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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    ST-70: About the Increased Capacitance in Film Caps on the PC-3 Board

    lorne
    lorne
    lorne


    Posts : 21
    Join date : 2015-01-10
    Location : Sendai, Japan
    • Post n°1

    ST-70: About the Increased Capacitance in Film Caps on the PC-3 Board Empty ST-70: About the Increased Capacitance in Film Caps on the PC-3 Board

    Post by lorne Sun May 08, 2022 4:04 pm

    Greetings everyone. I've been away for a long time — long story and other projects. Now I'm rebuilding an old friend I've had for around 35 years — an early Dyna Kit ST-70. I have installed the bigger power X-former, an SDS cap board I had in stock, a new glass PC-3, new ceramic sockets and a set of new tubes set to go including four NOS RCA 7199's

    QUESTION: relating to the film capacitors -

    STOCK FILM CAPACITOR RATINGS
    4x .10uF @ 400 Volts
    2x .50uF @ 400 Volts

    I'm sure some you members can speak to this: what is you experience with increasing capacitance in the PC-3 film caps?:

    INCREASE
    4x .22uF @ 400 Volts
    2x..10uF @ 400 Volts

    I do have the latter in my current stock. (Nothing fancy).

    And ... I could also choose to install all six as .10uF @600 Volts — and the outer foil sides are marked.

    Any thoughts, suggestions, reflections very much appreciated in advance. Has anyone heard some better performance from the increase? Is there any greater durability in the 600 Volt rating?

    (BTW: I did a search on this and could not find what I wanted) ... Lorne
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    Bob Latino
    Bob Latino
    Bob Latino
    Admin


    Posts : 3263
    Join date : 2008-11-26
    Location : Massachusetts
    • Post n°2

    ST-70: About the Increased Capacitance in Film Caps on the PC-3 Board Empty Re: ST-70: About the Increased Capacitance in Film Caps on the PC-3 Board

    Post by Bob Latino Sun May 08, 2022 5:12 pm

    lorne wrote:Greetings everyone. I've been away for a long time — long story and other projects. Now I'm rebuilding an old friend I've had for around 35 years — an early Dyna Kit ST-70. I have installed the bigger power X-former, an SDS cap board I had in stock, a new glass PC-3, new ceramic sockets and a set of new tubes set to go including four NOS RCA 7199's

    QUESTION: relating to the film capacitors -

    STOCK FILM CAPACITOR RATINGS
    4x .10uF @ 400 Volts
    2x .50uF @ 400 Volts

    I'm sure some you members can speak to this: what is you experience with increasing capacitance in the PC-3 film caps?:

    INCREASE
    4x .22uF @ 400 Volts
    2x..10uF @ 400 Volts

    I do have the latter in my current stock. (Nothing fancy).

    And ... I could also choose to install all six as .10uF @600 Volts — and the outer foil sides are marked.

    Any thoughts, suggestions, reflections very much appreciated in advance. Has anyone heard some better performance from the increase? Is there any greater durability in the 600 Volt rating?

    (BTW: I did a search on this and could not find what I wanted) ... Lorne

    Hi Lorne,

    You can up the four coupling caps from .10 uF to .22 uF and the two .05 uF interstage caps (not .50 uF as you mentioned) to .10 uF and it really won't make any difference in the sound of the amp. The real problem with the PC-3 board circuit is that Dynaco used that "one 7199 driver tube for each channel" to save a tube. You really need THREE driver tubes - one dual triode 12AU7 tube as a voltage amplifier for BOTH channels and two 12AU7 dual triodes (one for each channel) for phase splitter/inverter duties to create a good driver circuit. As I mentioned earlier here, this circuit on the PC-3 board was designed to keep manufacturing costs down and was not designed for best sound. The other problem is that the 7199 driver tube is not made anymore and those left in circulation keep rising in price. Understand that there is no direct replacement of the 7199 tube out there now. Yes - you can use an adapter and use 6GH8A tubes in that socket. (A 6GH8A is basically the same tube as a 7199 but has a different pin out).

