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    ST-70 Power Supply Capacitance - How much is too much?

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    profgrif
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    ST-70 Power Supply Capacitance - How much is too much?

    Post by profgrif on Wed Dec 23, 2009 4:38 pm

    I'm about to dive back into my ST-70 after 15 years of happy listening since it's first re-build. The power transformer just got too noisy (not to mention hot!!) and I figured in the name of safety and reliability, I should replace it. I'm going to use the larger PA-060S replacement.
    I figured I would replace all the elecrolytics while I'm under the hood anyway.
    Currently my ST70 has a Mallory replacement 40-30-20-20 uF @500v cap, with an extra 100uF at 450V axial cap used after the choke, and a couple of Solen 5.1uF film caps as by-passes inside the chassis. I picked up a new 80-40-30-20UF @525, which is almost as much capacitance as the old quad cap with the 100uF axial, but I'm wondering if I should add still more, and if so, how much? Also, the current bias supply is just a 1N4007 diode and two 100uF caps, should I beef this up as well?
    BTW, I'm still using the GZ34 tube rectifier, the driver board is a Curcio differential cascode stage with Tesla 6922's, and I'm running the outputs in triode for a whopping 17W/ch.

    Any input is appreciated. Very Happy

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST-70 Power Supply Capacitance - How much is too much?

    Post by Bob Latino on Wed Dec 23, 2009 5:52 pm

    profgrif wrote:I'm about to dive back into my ST-70 after 15 years of happy listening since it's first re-build. The power transformer just got too noisy (not to mention hot!!) and I figured in the name of safety and reliability, I should replace it. I'm going to use the larger PA-060S replacement.
    I figured I would replace all the elecrolytics while I'm under the hood anyway.
    Currently my ST70 has a Mallory replacement 40-30-20-20 uF @500v cap, with an extra 100uF at 450V axial cap used after the choke, and a couple of Solen 5.1uF film caps as by-passes inside the chassis. I picked up a new 80-40-30-20UF @525, which is almost as much capacitance as the old quad cap with the 100uF axial, but I'm wondering if I should add still more, and if so, how much? Also, the current bias supply is just a 1N4007 diode and two 100uF caps, should I beef this up as well?
    BTW, I'm still using the GZ34 tube rectifier, the driver board is a Curcio differential cascode stage with Tesla 6922's, and I'm running the outputs in triode for a whopping 17W/ch.

    Any input is appreciated. Very Happy

    Hi,

    I would install the 80, 40, 30, 20 in the amp and keep your 100 uF after the choke on the second section which has the two red output transformer leads. This is the section that faces the back of the amp. You are also (IMHO) doing the right thing in adding BYPASS capacitors to the power supply. Some ST-70 owners like to add a LARGE amount of capacitance to the amp (after the choke) in hopes that it will play louder or better in some way. After a certain point the amp just won't use all the extra capacitance and you can actually do the amp harm by adding too much. The amp will start to sound a little "slow" and you may actually lose some high frequency response with too much capacitance on the amp due to resonances. This problem can be addressed, however, by the use of a bank of smaller bypass capacitors which will provide a low impedance path to ground for the entire power supply over a wide range of frequencies. You may want to try instead of two 5.1 uF caps a bank of a 5.1 a 1.0 and a .1. The use of a bank of three DIFFERENT bypass capacitors will usually break up any resonances in the audible range.

    You are smart to upgrade the power transformer also. With an upgraded power transformer you can now use, in addition to the standard EL34, 6L6, KT66 and KT77 tubes for the ST-70, either KT88 or 6550 tubes in your amp. The latter two tubes will draw about 30% more current and should not be used with a STOCK PA-060 power transformer.

    Bob

    profgrif
    Guest

    Re: ST-70 Power Supply Capacitance - How much is too much?

    Post by profgrif on Thu Dec 24, 2009 11:54 am

    Awesome, thanks for the input Bob.

    Have a very Happy Holiday!! Very Happy

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