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    Noobie re-building an ST-70

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    cc43fan

    Posts : 29
    Join date : 2012-01-30
    Age : 53
    Location : Upstate New York

    Noobie re-building an ST-70

    Post by cc43fan on Sun Feb 26, 2012 10:10 pm

    Hi,
    I'm a relative rookie when it comes to vintage restoration (my first project was restoring a Fisher 500-C). I'm in the process of gathering the parts and info to restore a ST-70. My homework had led me to purchase a VTA driver board. I'd like to hear opinions on the parts list for this board. For example which caps would most use for the coupling caps? Would some use various types of caps throughout the board? The goal is for my choices is to produce the best sound quality.

    My research has also led me to think going with an SDS power board will offer more benefits over replacing the multi-cap can (better bass response, longer life, less stress on the power transformer). I've done my best to price out the SDS board and parts separately, and it appears to make sense to purchase the complete kit vs. sourcing the parts myself. I've also read the thread on the "tube rectifier diode mod" and it appears this applies to those using the SDS board, am I correct?

    Could those in the know please comment on my thinking? I've been pouring over the threads here, and if my questions have already been answered, I apologize for making more work, but my head is swimming from trying to absorb the knowledge here.
    I'm really looking forward to this project, and if all goes well, I'm expecting this amp to be the daily driver. I would enjoy posting pics of my progress if anyone would like to see.

    Thanks to all in advance for any and all thinking, and I hope my project will add value to the Dynaco forum!
    Chuck

    stewdan

    Posts : 160
    Join date : 2010-03-07
    Age : 78
    Location : Houston Texas

    Re: Noobie re-building an ST-70

    Post by stewdan on Sun Feb 26, 2012 11:44 pm

    Hi Chuck -- Welcome aboard!!

    In response to some of your questions --

    The Rectifier Diode Mod is applicable to the Rectifier and is not dependent on the Quad Cap or the SDS Cap Board. It just provides more security for the rectifier by pre-filtering the voltage to the tube.

    Some people replace the coupling caps with Spraque Orange Drops, some with Russian Paper-in-Oils, some with exotic caps. It is all a function of what you hear. The Orange Drops are inexpensive, the Russian PIOs a few bucks more per cap and the exotics --- well, the sky is the limit. I have used both the Orange Drops and the Russian PIOs and I prefer the PIOs. After break-in they are extremely smooth and quiet. There was a post the other day and they talked about the Russian PIO caps. Take a look at the posts.

    On a side note, what restoration did you do to your Fisher 500-C? I also have one.

    Stew

    heyraz

    Posts : 58
    Join date : 2011-10-26

    Re: Noobie re-building an ST-70

    Post by heyraz on Mon Feb 27, 2012 3:24 am

    Morgan Jones authored 2 books, "Valve Amplifiers" and "Building Valve Amplifiers". I found them an invaluable source of information while rebuilding my ST70 (my first ever). In fact, after reading the books, I would say the ST70 is a "textbook example" of amplifier design. It's very straight forward in design and an excellent platform to learn from. I took my time, referred to the books and understood every step as I built my amp. For example, the books explain with examples and diagrams the right and wrong way to lay out heater wires. It was like taking a class in "Amplifier design and construction" with lab.

    I used to wonder "why did the designer choose THAT value for THAT component", and now I understand. In fact, going step by step with the book as reference I understood the amp so well I didn't need to follow the kit instructions. I was able to check my work against the schematic. It all made sense to me.