    If you really want to increase the sound quality of an original Dynaco ST-70, get rid of the original (or replacement) PC-3 driver board and install a VTA ST-70 replacement driver board. The VTA driver board has its own power supply and does not have to draw power from the quad cap. If you use the VTA driver board, the quad cap only powers the two output transformers. In the original Dynaco ST-70 the quad cap had to power both the two output transformers and the PC-3 board. With the VTA driver board you also get into 12AU7 driver tubes which are still in production and easily accessible. The VTA driver board also has its own on board bias system with individual biasing of each output tube independently.

    Bob

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    lorne
    lorne
    lorne


    Posts : 21
    Join date : 2015-01-10
    Location : Sendai, Japan
    • Post n°3

    ST-70: About the Increased Capacitance in Film Caps on the PC-3 Board Empty Re: ST-70: About the Increased Capacitance in Film Caps on the PC-3 Board

    Post by lorne Sun May 08, 2022 6:19 pm

    Thanks for your reply Bob. And yes .05uF not .50uF — a flub I made in the groggy wee hours here in Japan.

    I read your comments about the deficiencies of the original board some time ago. I took your advice seriously. But, I began this project before this site even began. The worked stalled for a long time due to other projects. During that that time, I had bought a new stock glass board and a supply of NOS 7199's. Everything I need is in my stock bins.

    So, I am going to build the amp using the stock topology. Later I will work up to a VTA board and parts. ($$ and ¥¥ are in short supply, moreover shipping plus the monetary exchange rate is savage these days.)

    And thanks for the advice about the caps. I may go with the 600 Volt .10 cpas for all six film caps. And just for the hell of it, I'll point all the marked outer foil sides in the same direction as the striped, factory oil caps.

    Thanks again for the considerate reply — Lorne
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    yel17
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    yel17


    Posts : 9
    Join date : 2015-04-23
    Age : 75
    Location : Chicagoland
    • Post n°4

    ST-70: About the Increased Capacitance in Film Caps on the PC-3 Board Empty Re: ST-70: About the Increased Capacitance in Film Caps on the PC-3 Board

    Post by yel17 Sun May 08, 2022 6:38 pm

    During the past 40 years I’ve owned three original Dyna ST-70’s. I upgraded the coupling and feedback caps on all of them and was never really happy with the sound. The upper mids have a lisp type of sound. Then in the early 90’s I picked up a pair of McIntosh MC-60’s and upgraded the capacitors.
    I’m retired and have plenty of time so I built a VTA-70 with the 12AU7 driver board. I’m running Gold Lion KT77S. I am very happy with the sound of the VTA-70. It is a close second behind the MC-60’s in terms of overall sound quality. I could not recommend it higher.

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    lorne
    lorne
    lorne


    Posts : 21
    Join date : 2015-01-10
    Location : Sendai, Japan
    • Post n°5

    ST-70: About the Increased Capacitance in Film Caps on the PC-3 Board Empty Re: ST-70: About the Increased Capacitance in Film Caps on the PC-3 Board

    Post by lorne Mon May 09, 2022 9:12 am

    yel17 wrote:During the past 40 years I’ve owned three original Dyna ST-70’s. I upgraded the coupling and feedback caps on all of them and was never really happy with the sound. ....
    ... I built a VTA-70 with the 12AU7 driver board.  .... I am very happy with the sound of the VTA-70. It is a close second behind the MC-60’s in terms of overall sound quality. I could not recommend it higher.

    Thank you for the comment. It inspires confidence in regards to my plans to keep the values stock. My cap supply is very ordinary, metalized film caps rated for 600 VDC.

    I regard this amp as a work in progress; it may evolve in the future.

    I've owned this Dynakit since 1988. I have a special attachment to it. I bought it from a very close friend who died 20 years ago. Before I moved to Japan from Vancouver, this ST-70 ran with a Melos GK 1+1 preamp and big Magnaplanars. The sound was terrific, despite the fact that the speakers really demanded much more power. The poor Dyna pretty well boiled itself crazy, but it still worked Ok when I tore it down. I still have the Melos, and the pair will be reunited once I finish the resurrection. They do mate up very well with each other.

    Your MC-60's have to be the grail outfit many of us can only dream about. So, your comparison of the VTA amp has to be very high praise indeed.

    Lorne
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