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How good can an ST-70 be made?

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summilux

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Post n°1

How
good can an ST-70 be made?

by summilux
on Thu May 07, 2009 10:16 am

I'm new here but I am NOT trolling.

I have a few DIY amp projects
under my belt, a scratchbuilt 300B using a Hammond OPT (JE Labs), a kit
Assemblage ST-40 (an EL34 amp) and some hotrodding of classic Naim amps. In my
home I alternate between the two tube amps because each has their shortcomings:
bass in the 300B and midrange sweetness in the ST-40. What I miss in one I can
pretty much find in the other. I listen to classical, folk and instrumental
music mostly although I have recently discovered a strange compulsion for
Kwaito.

I am mulling about new projects to take on and I am considering
an ST70, mainly because I've been reading some good things about its OPT. If I
do this, I will probably be buying the amp solely for the OPT. Likely, I'll put
a new driver board in, change all the connectors, tweak the PS and probably put
in a new power transformer as well.

Once this is done, how good will an
ST70 be? Not good for the money, not good for a vintage amp, but good in an
objective sense. An example may be the Denon DL103 cart, which to me is a really
good moving coil. Its $160 price is of secondary concern to its "goodness"

Maybe what I am asking is whether or not an ST70 can be made great based
on the virtues of its OPT.

Tube Nube

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Post n°2

Re:
How good can an ST-70 be made?

by Tube
Nube on Thu May 07, 2009 12:21 pm

Welcome to the board, Summilux.

I can't answer your question in
detail, though I think other knowledgeable folks on the board will jump in and
be able to give you a very good sense of how a modern ST70 rates, along with a
sense of value for the money.

I just wanted to comment that you're
describing an ambitious project there, but obviously you've got the DIY bug that
has returned some great results for the effort invested.

I was
wondering, though, are you talking about designing your own driver board, or do
you mean you'll be choosing one of the available modern drivers to modify a
stock ST 70?

I was persuaded that there are 2 or 3 modern driver boards
available that are truly magnificent. Joe Curcio offers one, but I some how came
to the conclusion (can't remember exactly how, now) that the VTA board was a
better choice.

If you check out the version of the ST 70 available from
the accompanying site tubes4hifi.com, you'll find it comes with upgraded
transformers, caps, and a few other goodies. Essentially, several desirable mods
all in one package.

That's what I opted for.

Now, I wish I could
tell you how it compares to other tube power amps in the $1k to $5k price range,
but I can't.

I can tell you that compared to my Naim Nait2 (which I
dearly love), the Dynaco is mouth wateringly good.

FWIW

Brenton

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Post n°3

Re:
How good can an ST-70 be made?

by summilux
on Thu May 07, 2009 12:49 pm

Brenton,

I won't scratch build something. I'll use someone else's
board. Based on internet reviews, I'd probably go for the VTA board although
there is a tempting Welborne/Kimmel board on the 'bay right now.

The
Naim Nait1 was my first "real" amp and I still have it. My Flat Earth system is
a Naim NAC32.5/SNAPS and mono-ed NAP110. All of them have been given an
audiophile going over: better caps, better supply regulation based on info from
PinkFish. I hook it up with some Linn Kans and an LP12. It sounds pretty nice,
I must admit that Naim/Linn do the PRaT thing very well, but its nothing like
TOOBES!

Jonathan

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Post n°4

Re:
How good can an ST-70 be made?

by Tube
Nube on Thu May 07, 2009 1:18 pm

Yo John,

I've been spending a lot of time on Pink Fish, finding a
lot of information and assistance there.

I, too, have an LP12, and
thanks to guidance on PinkFish, had the confidence to do some upgrading to it
with new springs, Cetech Subchassis, acrylic baseboard, and a used Ittok from
one of the PFM members that I sent right off to j7 for the foam and wiring
treatment for which he is so highly praised.

Speakerwise, I'm still with
my 1991 original Linn Index II's. I'm working on something I hope will be
better. If not, I'll be buying. How do you like the Kans?

Always loved
the ... impact of the Nait. Maybe that's the PRaT thing to which you and others
refer.

I quite agree with you, though, about how tubes is
different!

summilux

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Post n°5

Re:
How good can an ST-70 be made?

by summilux
on Thu May 07, 2009 1:58 pm

I LOVE the Kans (Mk I). Because I listen to music in a small room 10x12,
my 300B and ST-40 drive them pretty well and they are quite musical. Using them
with a tube amp helps tone down the brittleness they can have with solid state.
They are no match for my homebrew FOostex with respect to clarity, but sound
good nonetheless.

