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Old 16th April 2003, 10:41 PM   #1
toenail is offline toenail  United States
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Default Dynaco st-70

Bear with me as I have little knowledge of tube amplifiers and would like to get my feet wet SLOWLY as I learn.

I have a Dynaco st-70 (the first one) that was handed down by dad. I believe he purchased it new in the late'60's. I'm aware that there are some common parts that people replace on these to improve their accuracy/output. I've also read that if the amp is causing a hum in the speaker that parts need to be replaced as well. Mine hums. At a glance it looks like most of the work that needs to be done would be plug-in or solder type and I'm hoping that I can at least handle that part.

The trouble is that different resources recommend different parts and I'm unsure of which to use. For example some tout a particular tube as being the end-all be-all replacement, while others insist that a different brand is the one to go with. One guy says use orange drops (whatever those are) and another insists that brand Y are the ones to use. I've also seen "ugrade" kits that give you all the typical problem parts to replace and you supply the labor.

I don't mind piecing something together if the end result will be better than the available kits. What I need is some guidance about the choices available and probably a little help when I get down to doing the work. I'd prefer to keep cost low if possible.

Any ideas?
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Old 16th April 2003, 10:44 PM   #2
SY is offline SY  United States
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At a minimum, replace the electrolytic capacitors. And replace the selenium rectifier in the bias supply. Good luck finding 7199s.

At a maximum, there are a host of ST 70 rebuilds out there. The best I've found so far is the Curcio mod- full regulation of all stages, new driver stage with easy-to-find tubes.
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Old 16th April 2003, 10:52 PM   #3
EC8010 is offline EC8010  United Kingdom
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What it comes down to is that you have a thirty-year-old amplifier that is still working, but is a bit poorly. Name me one transistor amplifier that achieves this longevity!

The reason that there is lots of conflicting advice is that most people assume that you want to improve the amplifier beyond its original performance. You might just want to fix it first, then tart it up. As SY says, replace the electrolytic capacitors (they're the ones with values >0.5uF), and definitely replace the selenium rectifier (looks like a cigarette packet) before it goes short-circuit and takes out the mains transformer. I would also replace the coupling capacitors (probably 0.1uF) because if they fail, they will take out the output valves.

Meanwhile, try reading about how valve electronics works. then you can make your own decisions.
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Old 17th April 2003, 12:15 AM   #4
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>...have a thirty-year-old amplifier that is still working, but is a bit >poorly. Name me one transistor amplifier that achieves this >longevity!

Ok, so it's only 29 years since I built it, but my solid state Dynaco SCA-80Q is still going strong with the following repairs:
>Neon bulb in power switch went intermittant, replaced.
>Controls and switches cleaned once or twice.

The Fisher 800C needs a bit more (1968, though I just got it) - lights, a 6HR6 tube, matched output tubes (7591$), a power switch, and maybe a good cleaning of the sockets will bring the intermittant tuner back...
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Old 17th April 2003, 12:23 AM   #5
EC8010 is offline EC8010  United Kingdom
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Well, I suppose I just asked for that.
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Old 17th April 2003, 11:31 PM   #6
toenail is offline toenail  United States
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Thank's for the responses. I did check out the Curcio site. It appears that they have several kit versions at just over $100 that allow you to be more flexible with tube replacement. Providing that this doesn't hamper the sound quality I may go this route.

I need to hunt down that post I saw the other day that had the Tube Primer link in it. I forget to bookmark it.

And SY, my LSDC finally showed up. Thanks for the help with that.
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Old 17th April 2003, 11:37 PM   #7
SY is offline SY  United States
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That ought to keep you off the streets and out of trouble for a few weeks!

The Curcio stuff not only doesn't hamper the sound, it's the best off-the-shelf ST 70 input stage I've seen. I've got one myself, and it's been working just fine for about a decade.
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Old 18th April 2003, 01:20 AM   #8
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Quote:
I need to hunt down that post I saw the other day that had the Tube Primer link in it. I forget to bookmark it.

drop me an email. I'll send you a list of online diy tube-audio stuff.

Cheers
Craig Ryder
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Old 18th April 2003, 01:48 AM   #9
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toenail,

I rebuilt one of those and a couple Mk3s as well. I used teh Sheldon Stokes kit to rebuild the power supply and replace the somewhat unavalable rectifier. I also use Triodeal's replacement boards and parts. It all works very nicely and had good instructions. For sure get rid of that rectifier, when it goes you cant run fast enough to shut things down. A nice advantage of buying a new main board is that you get a good base to solder to the old boards are usually warped and very weathered.
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Old 19th April 2003, 11:08 AM   #10
toenail is offline toenail  United States
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Thanks for the suggestions. One thing I can see that would be beneficial is replacement of the board. I'm afraid I'll trash the exsisting one trying to replace the caps.

Are you guys familiar with this site? Good recommendations?

http://www.angelfire.com/ma2/AdamVon/dynaresto.html

It seems to sum up many of the points you've raised.
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