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Wild mods on Dynaco ST70

SKS

SKS

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With out question, the Dyna 70 is the most widely Modified tube amp ever. I still have my Dyna70 my first tube piece.
most are pretty dam good due to their excellent output Iron
 
looks like some flavor of 3 legged device strapped across the current sense resistor on the cathode. I'm assuming thats the auto bias circuit. Probably aims for some fixed voltage drop across that cathode resistor and it shifts the grid voltage accordingly to obtain that.
 
SKS said:
Saw this posted on CL near me. https://sfbay.craigslist.org/sby/ele/d/san-jose-dynaco-dynakit-st-70-tube/7370428384.html

View attachment 2335568 View attachment 2335569

Filter cap is bigger than the PT (cl post says 20x larger than stock). Has a giant U shaped bus bar, and a bunch of components i cant even identify. On one hand I respect anyone who can figure out how to mod things on their own, but this thing looks like a hot mess. Any thoughts on whats going on here?
Click to expand...

Maybe a film cap in the PSU? Equivalent ones to electrolytic caps can be huge.
 
^^ tends to be my opinion of a lot of these things, especially when there is no documentation, but its a bit like a train wreck, can't look away.

I kind of enjoy looking at different things just to get ideas, either on things to do or things to never do. I wouldn't want to buy it myself but picture-wise its more interesting than yet another ST70 in one of the very well known configurations that get discussed to death.
 
The real question is: Does it meet original specs.

If it performs better, cool. Usually these mods/hacks are not well thought out. Their poor soldering technique and lack of fasteners usually give them away.

In all honesty I was green once. I was convinced that dumping huge sums of money at a bolt on dynaco mods was going to yield the world's greatest tube amp, and boy was I lucky to have one right here to perform all of these "upgrades" on.

It was very pretty
Looked like a million bucks
Performed very poorly.
The "board kit" I used eliminates the feedback loop, which should have been my first clue...
Live and learn and don't cry over botched dynacos.
 
Personally i would change the title to "Wild Mods on Former Dynaco ST70 " this to avoid any confusion
 
From the pics, the chassis looks unmodified, a new driver board,and a rectifier socket, and it looks reversible.
I just looked at the AD, the price wouldn't be right for me to think about it. :crazy:
 
s-petersen said:
From the pics, the chassis looks unmodified, a new driver board,and a rectifier socket, and it looks reversible.
I just looked at the AD, the price wouldn't be right for me to think about it. :crazy:
Click to expand...
Then it should be priced considerably lower then an unmolested st70 ?
 
In my opinion.... it is not worth as much as a good to excellent original ST-70...
It may be undocumented, unsupported, and the specs may not have been tested, so you really don't know what you are getting.
 
I just read the ad.
I hope dude is stuck with it.
The ad mentioned wanting to trade for another dynaco plus cash. Apparently they feel they deserve to get paid to continue doing this...
Well opinions will vary on that...
 
The price is always the determining factor. I'd consider it if it were the same or less money as a chassis and a pair of transformers, because thats really all you're buying there. If the transformers are good and the chassis is not molested it can be stripped out to nothing and rebuilt as any ST70 variant you like. I'd have to reverse engineer the thing to see exactly whats going on there, and if it functions, measure it to see what it actually does when the rubber hits the road. I wouldn't immediately condemn it as hacked up useless garbage without an evaluation.

I know some folks claim heresy whenever a Dynaco shows up in anything other than as-designed by Saint David but consider how many people probably got into electronics work by building and modding these things. At one point they were simply old obsolete trash that was basically free, so it was a good test bed for experimentation. They aren't rare, they still aren't worth any real money beyond just what they are worth as an amplifier. Anyone that wants a stock one can buy all new parts and build one today.
 
Here's the Canadian version of stupid, presently on sale on the idiot-mart:

3439981-df9f382a-dynaco-stereo-70-updated.jpg
 
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