mod st70 sound?

[K-MAN]

I was wondering considering all the great mods people have come up with for the st70 over the years how they sound compared to the original? It seems like the design has been pretty much tuned and tweaked as much as it can be since they have been around for almost four decades. I see there's allot of different tube combos people use as well, does that make a significant change to it's sound or more subtle? I've been looking for another dynaco since I've finished my SCA35 and I cant pick between one of the kits for a SET amp or buying a st70 that needs work and installing some of the upgrades.

[kheper]

To keep the price of the st70 kit down, the
engineers cut corners with the drive circuitry.
It had very serious flaws. The board is made
of cheap phenolic and degrades over time.
The sound of an un-modded st70 is usually
described as muddy; This muddiness is, no
doubt, the main reason for the existence of
so many after-market boards.

The capacitance in the original design of
the power supply is way too low, which
further contributes to the muddiness. Newer
cap-boards, with discrete electrolytics, make
up for this, allowing the bass to be truely
punchy and precise. The rectifier tube is
operating at about its maximum current
rating. So, by using solid-state rectification,
the capacitance can be taken even higher,
than with the cap-boards.

If the iron in the st70 was not sufficiently
excellent, to hold up over time, the st70
may have passed away unnoticed.

[Shannon Parks]

I was wondering considering all the great mods people have come up with for the st70 over the years how they sound compared to the original?

My opinion is that the original design performs well and sounds great. It's biggest problem is that NOS RCA 7199s are pretty scarce and expensive. I haven't messed with the supposed 7199 subs as the mod boards add a needed upgrade to the phenolic original (as kheper mentions) to keep the ST70 rocking along for another 40 years.

[Sal Brisindi]

I just recently finished restoring my ST-70 and to tell you the truth, It sounds absolutely awesome to me. My ST-70 still has its original circuit board, I did replace the caps on it. Now I am not an audiophile but I do listen to music. If it sounds to me, then I am happy.

Regards,
Sal Brisindi

[TerrySmith]

IMHO, the ST70 is kinda like a '57 Chevy, meaning you can buy every part and build a brand NEW ST70. Some parts are better, like the chassis from Dynakitparts and Soundtastick, and some not as good- like the repro Mullard and Chinese EL34.

Over the years there have been many different driver boards, original replacements, upgrades, and even a solid state board! Some look and fit like a science project, and a few fit and sound very nice.

[K-MAN]

Is it cheaper buying a junky st70 and salvaging the iron for a new one or to just buy new transformers online?

[kheper]

You can usually buy two, used A470s on
e-bay for abound $125. New dynaclone
A470s are $170. The current production runs
of the PA-060s are superior to the old ones,
and cost $100. If you only need transformers
and have no desire to salvage the chassis
and other parts, it may be worth your while to
go the parts-route, rather than the
salvage-route.

[Uncle Ned]

To keep the price of the st70 kit down, the
engineers cut corners with the drive circuitry.
It had very serious flaws. The board is made
of cheap phenolic and degrades over time.
The sound of an un-modded st70 is usually
described as muddy; This muddiness is, no
doubt, the main reason for the existence of
so many after-market boards.

I would attribute the "muddy" sound to the cheap components
degrading over time. ST70's sold for $99 list and as little as
$79 on sale at EJ Korvette's, so Dynaco wasn't going to pile expensive
parts in it. Bear in mind that the ST70 had plenty of competition
at the time it was made and still was and is the best selling
hi-fi tube amp of all time. Most of the people who bought them had the
opportunity to listen to other amplifiers, Korvette's, Allied, etc, did
have demo setups.

I would guess that Korvette's, Allied Radio and Muntz Stereo
together probably accounted for 75% of Dynaco sales both
tube and solid-state. None of those outlets were exclusive Dynaco
outlets.

The capacitance in the original design of
the power supply is way too low, which
further contributes to the muddiness.

This is typical of 1940's/50's amplifiers, where engineers were trained to
calculate minimum decoupling values in power supplies.

And can capacitors with a few exceptions really weren't
any great shakes, either. The idea of cramming multiple capacitors
in one can was to save money, not to make a better capacitor.

[Bob01605]

Is it cheaper buying a junky st70 and salvaging the iron for a new one or to just buy new transformers online?

