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

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    RockyAM

    Posts : 26
    Join date : 2008-12-14

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

    Post by RockyAM on Wed Dec 12, 2012 12:33 pm

    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
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    Bob Latino
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    Join date : 2008-11-26
    Location : Massachusetts

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

    Post by Bob Latino on Wed Dec 12, 2012 3:48 pm

    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
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    Tube Nube

    Posts : 668
    Join date : 2008-12-06
    Age : 53
    Location : Calgary, AB

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

    Post by Tube Nube on Wed Dec 12, 2012 6:07 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!
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    PeterCapo

    Posts : 421
    Join date : 2008-12-05

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

    Post by PeterCapo on Thu Dec 13, 2012 4:14 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
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    Bob Latino
    Admin

    Posts : 2543
    Join date : 2008-11-26
    Location : Massachusetts

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

    Post by Bob Latino on Thu Dec 13, 2012 8:15 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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    Tube Guy

    Posts : 4
    Join date : 2017-02-19

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

    Post by Tube Guy on Tue Feb 21, 2017 7:29 pm

    Agree with you comment on great sound.
    Recently constructed VTA-ST70 is Da Bomb. In the past, an EL-84 Eico and Pilot amp fan.
    The VTA with Blackburn XF2 output tubes, Tele Long plate phase inverter and RCA Cleartops works well for my ears.
    Modded Braun L810A speakers, Nothing fancy, no subs, not bi-amping, not necessary IMHO.

    Thx, Roger

    Ed Chambers

    Posts : 6
    Join date : 2017-06-20

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

    Post by Ed Chambers on Thu Jun 22, 2017 11:27 am

    I recently joined this forum after completing a " retro" system which includes Bob's excellent VTA -120 amp. I love it for the way it performs and its handsome looks. I have only a few comments about the chassis layout, which Bob retained. Why did the original Dynaco have the input jacks on the front and the on/off switch in the rear? Seems backwards, but it's certainly no big deal. Ed
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    Bob Latino
    Admin

    Posts : 2543
    Join date : 2008-11-26
    Location : Massachusetts

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

    Post by Bob Latino on Tue Jun 27, 2017 7:53 pm

    Ed Chambers wrote:I recently joined this forum after completing a " retro" system which includes Bob's excellent VTA -120 amp.  I love it for the way it performs and its handsome looks.  I have only a few comments about the chassis layout, which Bob retained.  Why did the original Dynaco have the input jacks on the front and the on/off switch in the rear?   Seems backwards, but it's certainly no big deal.     Ed

    By having the input jacks on the front, there is a very short run of wire from the input jacks to the driver board input eyelets. You need less than two inches of wire and the wire need not be twisted. If you place the input jacks on the rear, you have to have two long runs of wire around the sides of the amp to any input jack you may want to place on the rear face of the amp. Understand that long runs of wire to the input jacks exposes these input wires to picking up hum and noise from the power transformer. If you do run the wires to two input jacks on the rear face of the amp, you must either use shielded cable or twist the two wires to each input jack. Having the input jacks on the front face is not really a major problem. On my own VTA ST-120 I run the two interconnects around the sides of the amp to the rear and to my preamp. You can also run the interconnects UNDER the amp. The four chassis supports raise the amp up about 3/8 of an inch above the supporting surface.

    Power switch ... A few customers have used the unused stereo/mono switch on the front face of the VTA ST-70/120 amp as their power switch. They tell me that after they did this mod there was no increase in noise to the amp. Another thing that you can do is to leave the switch on the rear face of the amp ON all the time and plug the amp into a power strip that has a switch on it. You place the power strip on the floor near the front of your music system. Every time you close the switch on the power strip, the amp comes on.

    Bob


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