Jonathan

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Post n°6

Re:
Dynaco OPT

by Bob Latino on Thu May
07, 2009 2:09 pm

Jon,

RE: what you said above ....

"I am mulling about new
projects to take on and I am considering an ST70, mainly because I've been
reading some good things about its OPT. If I do this, I will probably be buying
the amp solely for the OPT. Likely, I'll put a new driver board in, change all
the connectors, tweak the PS and probably put in a new power transformer as
well."

The
only two really good things about the original Dynaco amps were the OUTPUT
transformers and the Mullard tubes. The myth that "that old iron just can't be
beat" is that - just a myth. I have listened to OPT from Dynakitparts and they
are every bit as good as the original Dynaco OPT. It is not that hard to take an
output transformer apart and "reverse engineer" the transformer noting stack
height, size of wire and winding technique and replicate that transformer and
have it sound identical to the original Dynaco transformer. You can buy a used
A-470 output transformer on Ebay for $60 when a new one goes for $90 but what do
you get? A transformer primary that has looked at 440 volts for maybe 50 years,
brittle wires that the strands break, cloth insulation that is hard to strip and
leads that are probably now too short because when "Bubba" removed it from the
amp he just cut the leads instead of desoldering them.

To buy an ST-70
for $300 - $350 just for the output transformers IMHO just doesn't make sense.
If all you want are just the output transformers then use the $300 and buy NEW
all THREE transformers and you will also get along with the two new OPT a far
superior POWER transformer that won't hum, buzz and vibrate like many of the
original undersized Dynaco PA-060 power transformers. It will also run cooler
because the new ones have about a 5/8 inch higher stack with about 2 pounds more
iron in there to dissipate the heat.

Bob

Last edited by Bob Latino on Tue Jun 02, 2009
12:09 pm; edited 1 time in total

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Post n°7

Re:
How good can an ST-70 be made?

by Tube
Nube on Thu May 07, 2009 5:26 pm

There's some of that expert advice I was talkin' about.

Correct me
if I'm wrong, Bob -- and I know I'm inviting you to offer a self serving
opinion here -- but fun as it may be to get an old Dynaco and rebuild it,
wouldn't the quickest and perhaps cost-effectivest (sic) route to a state of the
art ST 70 be to get one of your kits?

John, I'm interested to hear that
the tube jobs really smooth out the Kan speakers. I drooled over the Kans back
in the early 90's.

I'm interested to know what you built with the Fostex
drivers.

Brenton

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Post n°8

Re:
How good can an ST-70 be made?

by summilux
on Fri May 08, 2009 6:30 am

I used the Fostex FE127 PAWO plans at planet_10 hi fi. The not only sound
good but have a very good wife acceptance factor.

Bob
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Post n°9

Re:
How good can an ST-70 be made?

by Bob Latino on Fri May
08, 2009 8:00 am

Tube Nube wrote:There's some of that expert advice I was
talkin' about.

Correct me if I'm wrong, Bob -- and I know I'm inviting
you to offer a self serving opinion here -- but fun as it may be to get an
old Dynaco and rebuild it, wouldn't the quickest and perhaps cost-effectivest
(sic) route to a state of the art ST 70 be to get one of your kits?

John, I'm interested to hear that the tube jobs really smooth out the
Kan speakers. I drooled over the Kans back in the early 90's.

I'm
interested to know what you built with the Fostex drivers.

Brenton

Brenton,

As I have mentioned a few times on some of these posts about the only thing
you can be fairly sure of when you buy an old original ST-70 is that the two
output transformers are probably as good as what can be produced today. The rest
of the parts, although they were probably "good" for that era, in light of what
is available today, are only of mediocre quality.

1. The power
transformer was undersized.
2. The quad cap only has 20 uF to run the two
output transformers - way low ..
3. The driver board was made of an
inexpensive phenolic material that would turn black from the heat under and
around the 7199 driver tubes.
4. The carbon composition resistors throughout
the amp by now have probably drifted from their original value. Carbon
composition resistors also tend to get "noisy" after a while. What you think is
"tube hiss" in your amp may be coming from one or more resistors.
5. The tube
sockets were a cheap plastic material.
6. The bias system is crude by today's
standards.
7. The driver circuitry with the 7199 tubes was not linear at all
drive levels. As you ran the volume control UP on your preamp increasing the
amp's drive level > the amp's high frequency response would go DOWN.
8.
The stereo/mono switch only worked correctly if you ran a wire from one output
transformer to the other. (16 ohm to 16 ohm if you had 8 ohm speakers and 8 ohm
to 8 ohm if you had 4 ohm speakers) People would flip the switch to "mono" and
think the amp was in mono but it wasn't - what they had now was two 35 watt
channels carrying the same signal. You have to wire the two output transformers
together to actually have the entire ST-70 run in mono.

