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    Slightly increase size of first cap on stock ST-70??

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    SuperB

    Posts : 15
    Join date : 2012-01-01

    Slightly increase size of first cap on stock ST-70??

    Post by SuperB on Wed Feb 01, 2012 10:39 am

    The first cap right after 5AR4 is 30mf, some of my friend suggest I raise it to 40mf, but I've seen someone from other forum says that better lower it down less than 30mf for more voltage drop.

    I know the VTA ST-70 raise the first cap in the PS to 40mf.

    Any thought?

    Sal

    Posts : 223
    Join date : 2009-02-05
    Location : Central New Jersey Dynaco-ST70.com

    Re: Slightly increase size of first cap on stock ST-70??

    Post by Sal on Wed Feb 01, 2012 10:49 am

    You can raise that section to 40uf and the section after the choke to 80uf or more, but don't go to crazy high after the choke where the red leads of the audio transformer connects to.

    Sal

    SuperB

    Posts : 15
    Join date : 2012-01-01

    Re: Slightly increase size of first cap on stock ST-70??

    Post by SuperB on Wed Feb 01, 2012 11:01 am

    What aboutn 40mf for the first cap and then increase to 100mf after the choke for the output section? And the ones feeding driver and input section will remain the same -- 20mf for each section as the stock does.

    Bob Latino
    Admin

    Posts : 2374
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Slightly increase size of first cap on stock ST-70??

    Post by Bob Latino on Wed Feb 01, 2012 3:57 pm

    SuperB wrote:What aboutn 40mf for the first cap and then increase to 100mf after the choke for the output section? And the ones feeding driver and input section will remain the same -- 20mf for each section as the stock does.

    SuperB,

    That will work OK ... Just make sure that the caps are rated 525 VDC or higher. If you just go to 500 volted caps like on the SDS or Triode board you are cutting it close on "headroom" for that cap after the choke.. The ST-70 runs at about 440 volts on today's 120 volt input AC line current but may surge near 500 volts on turn on. Placing more than 100 uF on that section after the choke will not help the amp any further - and - EXCESSIVE capacitance can slow the high voltage capacitors ability to recharge quickly.

    Bob

    SuperB

    Posts : 15
    Join date : 2012-01-01

    Re: Slightly increase size of first cap on stock ST-70??

    Post by SuperB on Thu Feb 02, 2012 1:37 am

    Hi Bob!
    This is the story. I just helping my friend clean up his mini storage and it has "tremendous" amount of unused exotic parts there, such as Black GAte WKZ series, Elna Cerafine, ICAR SP25 paper in oil (I have some as well), Sprageu Vitamin Q door knob type.......And he give me some Sprage those are NOS. The 40mf that referring to is ICAR SP25 Paper in Oil type and rate at 1000V. However, is that ok if using those position for "20mf" as cap rate at 330VAC (around 400VDC) since the Sprague rated at 330VAC?

    As I said very beginning of this thread, this is a rebuilt of my stock Dynaco ST-70. First, the output section supply will have Unlytic 100mf 750V (also unusded parts from my friend) there and the input section has 100mf as well (2 x ASC X386S 50mf 440VAC in parallel), but I want to use the Sprague Vitamin Q in the supply for the "driver" section (the triode section 7199 for phase spliiter). Unfortunately, I don't have that much Sprague door knob from my friend only 20mf (2.5mf x 8 ).

    As a result, it has 100mf for the output section; 100mf for the input (pentode section of 7199) and 20mf for the driver (7199 triode section of 7199) in the supply. IS 20mf for the driver, alright??

    I remembered that I read some article from DHTRob and he suggested that avoid large values when using PIO cap, and it will slow down the sound.

    I have more question about the VTA M-70, but better start a new thread to avoid confustion....... bounce

    Thanks!



    SuperB

    Posts : 15
    Join date : 2012-01-01

    Re: Slightly increase size of first cap on stock ST-70??

    Post by SuperB on Thu Feb 02, 2012 1:44 am

    I forgot to meniton, it rebuilt on a new chassis, and this is the reason has lots of space for that.

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