mod st70 sound?

[EWBrown]

I even have my doubts about re-using old PA-060s andPA-774s, I've had some that, even though they still worked, they had that oh, so aromatic unmistakable "burnt transformer" smell, and some of the really old ones were tar-dipped, probably in an attempt to keep the lams and windings from buzzing, and they are generally in the condition of having a "near death experience" by the time I get 'em... Those get the old heave-ho, the other less smelly and sticky ones get re-used in "light duty" assignments.

The other consideration was that typical US AC line voltage was closer to 115VAC in the 50s and 60s and is at least 120VAC, and up to 128VAC in the 21st century. So an old tired and semi-toasted power trannie will have a short service lifetime on today's higher primary voltages.

/ed B in NH

[opc]

Hi Guys,

I know this thread has been dead for a little while now, but I think there's a very important and fundamental point that's missing from the whole "new parts/old junker" argument.

The ST-70 was the very first tube amp I ever worked with, and it was back in the day when I didn't have the skill to build an amplifier from scratch. When you say that it's better to buy new iron than to buy an old original, you're completely missing the fact that when you buy an old Dynaco, you're buying a complete tube amp, and not just three transformers. If you do the math, the truth really starts to come out.

The "New Parts" ST-70:
Output XFMR's: $140
Power XFMR: $100
Chassis (TE): $200
Choke: $30
All new parts: ~$160
Tubes: ~$100
Hardware: ~$30
Driver Board: $30
Cap board: $30

Total: $820

I bought my first Dynaco on eBay about 8 years ago for $248 and it was in fully functional condition, but missing the two 7199 tubes. I bought both of those along with a pile of Holco resistors and some Sprague coupling caps for $70 So I had a fully functional ST-70 for less than half the price of the new parts option.

Now, for a beginner like I was, having an already finished amplifier I could listen to was a priceless advantage. Sure it didn't sound as good as the new parts would have, but I learned a huge amount in the process, and I continued to upgrade parts as I learned more about tube amplifiers and what the weaker points of the ST-70 were. It's this whole experience that makes the ST-70 special amongst other tube amps.

Now, 8 years later, the Dynaco has long since been replaced with a whole slew of different, and generally much better, amplifiers. If you're looking for a really good tube amp, I'm not sure the Dynaco is really the best choice. For the $820 in new parts, I think you'd be much better off building a different circuit all together. Look into something single-ended, or try a more exotic tube like the 6C33C. The world is your oyster if you have $1000 and want to build a tube amp.

If you want the "Dynaco Experience" though, then in my opinion, the only way to go is to buy an original Dynaco. Not because of the cloth lead transformers or the original tubes, but just because it's the only choice that really makes sense. It's a tube amp on a budget, and it's the perfect learning platform. If you take those things away, all that's left is a dated design from the 50's and a mediocre set of iron. In the end though, had it not been for the ST-70, I wouldn't have the love of tube amps I have today, and I wouldn't have the knowledge of tube circuits gained from the time I spent working with the Dynaco.

It's like owning a classic British sports car, you don't start from scratch and build one from the ground up with all new parts. That takes an immense amount of knowledge, effort and tools. You find an old beater, restore it, and live with its faults and quirks as part of the experience. If you're looking for an every day commuter, or something to use on a race day, you'd be better off spending your money on a Toyota Corolla or a Radical, it'll be faster and more practical. Cheaper too, in the long run.

Anyhow, just some food for thought...

Cheers,
Owen

[crispycircuit]

Unless you can see and listen to an old ST 70, your taking a big chance. Most of the cloth covered wire tranny wires are brittle and stiff as a rock and break very easy when moved. The power tranny is undersize. They can vibrate up a storm shaking the whole chassis making you set it on soft feet so the shelf you have it on doesn't radiate 60 hum to the shelf. The chassis are always pitted or rusty. So to me new is best bet......

[Ty_Bower]

I bought my first Dynaco on eBay about 8 years ago for $248 and it was in fully functional condition, but missing the two 7199 tubes. I bought both of those along with a pile of Holco resistors and some Sprague coupling caps for $70 So I had a fully functional ST-70 for less than half the price of the new parts option.

Yes, but that was 8 years ago. Have you seen the going price for a ST-70 on eBay these days?

[GrooveGrinder]

IMO, it depends upon what you're trying to do.
If what you want is a learning project or something
to tinker on endlessly, go with a crusty oldie.
If, instead, what you're after is a good working/sounding
amplifier.. new/clone is best.

