I recently joined this group and bought my first toob power amp, a
Dynaco ST-70. I fully intended to start modifying it right away, but
it sounds so much better than the solid state amp I was previously
using that I think I will just listen to it for awhile before tearing
into it.

How many ST-70 will be left, after you molest yours?

wrote:
How many ST-70 will be left, after you molest yours?


Quite a few I suppose.

But what if he's only doing the cap and other things that decay? It is
kinda necessary, whether we like it or not.

in article ,
at wrote on 5/10/05 5:10 PM:

How many ST-70 will be left, after you molest yours?



Ya gotta die in order to resurrect, Harry.

The choice is between leaving a thing vintage and original and silent vs.
fixing what needs to be fixed and enjoying the music. I have second
thoughts about deciding to perform surgery on an amp as common and
pedestrian as a Dyna ST-70.

And while you're at it, you ought to try the Welbourne Labs mod. Done two
that way and both sounded fabulous!

Jon

in article , Jon Yaeger at
wrote on 5/10/05 6:03 PM:

in article ,
at wrote on 5/10/05 5:10 PM:

How many ST-70 will be left, after you molest yours?



Ya gotta die in order to resurrect, Harry.

The choice is between leaving a thing vintage and original and silent vs.
fixing what needs to be fixed and enjoying the music. I have second
thoughts about deciding to perform surgery on an amp as common and
pedestrian as a Dyna ST-70.

And while you're at it, you ought to try the Welbourne Labs mod. Done two
that way and both sounded fabulous!

Jon

That is, I "never" have second thoughts about it . . .

"Billy Boy (78's Rule!)" wrote in message
ups.com...
I recently joined this group and bought my first toob power amp, a
Dynaco ST-70. I fully intended to start modifying it right away, but
it sounds so much better than the solid state amp I was previously
using that I think I will just listen to it for awhile before tearing
into it.

Long as I can remember, I've been a toob amp hobbiest. The first amp I built
was right out of the back of an RCA RC26 tube manual that I lifted out of a
college radio station in the mid '70's. It was the 30 watt job; 7868's,
7199, Stancor iron, Bud chassis and all. Only built one monoblock, worked
great! Still, when the funds came available, I went and bought an expensive
HK SS job to use with my new Martin speakers. One day, at a "friend of a
friend's" house, I saw he had an ST-70 & PAS in his living room. When I
asked about them, he said "want to buy 'em?, $50; I want to get something
more modern". Turns out his dad put them together in the mid sixties. This
was around ~1982, so $50 for pristine Dyna's wouldn't be the jaw dropper it
would be today. In any case, after $50, getting them home, a cleaning and a
bias adjustment on the power amp, I connected them to my Martins and
turntable. After spinning Santana's Zebop, I was simply stunned! Well, after
reconnecting the HK one more time to make sure I wasn't going nuts, the
ST-70 stayed. Although I'm using my own design amp and preamp these days, I
still have the ST-70 & PAS.. About 4-5 years ago, I stripped the ST-70 down
to the bare chassis, cleaned and polished the chassis, reassembled the amp
with new resistors, caps, and NOS tubes. Point is, whatever your opinion is
regarding the ST-70, that damn little jewel might be responsible for more
Toobie converts out there today than any other consumer amp. Probably
because there are so damn many of 'em and, while not perfect, they do have
the Magic.

wrote:
How many ST-70 will be left, after you molest yours?


Mine was in 100% stock form when I got it (except the tubes),and
functioned beautifully.
I later started having problems with arcing Sovtek 5AR4's in it..
Thought it might be the can-cap,but leakage current in it was very
reasonable (still in good shape,no hum heard either.) But I replaced it
with the SDS labs cap board anyways,as a replacment for the can-cap,and
to 'hot-rod' it a bit..Also put new (Sprague) coupling caps in it.
After the SDS board,and new coupling caps,I enjoy it alot,and don't
intend to do any more work on it...except maybe replace those old carbon
resistors one of these days.

All amps need the usual "routine maintenance" after 50 years..
(Caps+resistors,selenium rectifiers,power cord,etc.)