About the only thing still good on most older Dynaco ST-70's are the OUTPUT transformers. Most of the other parts, as has been mentioned above, were made to sell at a price point and as such were not designed for long term durability. Sure, the Mullard output and rectifier tubes were great also but on most older ST-70's they have long since been replaced and are rarely found on an older ST-70 you may pick up on Ebay. The POWER transformers were undersized and many of the old ST-70's I see are beset with mechanical vibration, buzzing etc. This is not to mention that the two heater legs were only rated at 3 amps per leg (6 amps total)

IMHO it is not worth the effort to pay $300 for an older junky ST-70 when about the only good thing you can count on are the OUTPUT transformers. Newer output transformers from Triode or Dynakitparts are well made and sound identical to the older A-470 output transformers. For the same $300 you will pay for an old ST-70 you can get a new POWER transformer AND a new set of OUTPUT transformers. Get a new chassis also and start fresh.

I hear all the time on the net the "myth" that those original "cloth lead" Dynaco A-470 output transformers are so "desirable". What they REALLY mean is that in all the pile of mediocre parts that the ST-70 was made from, those original A-470's (as well as the original Mullard tubes) are the only parts that can be counted on and should be salvaged on a rebuild. I don't know how Triode Electronics went about in creating their "DynaClone" transformers but I have had a number of conversations with the owner of DynakitParts. He mentioned to me that their A-470 was completely "reverse engineered" by taking apart an original A-470 and then using the same sized and same length of wire and using the same interleaved layer winding technique. They also have the same M6 grain oriented laminations. I have used over FIFTY of the DynakitParts A-470's in the past year and found them to be of uniformly excellent build quality. Their sound quality also (as has been mentioned above) is every bit as good as the older original A-470's.

Bob Latino

[TomMcNally]

Let's keep in mind that the components Dynaco used back
when the ST-70 and MK-III amps were first built were
pretty much the best available at the time ...

The following had not been invented yet:
fiberglass PC boards
gold RCA jacks
1% metal film resistors
gold plated ceramic pcb mount tube sockets
IEC power connectors
boutique audiophile capacitors
silicon diodes were barely on the market

So I must disagree that the Dynacos were cheap
junk ... there wasn't much better available.
The big advantage I see to the Dynaco kits
vs factory assembled brands, was the build
quality was better for a couple of reasons ...
nuts and bolts were used rather than rivets,
and generally, factory workers rushed through
the build because they got paid by the piece.
Much of the old tube equipment is a nightmare
to work on, because it's a mess under the chassis.

I take care of a 1962 vintage RCA radio transmitter,
which was THE BEST in its day ... and yeah, it has
the phenolic pc boards with the same type components
Dynaco used. State of the art back then.

[Bob01605]

Let's keep in mind that the components Dynaco used back
when the ST-70 and MK-III amps were first built were
pretty much the best available at the time ...

The following had not been invented yet:
fiberglass PC boards
gold RCA jacks
1% metal film resistors
gold plated ceramic pcb mount tube sockets
IEC power connectors
boutique audiophile capacitors
silicon diodes were barely on the market

So I must disagree that the Dynacos were cheap
junk ... there wasn't much better available.
The big advantage I see to the Dynaco kits
vs factory assembled brands, was the build
quality was better for a couple of reasons ...
nuts and bolts were used rather than rivets,
and generally, factory workers rushed through
the build because they got paid by the piece.
Much of the old tube equipment is a nightmare
to work on, because it's a mess under the chassis.

I take care of a 1962 vintage RCA radio transmitter,
which was THE BEST in its day ... and yeah, it has
the phenolic pc boards with the same type components
Dynaco used. State of the art back then.

Tom,

Yes - but the question asked was "Should I buy an old ST-70 for the transformers OR buy new reproduction transformers" and the answer is IN RELATION TO WHAT IS AVAILABLE TODAY the quality of parts on an older ST-70 is not good. True - a lot of what you mention above was not available in 1959 when the ST-70 was made. But - Why should someone buy a pile of "old technology" just to get a pair of output transformers when you can get equal quality iron without having to deal with pulling an old amp apart. If ALL YOU WANT are three transformers and you have to pay $300 for a complete older ST-70 on Ebay > why do it? Just buy three new trannies for the $300.