Having just completed one rebuild, and just
starting another,
(had 2 70s since the late 70s) I have some
strong feelings about the matter.
If I had to do it over again, I'd do differently.
Given that I would like to have a good working
70, I'd sell the 2 oldies and buy a new clone kit.
I found the condition to be THAT bad.

By now (2008), virtually ALL the chassis are nasty/filthy
and rusty/pitted. (to one degree or another)
Connectors, wiring, switches/pots, pwr cord
are worthless... or need a lot of work/time.
The board isn't worth attempting to rebuild..
at least most ppl feel.
Its almost universally recommended to change
the tube sockets... since pitting/tensioning can
cause tube-runaway and the like... not to mention
the effect that has on sound....
Bias supply needs rebuilt.
Can cap replacement is almost certainly a must,
and new crummy can caps cost 40 bux.
Those "very desirable" cloth covered transformer
leads are very likely to break off in your hands
when you go to unsolder them.
After they break off, the short portion will rain
powder on your bench causing you to examine
them.
The only thing you'll then notice, is that the copper
is green.
The PA060 is current-limited, killing any hope
of headroom, and really won't like your line voltage.
Ugh.
Nope... I wouldn't do it again.
Not when there's new and at least a bit improved
available... And for VERY reasonable price

By the time you get done fixing all that needs to
be fixed.. you're in for SO much, it doesn't
make sense to my mind.
You'll still have a dirty, nasty oldie with new problems
that'll come up as you're going along...
Sometimes causing failures that take out
expensive parts.

Do what, for example?
... clean and shine the chassis and leave the old
rusty wiring/connectors/sockets?
Try cleaning the original board...
It doesn't come clean. Try cleaning around
the 7199 sockets.
So... nice clean (relatively) shiny chassis with
filthy circuit board?
Hey.. if that works for you, gawd bless......

If you happen to have an old 70 already in your
possesion, that changes it a bit.
*Might* be worth re-doing, but the parts cost
is so high, why not dump the oldie and get the
new shiny chassis and irons.... the only things
worth salvaging.
Unless you have some fetish-like desire for maintaining
a museum piece. (more pwr to ya.. well less actually)
Yeah yeah, I know... the original Mullard tubes.
How many 70 out there still have them?
How many of those don't have thousands of hours
on them?
Oh sure... there might be a handfull...
That you can count on one hand.

7199s...
Parts Express is the only place (that I know of)
that still lists them as a catalog part.
(2008 catalog)
JAN $45.00 per tube.
Other places that happen to have a few left in
stock, usually get $75.00.
I think thats for a pair of Sovtek, but I'm not
certain.
Sovtek, the only new producer, has ceased production
permanantly, btw.

I could go on and on...
(as you can see :-)
--------------------

No... not $820.00.
$585.00!
A heck of a fair price, I think!
A heck of a value too, considering the hundereds
of hours along with the $$$ to rebuild an oldie.
The same basic experience as building an original
back in the day as a bonus.
Brand new, top quality parts too.


"Dyna-Clone ST-70 Tube Amp Complete Build Kit"
http://store.triodestore.com/st70buildkit.html

All the nasty no-longer-used ugly holes plugged
up.. cap board, so hole for can cap gone...
Punched for modern connectors..

Heh..
To me, its a no brainer!
But thats just me...

Bear in mind the original poster's question.
He wanted to have a good working amp for a reasonable
price and was considering other alternatives
to build from new.
Not a perenial maintanance project or a scrounging
adventure.

I always tried to keep my 70s in good condition.
Never abused.
The rust set in anyway.
Even kept after 'em, trying to keep them running.
Up to a point... which came when there was just
to much wrong.
To unpredictible too... ie: was it the can cap
or tube sockets causing the random EL34 flareups,
and 5AR4 flashovers?
Or maybe the bias supply...
Maybe the Selenium..
..or maybe all of the
above.
To my mind, a total rebuild was called for.

Even right now, I'm struggling with the second
of my 70s...
Its all apart.
Wire-wheeling/painting the irons, before heat
shrinking the leads.... knowing the PA060 isn't
gonna cut it, for my needs.
I'm at an impasse with the chassis.
It had a fair case of "cinnamon-sprinkle-itis.
I've cleaned it up as best as I can... many hours
worth, and it still looks bad
Very hazy and the pits come thru.
Half of the writing is gone.
All the time/cash I'm about to put into it...
it needs to look better than that............
$100-150 to re-plate?
Then, no writing and no original value...
Doesn't make sense.
Slap it back together, spending the minimum
then dump it?
That sounds better but what a shame.

