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    low gain VTA board capacitor values

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    icky

    Posts : 5
    Join date : 2009-07-13
    Age : 61

    low gain VTA board capacitor values

    Post by icky on Wed Jan 13, 2016 6:55 pm

    I know this has been discussed at length in the past (ad nauseum, with electronic theory affraid ), but what is the value range suitable for the Interstage caps (C1 & C2 ) in µF? I know they are to be 400 volt rated and the parts list calls for 0.10 µF, but do they have range similar to the coupling capacitors, C5-C8?

    Thanks, Jeff

    arledgsc

    Posts : 340
    Join date : 2012-11-30
    Location : SF Bay CA

    Re: low gain VTA board capacitor values

    Post by arledgsc on Thu Jan 14, 2016 12:03 pm

    At one time I did frequency response calculations with the low-gain cap schematic values and found they were spot on to provide decent lows without being too much. I'm sure 0.22uF would work OK as well in a pinch. But the larger the coupling cap value the closer you are to passing signals close to DC and can throw off bias of the next stage until these unwanted ultra-low excursions settle. This is known as blocking distortion if large enough to cut off the next stage. The larger the cap value the longer it takes near DC offsets to settle back to equilibrium. On the opposite end too low a cap value will limit low frequency response. I like a circuit -3dB response around 10 to 15Hz but that is just my preference.

    I'm sure Bob has tried other values and can comment about viability in this circuit.

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: low gain VTA board capacitor values

    Post by Bob Latino on Thu Jan 14, 2016 1:29 pm

    arledgc's post is spot on ... If you go TOO large on the value of a coupling cap (at least on the VTA amps) there is the danger of getting too close to DC on the -3dB of the stage's signal response. I have used .15 uF, .22 uF and .33 uF value caps at times in the C5 - C8 positions on the VTA amp kits. They all work well and cause no problems in the VTA driver circuit. The only other requirement is that these caps should be of 400 volt or higher rated on the VTA ST-70 and ST-120. The C5 - C8 caps look at 225 to 250 VDC on the inner lead and -40 to -55 VDC on the outer lead for a voltage swing of 300+ volts. On the VTA M-125's a 630 volt cap is used because each coupling cap is dealing with the higher current pull of TWO output tubes. Below is a link to an earlier post from 2010 on what the inside of a Russian PIO looks like. These caps may be gray or brown or green colored depending on the series. In the post (again 2010 dated) it mentions that I had used over 1000 of these caps. At last count, I am now up to about 5000 of these Russian PIO caps. I have had now just ONE failure of a single Russian PIO cap. They are, IMHO, dead reliable. They also give a very smooth output response and a holographic soundstage. You would be hard pressed to find a better coupling cap for the money ..

    Russian PIO coupling cap post from 2010

    Bob

    icky

    Posts : 5
    Join date : 2009-07-13
    Age : 61

    Re: low gain VTA board capacitor values

    Post by icky on Mon Jan 18, 2016 4:49 am

    Thanks arledgc and Bob for your replies.  Every time I delve into this forum I learn something new or my ancient electronic training comes roaring back!   study

    My 2008 VTA ST70 is now sporting a low-gain CCS driver board.  I used 0.22µF PIO Russian couplers.  I left the interstage caps 0.10µF.  Everything sounding fine.  I'll give it the 100-200 hour break-in on the capacitors and maybe start some tube rolling.  I'm starting with clear top GEs; they're economical (for now!) and sound good.  

    Thanks again,

    Jeff

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