Bob Latino

[EWBrown]

I'd probably go with all new iron, the "old junker" ST-70s on e-bay are a crapshoot. The better looking ones get big bucks, the grundgy rusty, crusty and musty ones can be cheap, but the iron can be shot, too...

One little "trick" I've used on e-bay, which takes some patience, is to buy single OPTs, they usually go for a considerably lower "unit" price than they do as a pair.

Also look for what are obviously A470s, but which have a different "house number". THis may take a bit of detective work, but it can be worth the effort.

Several years ago, I bought a NOS Dynaco pair that came from a Hammond Organ repair shop in Arkansas, for considerably less than half of the usual asking price for a pair of used A-470s. They were unused, and had all the correct impedances and lead colors, just nothing indcating "A-470" on them, and they have been serving exceptionally well in an ST-70 built of all new parts (except for those two NOS trannies and old chassis). Supposedly Hammond never used Dynaco iron in their products, but the shop had them in stock for some reason...

This amp, has the two "organ" OPTs, a Triode PA-060 and C-354 choke, the SDS cap board, the VTA-70 driver board (this was obtained well before Shannon's board was created), and only the chassis is the only original "Dynaco" ST-70 part. The OPTs, are Dynaco but not specifically ST-70 listed iron.

A couple years ago, used SCA-35s went for considerably less than usedST-35s (working or as carcasses) on E-bay, then someone finally figured out that the iron is the same thing, so the bargain prices evaporated.


/ed B in NH

[SDS-PAGE]

Can't always generalize that cheaper ST-70s on eBay are crappy. As with all other eBay finds, it actually is possible to get a decent ST-70 every now and then. And yes, patience helps. I got mine for around 300 bucks. Irons, chassis, filter caps and other misc. were in great condition. Beside I ended up eBaying the original board with tubes + octals for around 40 bucks, so it was an added bonus. I probably spent around 150 bucks on DIY mods, so I got mine done a whole lot cheaper than if you got everything brand new. Besides, there is something to be said about original Dynaco irons.

[Bob01605]

Can't always generalize that cheaper ST-70s on eBay are crappy. As with all other eBay finds, it actually is possible to get a decent ST-70 every now and then. And yes, patience helps. I got mine for around 300 bucks. Irons, chassis, filter caps and other misc. were in great condition. Beside I ended up eBaying the original board with tubes + octals for around 40 bucks, so it was an added bonus. I probably spent around 150 bucks on DIY mods, so I got mine done a whole lot cheaper than if you got everything brand new. Besides, there is something to be said about original Dynaco irons.

Re: Your comment "Something to be said about original Dynaco irons". I wouldn't overgeneralize about "original Dynaco irons". The PA-060 POWER transformer on the original 70's is not a good start for a complete rebuild which was what the original poster was attempting to do. After 40+ years or service many if not most of the original PA-060 power transformers have some issues like mechanical hums, buzzing, vibration etc. Would you put a 40 year old engine in a new car? Probably not ? That power transformer is the "engine" of that amp.

The A-470's on the old amps are probably quite serviceable and I wouldn't hesitate to use an original A-470 OUTPUT transformer on a new build. A statement like what you made perpetuates a "myth" that IMHO is not valid considering the fact that both Triode and Dynakitparts make NEW A-470's which are the equal of the older "desirable cloth lead" A-470's AND the replacement PA-060's that both companies sell is a MUCH BETTER POWER TRANSFORMER than the original PA-060.

Yes - I guess there is "something to be said about the original Dynaco irons" and IMHO it is this > The original PA-060 POWER tranny should not be used in a brand new build but the A-470 OUTPUT transformers are fine to use if they have been found to work OK in an older amp OR can be had at a good price.

Bob Latino

[SDS-PAGE]

The original PA-060 POWER tranny should not be used in a brand new build but the A-470 OUTPUT transformers are fine to use if they have been found to work OK in an older amp OR can be had at a good price.

Actually, I was referring to Dynaco OPTs, not PA-060s. I should have been clear on that. You need to look at what you want in a ST-70 before deciding to go w/ building new or restoration. As for me, if I can get a good sounding amp instead of spurgin' a small fortune buying everything new, I have achieved my objective. Others might prefer do it otherwise. It's whatever makes you happy. Is there a difference between two methods? Too many variables (hence the "myth") to make an assumption. I know what I need and want, so I don't have to make any assumptions.