I don't really need 2 70s.
For my amplification needs, I'm better off with
one and a pair of (posidon) MKIIIs.
I'd like to do a GSG someday too.
That'd take care of all of what I want to do.
So either way I will get rid of the second one.
Maybe the first too, eventually, in favor of an
up to date clone.

What should I do about a board for the second
70? (current project)
Since I'm going to dump it as soon as its running,
I want to spend as little as possible.
Even with Shannon's tremendously reasonable board
price.. it'd be nice to spend less.
Should I attempt to re-do the original?
Thats probably beyond my skill level.
Thats my current dilemma.

No....
I wouldn't do it again. No way.
Not when you can have new and improved for
a but under 600 bux.

Thats assuming your purpose is to have a good
working amp, thats very decent sounding and modernized,
for half of a thousand dollars.

I CERTAINLY would not buy ane oldie for upwards
of $350!
Not unless there were nothing new available
except for some parts...
and if they still cost ca $100 in working condition.
Then it might make sense to buy 2 or 3 for the
parts
....make 2 working units and keep the best
of them.
But hey... thats just me.
Whatever floats yer boat.

10 or 15 yrs ago, sure... going the other route
made sense.
But there coms a point where everything is
just to old.
IMO we've reached that point now.
Particularly since there's a new option and its
reasonable $$.

The british sports car analogy doesn't hold up.
IF you could buy a complete kit...
and IF it cost 10% more than a nasty oldie plus
needed parts..
and IF that kit yeilded a not very flawed with
decent iron result...
and all that for 600% less than comprable
preforming commercial units, and 400%
less than comprable kits....
Then it would hold up.
Those numbers are probably wrong but it makes
the point
____
End Of Rant
Yellow_Light_Colorz_PDT_09
Apologies for post length!
>donning flame retardant nomex long johns now..

[Wolf012558]

The price you gave above for the Triode Electronics ST-70 kit at $585 is without tubes. If you add in another $75 - $100 for tubes then you are at near $700 for the Triode kit which IMHO is not as nice as the kit that I got from Bob Latino last October at $695.

The chassis offered by Triode is a non standard chassis with no quad cap cutout which forces you to use and under chassis quad cap replacement board. It also has no silk screening of the Dynaco lettering and looks kind of "bald" to me. Bob's kit uses a standard chass that is identical to the old ST-70 chassis but like the Triode chassis is made of stainless steel. The Triode driver board has no provisions for measuring bias of each output tube individually. The VTA driver board on Bob's amp lets you adjust the bias on all four output tubes individually. Bob's kit also allows the amp to be operated in either the normal pentode ultralinear mode OR with the flick of a switch in triode mode. Triode's kit as supplied has no provisions for alternate triode/pentode operation.

Charlie Wolf

[burnedfingers]

In my opinion it all depends on what you want to end up with and how deep the pocket book it.

First of all I would have to disagree when it comes to output transformers.
Having both the cloth lead and the newer Asian version of transformers I can hear no difference between the two amplifiers. The frequency response is the same and the distortion figures between the different amplifiers run the same.

Given the age of the dynaco parts one should expect to take the transformers apart and solder on new leads and heat shrink over the solder connections. .....Problem solved.

The power transformer is fine unless one has the desire to pop in some 6550's for example. Then the transformer is not large enough to supply the additional filament current requirements. Solution... Install the larger Dynaco replacement transformer.

Cap problems are frequent and do require a solution. I've used two of the replacement cap boards now on both of my Dynaco's and believe they are the only logical solution to the problem. This also allows one to replace the bias rectifier at the same time.

Granted most Dynaco chassis have seen better days. Some people strip the nickle plating off and refinish with an automotive paint and others bite the bullet and purchase a new SS chassis.

Almost forgot about tubes.... the 7199's are getting expensive and a good and logical idea might be to modify the driver board to accept the 6GH8's that are available at lower prices. I just finished modifying one of my Dynaco's to use them. I can tell no difference between the 7199's and the 6GH8's. Another solution would be to replace the driver board with something in a newer design. I currently have a Mapletree driver board waiting on my shelf to be installed soon.

Granted there are other amplifiers that will deliver better sound/power but the cost is much more than putting a few dollars into a somewhat aged Dynaco.

Having owned 5 pairs of Quicksilver mono block amplifiers I thought they were some of the best sounding amplifiers made but after owning (3) Dynaco Stereo 70's that I have modified over the years I have come to realize they have a lot to offer. Dollar for dollar they are in my opinion worth the effort to rebuild and enjoy.

