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    Capacitor inductance

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    polarflux

    Posts : 40
    Join date : 2010-02-12

    Capacitor inductance

    Post by polarflux on Tue Feb 23, 2010 3:30 pm

    I found an interesting ST-70 article on the Audio by Van Alstine website that putting overly large capacitors on the driver board can cause excessive inductance leading to noise. I think this refers to the stock ST-70 board and not the VTA board, but perhaps this still applies. The second hand ST-70 I have came with huge Solen caps installed on the top of the board. It noticed, when the amp is not blowing fuses, that if I put my hand near the driver board it caused a buzz/hum noise to emit. I am wondering if anyone else has experienced this and if this could be due to inductance, or is this perhaps a symptom of 12AT7's going belly up.

    Thanks

    Bob Latino
    Admin

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

    Re: Capacitor inductance

    Post by Bob Latino on Tue Feb 23, 2010 4:49 pm

    Hi Phil,

    I think when you just replace that VTA driver board with a new VTA driver board from Roy at tubes4hifi.com your problems will be solved. Whoever "upgraded" that VTA driver board probably caused the issues with the replacement capacitor set that was installed. Problems such as you are experiencing come up when a self styled upgrade artist attempts to "upgrade" a design that Roy designed in the 1980's and has polished the design over 20+ years to what it is today. About the only modifications you can do to that board that make any audible difference are ...

    1. Larger power supply upgrade capacitors - C13 and C14 should be no larger than 120 uF while C15 and C16 should be no larger than 47 uF. If you play the amp loud a lot this upgrade is worthwhile.

    2. Upgrade the main coupling capacitors - (C5, C6, C7 and C8). Anything .22 uF or .33 uF at 600 volts or higher is OK here. Roy and I have found that the Russian PIO (paper in oil) K40-Y .22 @ 600 volts or higher work excellent here. They are not terribly expensive and IMHO (and Roy's) these military grade capacitors are equal to caps costing 5X what these caps can be had for. I personally test each and every one that I get from a fellow in Russia and then put them in matched sets of four. Not one of these (almost 1000 now ..) Russian caps has ever failed (that I know of?). Some customers have asked me about using these same Russian caps in C1, C2, C3 and C4 (phase splitter/phase inverter) section of the driver circuit. I have put them in there but they don't seem to change the sound at all in these 4 positions. They are no better in this position than the stock SBE 716P "orange drop" film and foil caps that are normally used in these 4 positions.

    3. Roll tubes in an out for best sound - Some tubes may sound better to you than other tubes. Experiment and try different brands. On the VTA driver board the CENTER driver tube is the initial voltage amplifier for BOTH channels. Changing this tube to another brand makes a much greater difference in the sound of the amp than changing out the two side driver tubes. In a stock ST-70 you can use, in addition to the standard EL34 output tubes, 6L6, KT66 or KT77 output tubes. If you have an upgraded power transformer you can also add KT88 and 6550 tubes to the list. Each tube has a slightly different sonic signature. You may find one type of output tube that you like better than the others...

    Bob

    polarflux

    Posts : 40
    Join date : 2010-02-12

    Re: Capacitor inductance

    Post by polarflux on Tue Feb 23, 2010 6:43 pm

    As always, thanks for the info. Regarding the quad cap, it looks like the 80,40,30, 20 525 VDC caps are out of production but a 475 VDC version is available. I noticed all the voltages for the quad cap are in the 430 volt range. Is there enough margin to use a quad rated at 475? With the other problems, I figure it can't hurt to have replacement quad cap if the new board does not correct things totally. Maybe I'm wrong, but I have a feeling my amp was subjected to continuous sadistic abuse by the previous owner. Should have bought your kit to start with, although overall this may turn out to be a richer experience (experience wise, not for my wallet). Hey, you know, you could have a Dynaco disasters thread on the forum. Starts with mine.

    -Cheers

    Bob Latino
    Admin

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

    Re: Capacitor inductance

    Post by Bob Latino on Tue Feb 23, 2010 6:59 pm

    polarflux wrote:As always, thanks for the info. Regarding the quad cap, it looks like the 80,40,30, 20 525 VDC caps are out of production but a 475 VDC version is available. I noticed all the voltages for the quad cap are in the 430 volt range. Is there enough margin to use a quad rated at 475? With the other problems, I figure it can't hurt to have replacement quad cap if the new board does not correct things totally. Maybe I'm wrong, but I have a feeling my amp was subjected to continuous sadistic abuse by the previous owner. Should have bought your kit to start with, although overall this may turn out to be a richer experience (experience wise, not for my wallet). Hey, you know, you could have a Dynaco disasters thread on the forum. Starts with mine.

    -Cheers

    Hi Phil,

    430 volts is just about right for the B+ DC voltage on the first section of the quad cap. The first section of the quad cap is the section that has a wire going over to pin #8 of the rectifier tube socket. If the quad cap was bad the amp would hum badly and/or the voltage would be down to well below 430 on the first section of the quad cap. Replace the driver board first. IMHO that should clear up all your problems with the amp.

    Bob

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