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Differences between the original ST-70 and my VTA ST-70 ?

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RockyAM

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RockyAM

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Join date: 2008-12-14

Post n°1

Differences
between the original ST-70 and my VTA ST-70 ?

by RockyAM
on Wed Dec 12, 2012 8:33 am

Hey Bob, I built your ST-70 kit like about 2 years ago. I run it every day
for a few hours. My wife, my son and I are constantly amazed at the great sound
from this amp. I did replace all the tubes early this year and I kept the old
ones for spares.

Last week my friend brought his original Dynaco ST-70
over here and we put the two amps side by side. Of course my amp looks newer and
the green driver board on my amp is not the same as the one on his amp. The
power transformer is thicker on my amp also. We played both amps one after the
other and even he agreed that my amp just sounded stronger and cleaner than his
amp. Now it probably wasn't a fair comparison cuz his amp is probably like from
the 1960's and he says that its all original. We both wondered what the
differences were inside the amps from his amp to mine besides what we could see
on the outside? RockyAM

Bob
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Post n°2

Re:
Differences between the original ST-70 and my VTA ST-70 ?

by Bob Latino on Wed Dec
12, 2012 11:48 am

If you place
the two amps side by side - yes you can see some but not all of the differences
between the original Dynaco ST-70 and the VTA ST-70. The design concept behind
the VTA ST-70 was to make the amp LOOK LIKE an original Dynaco ST-70 yet take
advantages of 50+ years of advances in electronics and have the VTA ST-70 SOUND
LIKE a modern tube amp. Efforts were also made to make the amp more reliable
than the original amp. Some things were not changed like the chassis layout. The
size and shape of the chassis and the parts layout of the VTA ST-70 is identical
to that of the original Dynaco ST-70. An original or aftermarket Dynaco tube
cage will fit this chassis perfectly. The silk screening of the Dynaco markings
on the front and rear face of the amp are also the same as on the original amp.
The OUTPUT transformers are the same size and shape and more importantly, are
wound the same way (interleaved/layer) as the original Dynaco A-470 output
transformers. The assembly manual for the VTA ST-70 is written in detail for the
beginner just like the original Dynaco ST-70 assembly manual.

The many
improvements over the original amp include ..

A. Chassis - The original
ST-70 chassis was made of 18 gauge nickel plated regular steel. Eventually most
of these chassis will develop rust spots. The chassis on the VTA ST-70 is made
of 16 gauge (.015 inch thicker metal) polished stainless steel and will never
rust. All the screws on the VTA amp are also made of stainless steel.

B.
Driver board and driver circuit - The driver circuit on the original amp with
the two 7199 tubes was done to "save a tube". In light of circuits available
today, the driver circuit can easily be improved upon. The original driver board
was made of an inexpensive phenolic material with open solder traces on the
bottom of the board. This phenolic material was not very heat resistant and the
board would many times turn black from heat under the 7199 tube sockets. The
open solder traces sometimes lifted from the board. The carbon composition
resistors found on the board would, after many years, stray from their original
values. The VTA board is made of epoxy/fiberglass and has no open solder traces.
It has plated through holes so that parts may be placed on either the top or
bottom of the board. It has an "on board" power supply using Nichicon low ESR
caps and an on board bias system that allows individual biasing of each output
tube. The new board also uses 1% metal film resistors that will not stray from
their original value over the years.

C. Power transformer - The original
ST-70 used an undersized PA-060 power transformer with a 1 1/2 inch stack that
usually runs pretty warm. The upgraded power transformer on the VTA ST-70 has a
2 1/8 inch stack, runs cooler, and has a much better power transfer
capability.

D. Choke - The original ST-70 C-354 choke was rated at 200
milliamps and would run very hot. Sometimes a "brown goo" would melt out of the
choke and deposit like candle wax on the bottom cover below. The C-24X choke in
the VTA ST-70 is rated at 240 milliamps and runs cooler.

E. Quad cap -
The 30, 20, 20, 20 uF quad cap on the original ST-70 has been replaced with an
80, 40, 30, 20 uF cap on the VTA ST-70. The total capacitance on the original
amp was 90 uF. On the VTA ST-70 the total capacitance is 464 uF.

F. RCA
input and output binding posts - The original ST-70 had aluminum RCA input jacks
narrowly spaced at 3/8". The VTA ST-70 has gold plated input jacks spaced at
9/16" and will accept modern thicker interconnects without the interconnects
touching. The output binding posts on the original amp were simple screw
terminals. The VTA ST-70 has modern gold plated binding posts which may be set
up for 4, 8 or 16 ohm speakers.

G. Wire - The original amp kit was
supplied with 22 gauge solid core PVC hook up wire. The VTA ST-70 amp kit is
supplied with 20 gauge (one size thicker wire) tin coated solid core irradiated
PVC wire. The tin coating makes for easier soldering and the irradiated PVC
insulation is a better insulator than the insulation on the wire in the original
Dynaco kit.

H. Triode/ultralinear operation - The original ST-70 ran in
ultralinear mode only. The VTA ST-70 runs in either ultralinear mode or triode
mode. The change for each channel can be made at the flick of a switch while the
amp is running.

I. Other optional items - The original amp had no options
like the VTA stepped attenuator to allow you to use the amp directly with high
level signal sources or VTA TDR (Time Delay Relay) which is a good addition if
you use solid state rectifiers.

Bob

Tube Nube

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Tube Nube

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Age: 51
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Post n°3

Re:
Differences between the original ST-70 and my VTA ST-70 ?

by Tube
Nube on Wed Dec 12, 2012 2:07 pm

Bob, you might consider this thread for. Stick-post!

A couple things
I think Bob elected to leave out warrant attention. Bob offers some upgrade
options, such as the Russian paper-in-oil capacitors, as well as a few tube
upgrade options.

You wouldn't expect Bob to emphasize this one, but
customer service is stellar and consistent, which is vital for anyone who is the
slightest bit squeamish about rolling their own from a kit. Bottom line: Don't
worry, be happy--and save thousands while doing so!

Wandering a bit
further a field, but still relevant for mentioning, this forum is a source of on
going support, education and cammeraderie. Spelled that wrong I think. Doubt
I'll get flamed for it here, though!

PeterCapo

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PeterCapo

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Post n°4

Re:
Differences between the original ST-70 and my VTA ST-70 ?

by PeterCapo
on Thu Dec 13, 2012 12:14 am

Bob,

When you sell the amp fully assembled, do you still include a
copy of the complete assembly manual along with other documentation like the
schematic, voltage chart and so forth?

Do you accommodate special
requests, for example, to build one with a particular brand of resistor or
capacitor, etc?

Thanks.
Peter

Bob
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Post n°5

Re:
Differences between the original ST-70 and my VTA ST-70 ?

by Bob Latino on Thu Dec
13, 2012 4:15 am

Hi
Peter,

A copy of the VTA ST-70 assembly manual is not included with a
wired amp. Those getting a wired VTA ST-70 receive a smaller 6 page users
manual. A schematic and voltage check chart is included with the users
manual.

I don't accomodate special requests for resistors, capacitors
etc. on a wired amp. If the customer wants special parts in their amp they will
have to build the amp themselves from a kit and make their own part
substitutions as the kit is being built.

Bob

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