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    Resetting the bias on an assembled amp?

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    Sprags

    Posts : 123
    Join date : 2013-02-27

    Resetting the bias on an assembled amp?

    Post by Sprags on Tue Mar 19, 2013 8:35 pm

    I am purchasing an assembled VTA ST-120 from Bob Latino. I'm hoping I take delivery in a few weeks. Excuse my anxiousness but I'm really looking forward to listening to fine music with an audiophile quality system and I'm thinking of all sorts of questions while awaiting the time.

    Today's question is: upon receiving the amp and installing the tubes, connecting the SP10 preamp and components and connecting my speakers should I reset the bias?

    And of course after I asked this question I realized the answer is probably yes since I do not know which tube was in each socket.

    Of course i will probably continue to have questions so I hope I don't wear out my welcome.


    Zimmer64

    Posts : 113
    Join date : 2013-01-29
    Age : 52
    Location : Switzerland

    Re: Resetting the bias on an assembled amp?

    Post by Zimmer64 on Wed Mar 20, 2013 1:51 am

    I am sure the manual will tell you. Bob's instructions are fantastic and bullet proof. In my ST70 manual it says check once a month. It also says, check, when you replace a tube. Since also your local line voltage may be different to where Bob lives, I would recommend to check / adjust bias in any case.

    Enjoy your amp! Hope you like it as much as i do my ST 70.

    Best

    Michael

    Bob Latino
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    Location : Massachusetts

    Re: Resetting the bias on an assembled amp?

    Post by Bob Latino on Wed Mar 20, 2013 7:47 am

    Hi Blake,

    When you get a VTA wired amp with tubes, the output tubes are marked as to where each one should go and the bias is preset. You should check the bias, as Zimmer64 has said, because sometimes your line voltage may be different the line voltage where the amp was tested. The bias may require a minor correction.

    Some bias tips ...

    1. Always check the bias in pentode ultralinear mode at idle with no signal running through the amp. Bias will drop very slightly when you change from pentode ultralinear mode to triode mode.

    2. Make an initial bias voltage check when you first turn the amp on and then again when the amp is fully warmed up. If the bias is a little low when the amp is first turned on, I would leave it alone because during the first half hour or so the bias will rise slightly as the amp warms up. Make your final setting after the amp is fully warmed up.

    3. As the output tubes wear a little, the bias will change very slightly. It is recommended that you check the bias once a month and make minor corrections if necessary.

    4. In some places in the USA the line voltage varies quite a bit during the day. If your bias does not seem to hold steady, it's probably your line voltage going up and down during the day.

    5. Don't be obsessed about getting the bias "perfect". On a VTA ST-120 the recommended bias setting is .550 VDC per each output tube which gives an idle current of 55 milliamps per each output tube. As long as all the output tube are between .540 and .560 VDC after the amp is fully warmed up, that is close enough.

    6. If ONE tube won't bias properly then that tube is probably bad and should be replaced. If ALL tubes give NO bias or a very LOW bias then the rectifier tube is probably bad and the rectifier should be replaced.

    Bob

    deepee99

    Posts : 1333
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Resetting the bias on an assembled amp?

    Post by deepee99 on Wed Mar 20, 2013 10:09 am

    Sprags,
    If I might add one caveat here from an M-125 owner:
    Should you change output tubes at some point (I swap between KT-120s and KT-88s) be sure, before you swap out the tubes, please turn the bias voltage all the way down. Different tubes require different bias settings, and you can fry KT-88s at the KT-120 setting pretty quickly. And yeah, you can can go nuts chasing voltages around. Just get it close (I leave it a bit under) the recommended setting after warm-up and you'll be fine.
    Just to be clear here, the voltages remain the same (in my case, 1.2) but the pots will need to be re-set to deliver that same level of juice to different tube configurations. So with the old tubes in there, dial the volts down all the way (in my case about 0.Cool, change the tubes, warm things back up, and then slowly bring the voltage up.
    A bit more about line voltage, if I might: I learned, after blowing a few rectifiers, that in the U.S.A. line voltage is, under current regulations, allowed to fluctuate 10% plus or minus, of 120V. So what's coming out of the wall can vary from 108 to 132 volts. Solid state gear seems able to handle this fluctuation but tubes cannot. Get a box (I recommend the Thurman, but it's spendy) that will clamp the voltage going into your amp and pre-amp to about 123V. I tried using a Variac to do the same thing but after two of the things blew up, went the expensive route and am not disappointed. Plus, the Thurman will take the noise of of your line voltage and you will hear an improvement.


    Last edited by deepee99 on Wed Mar 20, 2013 11:00 am; edited 1 time in total (Reason for editing : Clarity)

    Sprags

    Posts : 123
    Join date : 2013-02-27

    Re: Resetting the bias on an assembled amp?

    Post by Sprags on Wed Mar 20, 2013 12:44 pm

    deepee99 wrote: Get a box (I recommend the Thurman, but it's spendy) that will clamp the voltage going into your amp and pre-amp to about 123V. I tried using a Variac to do the same thing but after two of the things blew up, went the expensive route and am not disappointed. Plus, the Thurman will take the noise of of your line voltage and you will hear an improvement.

    I take it you are not referring to Uma Thurman the actress? What a Face

    I did a search for Thurman Line Conditioners and came up blank but I take it that is what you are recommending. I have a line conditioner that I aquired in the 90's. I believe it is one great big capacitor that weighs about 40 or 50 pounds. It was about $400 back in the 1993 or 1994. The company I worked for at the time purchased it for my lab equipment and when the R&D center closed down it was given to be by my supervisor. Will that work?

    I think what I may do is dig out the line conditioner and plug it in then check the voltage out using a Fluke multimeter.


    As an addendum to this post I did find Furman Power line conditioner and surge protector. I see they range in price from $60 top $600.

    I think I'm going to test the one I have since I got it to basically cover those two conditions of the juice coming going into my lab equipment.

    Thanks


    Last edited by Sprags on Wed Mar 20, 2013 12:59 pm; edited 1 time in total

    Bob Latino
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    Re: Resetting the bias on an assembled amp?

    Post by Bob Latino on Wed Mar 20, 2013 12:55 pm

    I think that you may be looking for Furman line conditioners ..

    Bob

    tubes4hifi
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    Re: Resetting the bias on an assembled amp?

    Post by tubes4hifi on Thu Mar 21, 2013 12:17 pm

    Furman's are excellent quality line conditions (and so is Uma Wink )

    Sprags

    Posts : 123
    Join date : 2013-02-27

    Re: Resetting the bias on an assembled amp?

    Post by Sprags on Thu Mar 21, 2013 12:33 pm

    Is there a Furman line conditioner in the $150 range that will perform adequately for regulating voltage but also reducing noise? Can someone recommend a model number?

    deepee99

    Posts : 1333
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Resetting the bias on an assembled amp?

    Post by deepee99 on Thu Mar 21, 2013 7:06 pm

    In that price range I would c/out an APC G5 unit, on Amazon here:
    http://www.amazon.com/G5Blk-9-Outlet-G-Type-Rack-Mountable-Conditioner/dp/B0028RCZSS
    You can set upper and lower voltage limits, and if your power goes out, it will delay a re-start of your amp(s) for up to a couple of minutes, which will prevent the rectifiers from launching through the roof into low-earth orbit.
    The cheapest Furman is going to be about thrice that much.
    I have a friend who says he could arrange a date with Una Furman, who looks just like Uma Thurman, but the price is $150 an hour plus the usual gratuity, and shipping and handling . . .


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