If you want an
ST-70 to have around for old times sake then get one off Ebay but IMHO this
shouldn't be the amp you play on a daily basis unless you take care to restore
or rebuild the amp.

Minimum things to do for reliability in order of
importance ..

1. If your amp has an original quad cap - replace it. A bad
B+ high voltage system can take out the rectifier and the output tubes. If your
amp has a low level hum that does not change with the volume control - the quad
cap is probably statring to go bad.
2. Replace the selenium rectifier with a
diode and change out the two capacitors and resistors on lugs 1-4 of the 7 lug
terminal strip. Your bias system is now much more reliable. A bad or unreliable
bias sytem can eat up output tubes.
3. Replace the 6 coupling caps on the
original driver board.

Optional for reliability and better sound
...

1. Replace the driver board with a modern driver board with all
triode circuitry. The amp will now SOUND better.
2. Replace the octal tube
sockets with modern octal sockets.
3. Replace the input jacks with modern
gold plated input jacks.
4. Replace the output terminal strip with modern
gold plated binding posts
5. (And this will cost you some money > $125)
Replace the power transformer with a new one if the old one mechanically
vibrates and/or has a buzz.

If you don't have an ST-70 and you want to
get one and PLAY THE AMP ON A DAILY BASIS it probably would be more cost
effective to build one yourself NEW from a kit. All the parts are new and most
kit vendors will warranty the parts for a year. The ST-70 in kit form is an easy
build. From 1959 to the mid '70's about 350,000 audio amateurs built the ST-70
from a kit. Many had never soldered anything before. Dynaco reported years ago
that only about 2 % of the kits were shipped back to them because the kit
builder couldn't get it to work correctly.

Bob

Last edited by Bob Latino on Tue Jun 16, 2009
3:12 pm; edited 2 times in total

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Post n°10

Re:
How good can an ST-70 be made?

by Tube
Nube on Fri May 08, 2009 6:40 pm

Johnathan,

Sent you a PM.

B.

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Post n°11

Re:
How good can an ST-70 be made?

by tubes4hifi
on Sat May 09, 2009 1:12 am

Hi Jonathan, Roy here, caught your question . . .
I started this
hobby/biz over 20 years ago, and during all that time, have bought, sold, and
rebuilt probably 100 hi-fi tube amps, and I know that Bob has probably done even
more than that.
Of course it's my opinion, and someone who doesn't know me
may think I'm biased, but IMHO
I would have to say that the amps Bob and I
sell would beat out 80-90% of what's on the market, and that includes the $5000+
spread. I'd say Bob's kit (with my VTA board in it) is an absolute steal at any
price from $600-800, and I've been told by several dozen customers that it for
sure beats out anything available for under $2500. I have a neighbor who's been
thru several AudioResearch tube amps the past half-dozen years and I even bought
one for myself to make a fair comparision, and it PALED!!! It was a ten year
old, not a new $4000 one, but I sold it fast for $1000 and haven't turned back.
Bob is also correct in saying a new one is a bargain compared to an older used
one. Several years ago I even offered a money back guarantee, never had anyone
take me up on it. That's hard to do on a complete amp with a low profit margin,
but on the kit's I was selling it helped to open up alot of disbelieving eyes
and minds.

Tube Nube

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Post n°12

Re:
How good can an ST-70 be made?

by Tube
Nube on Sat May 09, 2009 9:54 am

Bob, Roy,

You guys are gonna go crazy from insomnia, and / or
obsessive compulsive disorder from your meticulous attention to your customers,
potential customers, and forum-community members.

Don't get me wrong
here, I really appreciate your generosity here, and other members too, who
follow suit with friendly, well informed support.

But in the interests
of ensuring you guys are still gonna be here for me over the next 20 years
(since that's apparently how long I keep my amplifiers), I do hope you're taking
time for whatever takes the stress out of life for you.

If not, I insist
you get to it. Doctor's orders. And I don't want to hear any excuses.

;-)

Brenton

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