money you are starting to poor into it with another batch of upgrades, I would just go to a whole step up in amp. There are a load of good used amps out there.

Right now, stop worring about upgrading and enjoy your music. The constant need to move to a better mousetrap is often a game that cant be won. Right now you have something worth getting to know. Have some fun.

As for the preamp, some of the 70s/80s units were good as stated. I have not been a huge fan of Audio Research gear but the last version of their SP-8 had a nice sound...pretty good phono, too. The Premier versions of the CJ stuff was good but very warm sounding, a bit opposite of most of the Audio Research. Still, the SP-8 and SP-6c are not that bad. I think the CJ preamps are more rich sounding but some find them too golden in tone. I prefer the 6SN7 tube for preamp line stage. If you ever see a Cary SLP-70, it's a great pre used as it has an excellent 6SL7/6SN7 line stage with a great little 12AX7 based phono...with a seperate power supply (especially good for phono too). Trouble is, you dont see them come up used too often. But the 70/80s tube preamps did have phono stages that were pretty nice. You find something, give us a shout.

kh

Thanks! I agree that I think I have found a point where I can start to focus on enjoying the system and delay the equipment focus for awhile. I certainly have a lot of platters to spin.

I didn't do the expensive mods originally because I didn't want to sink a lot of cash and my friends time into this first adventure into "light bulbs" as my son calls them. I new the Dyna wasn't the top of the charts on your top 40 countdown, so I've kinda figured there would be a new list of pop stars when I became hungry for a new generation of sound...although I've read a lot of good things about the Curcio box referenced in this thread.

I'll start shopping for the pre's you've suggested and come before the elders with some candidates.

I read "Catch Me If You Can"--now movie which I haven't seen--on my flight home this week. Captivating! I recommend the book which is a quick easy read. I'd like to have some of his obvious charisma...and people skills although I would find more socially compliant ways of putting them to use.