    That being said, to answer some of your questions, I chose the K40y's by reputation and price. You get a lot of PIO bang for your buck compared to other PIO caps. I always check and match values with a meter before installation, and the K40Y's are very consistent. Even so, I usually order twice what I need to make certain I'll get my matches and to have spares in case of accidents or for other projects. I prefer PIO's because I think they're "less noisy" than the other types. My ST70 may not be as detailed as my other amp (GTA SE40), but I can listen to it all day long, it's just syrupy smooth and seductive. The only problem I had with the PIO's are their size, they're big. They will never fit adjacent to each other on the board and allow you to install the front tube sockets or access the mounting screws.
    I also used 600 volt wire for the build purchased from Antique Electronic Supply.
    http://www.tubesandmore.com/
    I chose cloth covered wires for that "retro" look and used different colors to simplify circuit tracing.
    I chose to use tube rectification rather than solid state for that "tube sound". If you decide to use solid state rectification you will need a delay board to prevent the sudden inrush of current before the heaters have had time to warm up. Solid state may sound "tighter" in the bass than tube, but that will depend on your choice of tube rectifier and capacitance. It's a tube amp that came with tube rectification, so I decided to retain that quality. More explanation about that in the books.
    I chose the twist lok cap for the look and simplicity. It's also "right there" at the "star ground".
    I use the low gain VTA driver board with a 6189 driver tube and two RCA Cleartop 12au7A phase splitters.
    Finally, I personally think the vintage tubes sound better than the new production tubes, especially for the rectifier. For this amp, I'm a Mullard man. Other favorite brand is RCA.
    Below are some images of my amp.

    Take your time and enjoy the build!!

    [url][/url]

    baddog1946

    Posts : 304
    Join date : 2010-02-03
    Location : Costa Rica

    Re: Noobie re-building an ST-70

    Post by baddog1946 on Mon Feb 27, 2012 9:03 am

    I don't think the VTA board is very compatible with the SDS cap board without some major changes to the board.
    Roy feeds the plates of the driver and phase inverter stages with one power supply rail. SDS uses a split rail system (like Dynaco) to feed the driver & phase inverter separately.

    Roy also added additional plate filtering other than just the 80-40-30-20uF quad cap with caps that are mounted on the driver board itself.
    The only real advantage I see of the SDS cap board would be the bypass caps and bleeder resistors for each stage to help the power supply.
    I personally would not change too much on Roy's board for the sake of a bit more capacitance.
    He has come up with some good things recently we can consider improvements like the CCS.
    If you keep the original Dynaco board or use a new Dynaco replacement board then you can easily install the SDS cap board but the VTA is not a good match with the SDS cap board IMHO.

    cc43fan

    Posts : 29
    Join date : 2012-01-30
    Age : 53
    Location : Upstate New York

    Re: Noobie re-building an ST-70

    Post by cc43fan on Mon Feb 27, 2012 5:44 pm

    stewdan wrote:Hi Chuck -- Welcome aboard!!

    In response to some of your questions --

    The Rectifier Diode Mod is applicable to the Rectifier and is not dependent on the Quad Cap or the SDS Cap Board. It just provides more security for the rectifier by pre-filtering the voltage to the tube.

    Some people replace the coupling caps with Spraque Orange Drops, some with Russian Paper-in-Oils, some with exotic caps. It is all a function of what you hear. The Orange Drops are inexpensive, the Russian PIOs a few bucks more per cap and the exotics --- well, the sky is the limit. I have used both the Orange Drops and the Russian PIOs and I prefer the PIOs. After break-in they are extremely smooth and quiet. There was a post the other day and they talked about the Russian PIO caps. Take a look at the posts.

    On a side note, what restoration did you do to your Fisher 500-C? I also have one.

    Stew

    Hi Stew,
    Thanks for weighing in. I read the post on the PIO's and is most likely the direction I'm going to go. As for my Fisher, I restored the entire power supply, signal path, and the electrolytics in the FM section. I also added an individual bias mod for the output tubes. Added new 7591's and the preamp tubes. It cleaned up pretty well, and sounds fantastic. I've added a couple of pics.

    cc43fan

    Posts : 29
    Join date : 2012-01-30
    Age : 53
    Location : Upstate New York

    Re: Noobie re-building an ST-70

    Post by cc43fan on Mon Feb 27, 2012 5:54 pm

    Morgan Jones authored 2 books, "Valve Amplifiers" and "Building Valve Amplifiers". I found them an invaluable source of information while rebuilding my ST70 (my first ever). In fact, after reading the books, I would say the ST70 is a "textbook example" of amplifier design. It's very straight forward in design and an excellent platform to learn from. I took my time, referred to the books and understood every step as I built my amp. For example, the books explain with examples and diagrams the right and wrong way to lay out heater wires. It was like taking a class in "Amplifier design and construction" with lab.