Mark S wrote:

"Billy Boy (78's Rule!)" wrote in message
ups.com...
I recently joined this group and bought my first toob power amp, a
Dynaco ST-70. I fully intended to start modifying it right away, but
it sounds so much better than the solid state amp I was previously
using that I think I will just listen to it for awhile before tearing
into it.

Long as I can remember, I've been a toob amp hobbiest. The first amp I built
was right out of the back of an RCA RC26 tube manual that I lifted out of a
college radio station in the mid '70's. It was the 30 watt job; 7868's,
7199, Stancor iron, Bud chassis and all. Only built one monoblock, worked
great! Still, when the funds came available, I went and bought an expensive
HK SS job to use with my new Martin speakers. One day, at a "friend of a
friend's" house, I saw he had an ST-70 & PAS in his living room. When I
asked about them, he said "want to buy 'em?, $50; I want to get something
more modern". Turns out his dad put them together in the mid sixties. This
was around ~1982, so $50 for pristine Dyna's wouldn't be the jaw dropper it
would be today. In any case, after $50, getting them home, a cleaning and a
bias adjustment on the power amp, I connected them to my Martins and
turntable. After spinning Santana's Zebop, I was simply stunned! Well, after
reconnecting the HK one more time to make sure I wasn't going nuts, the
ST-70 stayed. Although I'm using my own design amp and preamp these days, I
still have the ST-70 & PAS.. About 4-5 years ago, I stripped the ST-70 down
to the bare chassis, cleaned and polished the chassis, reassembled the amp
with new resistors, caps, and NOS tubes. Point is, whatever your opinion is
regarding the ST-70, that damn little jewel might be responsible for more
Toobie converts out there today than any other consumer amp. Probably
because there are so damn many of 'em and, while not perfect, they do have
the Magic.

The ST70 has a lousy power supply.
The OPTs are better than Quad II or anything from Leak.

So the potential for good or better sound than from the original
amp which was built down to a price and not up to a quality
is there, and a latter day perfectionist can take the
amp design a little further than Dynaco dared, or could afford.
As long as 3 mint specimens exist in 3 museum locations in the US,
the species will always be inspectable, even though it will largely have become
an extinct species, but then it will have evolved into something different.

ST70's bootstrapped input pentode and CPI was a bean counter's delight,
saving the maker the cost of a tube socket and tube and wiring and chassis real
estate for
each channel.
The input stage isn't what I call a very nice idea, and I prefer a triode V1,
and an LTP with triodes for V2 and V3.
Rather than keep the 5AR4, better to junk it and use the space for large amounts

of capacitance. Si diodes are fine, and with series resistors to drop the B+
down a bit to keep it as low as with the tube rectifier, and keep the peak
charge currents
into C1 of the CLC filter nice and low.
The choke in the CLC filter should be retained, but since it is only
1 or 2 henrys, it will be all the more effective when the C1 and C2 of the
revised CLC
filter have been increased to 235 uF with a total of 4 seriesed 350v x 470 uf
caps.

Then there will be virtually no hum in the rails and the IMD caused by PS ripple

during class AB operation will vanish.

The Magic isn't unique to Dynaco; Magic is sprinkled into ears all over the
world
by other tube designs, and is the principle reason why tube amps have not
gone the way of the wind up gramophone.

So basically, its very easy to improve on what Dynaco offered,
and those who revise such amps sensibly need not feel
remorse, guilt, or depression, since they do not warrant being
called sacred objects, and they will not spoil the Magic.

Patrick Turner.

I also like the Wellbourne mod, and the Van Alstine as well. Check out
both.

"Jon Yaeger" wrote in message
...
in article ,
at wrote on 5/10/05 5:10 PM:

How many ST-70 will be left, after you molest yours?



Ya gotta die in order to resurrect, Harry.

The choice is between leaving a thing vintage and original and silent vs.
fixing what needs to be fixed and enjoying the music. I have second
thoughts about deciding to perform surgery on an amp as common and
pedestrian as a Dyna ST-70.

And while you're at it, you ought to try the Welbourne Labs mod. Done two
that way and both sounded fabulous!

Jon