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    Biasing issues

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    tubes4hifi
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    Join date : 2008-11-30

    Re: Biasing issues

    Post by tubes4hifi on Mon Dec 02, 2013 5:41 pm

    what do you mean by NO reading on the meter, to me that means the meter is blank! Turned off.
    An out of range reading would either be zero ohms, or infinite ohms. If the reading is infinite (or OL on my meter) that means the resistance is higher than the range you have the meter set for, like trying to read 270K on a 200K range, so you need to increase the meter range to 2M or whatever your meter has. Or maybe your meter leads are not making a good connection on those particular resistors due to something on the leads (flux, oxidation, dirt, whatever).
    Make those checks again, it's impossible to have "no reading"
    A good capacitor will charge up as you are testing it for resistance until it finally is more than 200M or OL, otherwise it is bad (leaky if reading less than 200M, or if shorted reading less than 1M or even less than 1K.

    nerpissad

    Posts : 58
    Join date : 2012-09-16
    Age : 43
    Location : amsterdam

    Re: Biasing issues

    Post by nerpissad on Mon Dec 02, 2013 5:51 pm

    by no reading I meant a "1" on the display of my meter.  if I'm not measuring anything, the display will read "1", so it's kind of its idle value I presume?
    I have the meter set to 2M where it reads approx 290K (--> .290 on the 2M setting)

    here's a picture of it:


    even though it's pretty unclear, it does say .300 on the display Wink

    nerpissad

    Posts : 58
    Join date : 2012-09-16
    Age : 43
    Location : amsterdam

    Re: Biasing issues

    Post by nerpissad on Mon Dec 02, 2013 6:30 pm

    alright, after viewing a short youtube clip the problem appeared to be the person using the meter....

    all 4 1000 ohm resistors read approx. 985 and the four 270K ones read 270K.
    so I suppose that still leaves a bad contact on a tube pin, a bad solder connection or the bias pot itself.

    deepee99

    Posts : 1260
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Biasing issues

    Post by deepee99 on Sat Dec 07, 2013 6:14 am

    nerpissad wrote:alright, after viewing a short youtube clip the problem appeared to be the person using the meter....

    all 4 1000 ohm resistors read approx. 985 and the four 270K ones read 270K.
    so I suppose that still leaves a bad contact on a tube pin, a bad solder connection or the bias pot itself.
    By all means, re-tension the sockets. There's another thread on this topic, and it's a very low-tech solution to a multitude of ills including (in my case) red plating that was socket-specific. While you're at it, clean them with a little Deoxit. Might also squirt a drop of Deoxit into the bias pot and run the wiper back and forth a few times, power off while doing this of course. A weak solder connection on those bias pots is very difficult to catch visually, but try these easier fixes first before reheating those.

    nerpissad

    Posts : 58
    Join date : 2012-09-16
    Age : 43
    Location : amsterdam

    Re: Biasing issues

    Post by nerpissad on Sat Dec 07, 2013 9:31 am

    deepee99 wrote:
    By all means, re-tension the sockets. There's another thread on this topic, and it's a very low-tech solution to a multitude of ills including (in my case) red plating that was socket-specific. While you're at it, clean them with a little Deoxit. Might also squirt a drop of Deoxit into the bias pot and run the wiper back and forth a few times, power off while doing this of course. A weak solder connection on those bias pots is very difficult to catch visually, but try these easier fixes first before reheating those.
    that's what I did yesterday.  I desoldered everything around V7 and took it out of the amp, got rid of excess solder on the bias pot as well and resoldered everything together again.  I also tightened the tube sockets and got rid of a little bit of solder in one of the sockets. that little bit blocked one tube pin to not completely reach all the way down.
    my amp has been working perfectly ever since Wink steady bias, great sound.  this affair just made me a little OCD with my multimeter in hand watching the bias all the time...
    anyway, it seems that so far all of my issues with the amp were due to poor solder connections.  next time I run into something I'll be sure to just take out the problematic 'region' and redo that part again.

    thanks to everyone for all the help on this forum. novices like me cannot do without it!

    deepee99

    Posts : 1260
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Biasing issues

    Post by deepee99 on Sat Dec 07, 2013 10:05 am

    Wonderful to hear! (As is the sound of music through tubes). This forum is a great place for answers, and as a wise old editor used to say, The only dumb question is the one not asked.

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