    I used to wonder "why did the designer choose THAT value for THAT component", and now I understand. In fact, going step by step with the book as reference I understood the amp so well I didn't need to follow the kit instructions. I was able to check my work against the schematic. It all made sense to me.

    That being said, to answer some of your questions, I chose the K40y's by reputation and price. You get a lot of PIO bang for your buck compared to other PIO caps. I always check and match values with a meter before installation, and the K40Y's are very consistent. Even so, I usually order twice what I need to make certain I'll get my matches and to have spares in case of accidents or for other projects. I prefer PIO's because I think they're "less noisy" than the other types. My ST70 may not be as detailed as my other amp (GTA SE40), but I can listen to it all day long, it's just syrupy smooth and seductive. The only problem I had with the PIO's are their size, they're big. They will never fit adjacent to each other on the board and allow you to install the front tube sockets or access the mounting screws.
    I also used 600 volt wire for the build purchased from Antique Electronic Supply.
    http://www.tubesandmore.com/
    I chose cloth covered wires for that "retro" look and used different colors to simplify circuit tracing.
    I chose to use tube rectification rather than solid state for that "tube sound". If you decide to use solid state rectification you will need a delay board to prevent the sudden inrush of current before the heaters have had time to warm up. Solid state may sound "tighter" in the bass than tube, but that will depend on your choice of tube rectifier and capacitance. It's a tube amp that came with tube rectification, so I decided to retain that quality. More explanation about that in the books.
    I chose the twist lok cap for the look and simplicity. It's also "right there" at the "star ground".
    I use the low gain VTA driver board with a 6189 driver tube and two RCA Cleartop 12au7A phase splitters.
    Finally, I personally think the vintage tubes sound better than the new production tubes, especially for the rectifier. For this amp, I'm a Mullard man. Other favorite brand is RCA.
    Below are some images of my amp.

    Take your time and enjoy the build!!

    Wow, that is very nice work...I should have sent mine to you!! lol Thank you very much for the input and suggestions, I should have most of the parts and pieces sometime this week. My first order of business is getting the chassis cleaned up and trannies repainted. In my perfect world, I'd like the ST-70 to look mostly stock, with great sound and long lasting when it's powered up. Think COPO Camaro.....a sleeper at the light and you never knew what hit you. Smile

    heyraz

    Posts : 58
    Join date : 2011-10-26

    Re: Noobie re-building an ST-70

    Post by heyraz on Mon Feb 27, 2012 7:20 pm

    I used automobile chrome polish paste to rub the chassis. What a pleasant surprise underneath the grime!


    cc43fan

    Posts : 29
    Join date : 2012-01-30
    Age : 53
    Location : Upstate New York

    Re: Noobie re-building an ST-70

    Post by cc43fan on Mon Feb 27, 2012 8:02 pm

    heyraz wrote:I used automobile chrome polish paste to rub the chassis. What a pleasant surprise underneath the grime!

    That is stunning...any auto chrome polish will do? What is the material you used to mount the new RCA input jacks?

    I'm hoping mine turns out as nice.

    heyraz

    Posts : 58
    Join date : 2011-10-26

    Re: Noobie re-building an ST-70

    Post by heyraz on Mon Feb 27, 2012 8:58 pm

    Flitz is one chrome polish that comes to mind, you'll be surprised. I also used very fine steel wool for the final buff out.
    Most of my hardware was purchased from Dynakit parts:

    http://www.dynakitparts.com/store/product.aspx?id=210

    cc43fan

    Posts : 29
    Join date : 2012-01-30
    Age : 53
    Location : Upstate New York

    And so it begins...

    Post by cc43fan on Tue Mar 06, 2012 3:45 pm

    I thought everyone could use yet another ST-70 restoration...I'm in the early stages.




    The top is actually quite shinny, what you see is the reflection of my cluttered workbench.

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