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Old 16th February 2007, 05:15 AM   #1
kegger is offline kegger  United States
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Default Original Dynaco 70 circuit mods/upgrades?

Curious if anyone has tried any circuit mods or have links to some
on the original 70's curcuits that made any decent improvements?

I know the original isn't anything great and plenty of good circuits
out there for these amps (have done a few myself) but I have this
newer green pc board that's in nice shape and so I thought about
messing around with the original circuit to see if anything can be
squeazed from it any further.

Yah I've seen the Van Alstine little tweak for frequency limiting but
that's about all I found and frankly not what I'm after here, I'd just
leave it completly stock instead of that thing.
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Old 16th February 2007, 05:53 AM   #2
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1976 Audio Amateur
"Dyna Stereo 70 Amp Mods"
http://www.audioxpress.com/magsdirx/...ues/BISAAM.htm

H.L. Eisenson descibes replacing the tube rectifier with diodes, replacing some of the caps and diodes, upgrading some resistors, and dialing in the square wave response.

Sorry, I don't have a link to the article, but it is included in the "Audio Amateur Power Amp Projects" book. (I have the book, but have never had a Dynaco ;-)

in 1977 (and the book) an article: "Audio Research Modifies the Dyna Stereo 70" - well, they all but completely redesign it really.

the amp project book:
http://www.audioxpress.com/bksprods/products/bkaa40.htm

-Chris
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Old 16th February 2007, 09:31 AM   #3
kegger is offline kegger  United States
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Those might be cool, all those you have to pay for correct?

I've got a power supply rebuild I normally do, I'm just wondering about
the audio circuit and the original PCB as to any improvements to be had.
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Old 16th February 2007, 10:45 AM   #4
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Go with an aftermarket driver board. I used one that had (3) 12AT7's. It also had bias adjustment for each output tube instead of each pair. I also added more capacitence to the power supply.
There is also a triode modification you can make. I wired in a switch to flip from normal to triode mode. Some people swear by the triode mode on these things. It may have sounded slightly better but I'm not really sure.

Replace the diode in the bias supply for sure.

Just Google Dynaco Stereo 70 and you should hit some free information.
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Old 16th February 2007, 10:53 AM   #5
kegger is offline kegger  United States
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As I said I've used aftermarket boards and have designed my own which
work very well but I'm curious as to some tinkering on the original circuit.

I'd just like to see what a tweaked out/modded version of the original curcuit could do.

I'll probably have to devise my own mods to it but was curious any info allready done.

I'm looking for mods to the original PCB in the amp, plenty of replacement boards out
there but I'd like to see what can be done on the original.
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Old 16th February 2007, 11:06 AM   #6
SY is offline SY  United States
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I've redone at least a hundred ST-70s. The Ike Eisenson mods retain the basic circuitry but upgrade some parts and the power supply. Most bang for the buck. While you're in there, you should also replace sockets, switches, jacks, and put in a three wire power cord with the safety ground lead firmly attached to the chassis. Ike did not redo the goofy grounding scheme of the ST-70, but you should.

A step up is replacing the driver circuit board with something different. There are about a million of those out there. I've had limited experience with them since I generally do my own.

Another step up is putting in regulation for the power supplies. The best I've used is the Curcio design. I think regulation is the single most useful mod.

If it were me, I'd strip the chassis bare, replace all the low bid sockets and switches, redo the power supply, add a regulator, replace all the coupling caps and critical resistors on the stock driver board (be warned: you will destroy some traces so be prepared to do some repairs), then live with that for a while. It will be pretty darn good.
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Old 16th February 2007, 11:27 AM   #7
kegger is offline kegger  United States
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Thanks SY, before this gets out of hand, I to have done a bazillion mods
and or upgrades to the 70 but have never given the original circuit the time
of day and just once at least I'd like to see what can be done on the original
PCB to see if anything decent can come from it. (I've used it stock)

I'm not expecting earth shattering performance by any strech of the imagination
but I would like to give it a go and see how things work out, but I find little to no
work in this area on the 70 so I figured I'd post about it before I went and did it
myself anyway. Just figured if I could find a decent looking framework might make
my work a little less is all, but it looks as if I'll have to do this one on my own here.
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Old 16th February 2007, 11:50 AM   #8
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there is a mod to replace the 7199 with a 6GH8 as the driver tube. It requires cutting and jumping some traces. Other than what SY mentioned that probably about covers it.

You just can't make a silk purse out of a pigs ear.
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Old 16th February 2007, 11:56 AM   #9
SY is offline SY  United States
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You'll be surprised how well the unit works with the original 7199-based driver once the parts are refreshed and the supply tightened up.
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Old 16th February 2007, 12:01 PM   #10
kegger is offline kegger  United States
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Yah not trying to make these world class amps here, just want to experiment
with the original PCB/Circuit and see what can be achieved with it, that's all.

As I said, I personally have never tried to get the most from these original boards
and at least I'd like to give it a go once to say I've done it and see what happens.

----------------------------------

I hear yu SY, but I'm thinking there's probably still some tweaking that can be done on
the original PCB/Circuit to improve things a bit and like to try and find out what's possable.
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