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    Riddle me this?

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    polarflux

    Posts : 40
    Join date : 2010-02-12

    Riddle me this?

    Post by polarflux on Thu Feb 18, 2010 4:28 am

    Well, new here since I just recently picked up a second hand ST 70 built by Bob Latino. I really wanted to build one of Bob's kits myself, but the opportunity to pick one up second hand came up on Audiogon (with a cage!). I couldn't resists. I was especially excited since the seller claimed there were hundreds of dollars in upgrades that had been done to the amp and was supposedly in pristine condition. Well, buyer beware...sort of. I am glad to have and there is nothing that can't be fixed. As you can see in the photo, there are many boutique caps installed on the underside of the driver board by the sellers tech. There are additional large solen caps on the top of the driver board (not shown). It looks like all of the upgrade caps installed have be reupgraded? The problem is, though, the amp hums and makes fizzy sounds, and just blew a fuse tonight. I swapped tubes around, rebiased..In short, everything I knew to do prior to the fuse blowing. What I am wondering, compared to photos I have looked at of an upgraded driver board, are there caps that are not needed or could possibly be contributing to the hum and fizz? Hmm.

    A couple of other things. What is the bias value for the KT 88s on the ST 70?

    What are the little adjustment pots on the underside of the driver board all about?

    I wanted a project...Looks like I got a project!

    -Phil


    1MOR

    Posts : 19
    Join date : 2010-02-04

    Re: Riddle me this?

    Post by 1MOR on Thu Feb 18, 2010 2:06 pm

    Just some thoughts from a tubenubie. See if you can obtain the construction manual for your amp. Then remove the parts that were not in the original assembly and trace everything to put the amp back into its original configuration. You should have happy tube music when you are finished. Looks like the previous owner soldered himself into a corner and bailed on the project by selling it to you. Good luck!

    I am in the process of putting one of Bob's ST-70s together. Welcome to the club!! cheers

    polarflux

    Posts : 40
    Join date : 2010-02-12

    Re: Riddle me this?

    Post by polarflux on Thu Feb 18, 2010 2:29 pm

    Yeah, I agree with you. Like I said, I wanted a project to work on. Guess its a repair project. The problem is, the soldering job is really hackish and I am worried that the board itself may be corrupted. I'll find out when I get into it. Joy! I'm hoping bob will chime in and let me know if the z-caps in the middle of the board were installed by him or were installed by the previous owner. The soldering looks really good, so maybe the z-caps were from the original build. Thanks for the input.

    Bob Latino
    Admin

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

    Re: Riddle me this?

    Post by Bob Latino on Thu Feb 18, 2010 6:03 pm

    Yeah, I agree with you. Like I said, I wanted a project to work on. Guess its a repair project. The problem is, the soldering job is really hackish and I am worried that the board itself may be corrupted. I'll find out when I get into it. Joy! I'm hoping bob will chime in and let me know if the z-caps in the middle of the board were installed by him or were installed by the previous owner. The soldering looks really good, so maybe the z-caps were from the original build. Thanks for the input.

    Polarflux,

    Sorry about your problems with the amp ...

    That amp was "worked on" by someone and I do not know what was done to it. Those large "Z" caps are not part of anything that I would ever use and were not installed by me. The "Z" caps had to have been installed by a previous owner. The photo shows a mishmash of caps on the bottom of the driver board - none of which were installed by me ...

    The thing that really tics me off is that with the two cap upgrades that I supply as an option, that amp is taken about as far as you can go with "upgrades". The only thing that you can really do to "improve the sound" is to roll different tubes in there and see which tubes you like.

    What I advise is return the board back to it's stock upgraded form. In fact you might be better off just installing a new board and parts set with cap upgrades rather than take the time to pull all those other capacitors off there and then reinstall what should be in there.

    In an ST-70 (with an upgraded power transformer as you have in there) bias KT88's at .500 volts DC per output tube. If you had an ST-120 you would bias them at .550 VDC.

    The adjustment pots on the underside of the board are the 10K AC balance controls. These two pots adjust the phase splitter/phase inverter section of the driver circuit. The kit builder should set them at their center position which is always very close to their optimum setting. To set them more accurately you need a signal or function generator which most kit builders do not have. They are put on the bottom of the board so you won't "readjust them" accidentally when setting the four main bias pots on the top of the board.


    Bob


    Last edited by Bob Latino on Thu Feb 18, 2010 7:38 pm; edited 1 time in total

    polarflux

    Posts : 40
    Join date : 2010-02-12

    Re: Riddle me this?

    Post by polarflux on Thu Feb 18, 2010 6:24 pm

    Thanks for the reply...My gut is telling me the same thing that replacement of the VTA board may be necessary. It looks like there are caps installed in places where there should not be (compared with photos on your site). By the way, are the photos of the upgraded VTA boards on your site complete, are are they only to illustrate the upgraded components. Having a photo of a complete board to compare with would be good.

    Another thing, my amp has way too much gain for my Klipsch Fortes so I still need to do the low gain mod to VTA board anyway. Looks like my decisions are being made for me.

    Bob Latino
    Admin

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

    Re: Riddle me this?

    Post by Bob Latino on Thu Feb 18, 2010 6:46 pm

    Hi,

    Get in touch with Roy at www.tubes4hifi.com if you want information about or to purchase the low gain version of the driver board.

    There is a photo below of what a VTA driver board should look like with the power supply and the Russian PIO main coupling capacitors.

    Bob



    polarflux

    Posts : 40
    Join date : 2010-02-12

    Re: Riddle me this?

    Post by polarflux on Thu Feb 18, 2010 6:56 pm

    Well, I see that on my board the caps that the orange caps on the top have been replaced with the z-caps. Still does explain some of the other work. Sincere thanks for the info!

    polarflux

    Posts : 40
    Join date : 2010-02-12

    Re: Riddle me this?

    Post by polarflux on Fri Feb 19, 2010 7:29 pm

    Well, I decided it would be the best to replace the VTA driver board so I ordered another last night. Its the low gain version which will work better with my system. I did not order the Russian caps since I am kind of smarting from having to replace the VTA driver board after just picking up the amp. I did get the other, cheaper cap upgrade though. The large orange clarity caps on the VTA board in my amp (see photo in my original post) appear to be in good condition. Do you see any problem with using them in lieu of the stock caps or your Russian caps?

    Bob Latino
    Admin

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

    Re: Riddle me this?

    Post by Bob Latino on Fri Feb 19, 2010 8:05 pm

    polarflux wrote:Well, I decided it would be the best to replace the VTA driver board so I ordered another last night. Its the low gain version which will work better with my system. I did not order the Russian caps since I am kind of smarting from having to replace the VTA driver board after just picking up the amp. I did get the other, cheaper cap upgrade though. The large orange clarity caps on the VTA board in my amp (see photo in my original post) appear to be in good condition. Do you see any problem with using them in lieu of the stock caps or your Russian caps?

    As long as those Clarity caps are .22 uF or .33 uF and are rated at 600 volts or higher you could use them in place of the stock caps ...

    Bob

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