[burnedfingers]

The VTA driver board on Bob's amp lets you adjust the bias on all four output tubes individually.

Just for the record I used the VTA board on my first Dynaco and it is a great board. I would also like to point out it isn't hard to install two more pots and issolate the resistors on the driver board so that you can have individual tube bias on any amplifier. My other two Dynaco's have the added bias pots to allow for individual tube bias and the modification took less than 1/2 hour

[EWBrown]

As I remember in my College Days, of 1967-68, an ST-70 made a very efective dorm room heater.

Given that the on-site central forced hot water heating usually ran around subtropical temperature levels anyway, one could leave the window half-open even during the bitter coldest winter days, even without the additional BTUs generated by an ST70.. ...

Since there were (not too-well-known) under ground "service" passages between buildings, one could get away comfortably wearing T-shirt and shorts all year long, and avoid the bitter cold winter altogether Yellow_Light_Colorz_PDT_03

IIRC, in 1968 an ST70 kit was around $100, and a factory built unit was around $140, more or less. In 2008 dolarettes, $ 585 for a complete build kit is a bargain, as related with the four-decades old prices, given the economic changes (AKA the Shrinking Dollar) over the years

/ed B in NH

[Brinkman]

You can't put a price tag on the online tech support available to someone building an ST-70. As a learning tool, the ST-70 is unparalleled in this respect, and I think this should be factored into the equation depending on your skill level.
I hadn't ever soldered until I bought my ST-70 kit, and now the components in my home stereo set-up are all beginning to be replaced by upgraded and modified DIY kits. The pay-as-you-go nature of revamping an ST-70 is also highly favorable to those with a budget.
The wealth of info and support for those building and modifying an ST-70 makes it easy to succeed, and the sense of accomplishment is incredibly gratifying.
Of course some have enough spare cash to buy a pre-built, pre-modified ST-70 to check out the "ST-70 sound", and from that point of view, the ST-70's strengths and weaknesses can be judged accordingly. I would strongly recommend Bob Latino's ST-70 to anybody wanting to go this route.
I've had a couple good friends listen to music from my ST-70 and then start asking how much it costs to build one, could I build them one, etc...
Honestly, I would probably build it for free. They're THAT fun.

-Brinkman

[TomMcNally]

from the 1965 Lafayette Catalog

I read someplace that one of the early Dynaco factories was located close the the campus of Drexel University in Philadelphia ... and the "wired" versions were often build by a team of students working by the piece. So the factory wired versions aren't always the best, since many of them had never soldered before taking the "job" ...

[crispycircuit]

Tom is correct. The parts used by Dyna at that time was high quaility. The same as others of that time period, good stuff..... Tranny's don't last forever. The thin varnish coating on the windings tend to age poorly and short out when the power supply comes up to 450 volts or so. This can cause all kinds of problems like out of balance windings, etc. So new trannys are a safe bet and will be around in 30 yrs or more. I assume you don't want to keep repairing your project. Now you have many opinions and can choose your poison.......

[Ty_Bower]

Tranny's don't last forever.

I've heard some people say the same thing, but I usually hear more people say the opposite. I always figured there was nothing to wear out, and unless they were overheated or physically damaged they should be good for a long long time.

[crispycircuit]

What happens is the very thin coating winding wire (varnish or similiar) breaks down. Age is a concern. Then if the tranny was abused? For instance a shorted output tube drawing huge current through the primary. Or just the heat of running it for long periods, damp basements, hot attics, where ever it was stored can have a nasty effect. Some trannys are wound on paper bobbins and the paper absorbs moisture and corrodes the inner windings first. With all the newer better materials we have these days, the new stuff should last longer and sound better. I don't know why someone would want to pay big money on Ebay for a tranformer at the end of it's life. Many of these problems don't show up till 450 volts of B+ applied and some current is being drawn. It's depressing to have spent a great deal of time building your dream amp and have problems or poor sound... smoke is worse.... anyway that's my 2 cents

[burnedfingers]

Quote Tranny's don't last forever.

Well, I've never had a lot of problems with old power transformers and some I have date back to 1928. I've had a number of old radios and amplifiers and only had one transformer failure.

Sure there is a possibility that one might have a problem with an old transformer and also the same chance with a new one. If it ain't broke don't fix it. If you have the money to throw at a new transformer then go for it. Personally I don't think its good advice to try to convince someone that they need a new transformer based on the age of the used one.