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    Bias not stopping to zero

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    B&WTube

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    Post by B&WTube on Sun Nov 17, 2019 10:17 pm

    3 out of 4 of my bias pots have the same setting to reach .450 and the other is much lower on the pot dial to achieve.450. Upon playing with it this bias, I noticed that even if I turn it all of the way down- it still reads .160

    Also- this is the bias that controls the socket where I recently blew a power tube (already replaced).

    Furthermore there is considerably more hum coming out of my right speaker vs my left.

    Thanks, in advance, for advice I still have a lot to learn.
    Bob Latino
    Bob Latino
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    Post by Bob Latino on Mon Nov 18, 2019 9:25 am

    B&WTube wrote:3 out of 4 of my bias pots have the same setting to reach .450 and the other is much lower on the pot dial to achieve.450. Upon playing with it this bias, I noticed that even  if I turn it all of the way down- it still reads .160

    Also- this is the bias that controls the socket where I recently blew a power tube (already replaced).

    Furthermore there is considerably more hum coming out of my right speaker vs my left.

    Thanks, in advance, for advice I still have a lot to learn.

    When that power tube blew it also may have taken out the 10 ohm 2 watt resistor that goes from pins 1/8 to chassis ground on the tube socket. Open the amp up and measure the 10 ohm 2 watt resistor on all four tube sockets and see if the one on that socket measures different than the other three. If one of those resistors measures at zero ohms or it measures substantially higher than 10 ohms, then pick up a few off Ebay at the link below. NOTE - You have to pick "10 ohms" from the drop down menu on that Ebay page.

    10 ohm 2 watt 1% resistors on Ebay

    Bob
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    B&WTube

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    Post by B&WTube on Mon Nov 18, 2019 12:06 pm

    Thanks, Bob! I went ahead and ordered those. It is amazing that I can purchase and get them shipped to my house for $2.59 total.

    Do you know of any good tutorial that can walk me through the basics of what NOT to do/touch when testing internal voltages? I have never done anything inside of the amp with the power ON. I've heard there are things that if messed with have enough power to kill, and my kids still need me around for a few more years.
    Bob Latino
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    Post by Bob Latino on Mon Nov 18, 2019 12:16 pm

    B&WTube wrote:Thanks, Bob! I went ahead and ordered those. It is amazing that I can purchase and get them shipped to my house for $2.59 total.

    Do you know of any good tutorial that can walk me through the basics of what NOT to do/touch when testing internal voltages? I have never done anything inside of the amp with the power ON. I've heard there are things that if messed with have enough power to kill, and my kids still need me around for a few more years.

    You test those 10 ohm 2 watt resistors with the amp OFF so there will be no voltages involved. Just set your meter to the lowest resistance scale. On most meters it will be the 0 to 200 ohm scale unless your meter is an "autoranging" meter. If your meter is autoranging, just set it to ohms.

    Bob
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    B&WTube

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    Post by B&WTube on Mon Nov 18, 2019 12:24 pm

    Will do- thanks, Bob!
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    Dale Stevens

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    Post by Dale Stevens on Mon Nov 18, 2019 4:26 pm

    B&W, hold on; Be sure you read the sticky here on SAFETY.

    Bob, why not measure the bias res from the top without opening the amp??
    Just go from pin 8 on each power tube to ground? Dale
    Bob Latino
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    Post by Bob Latino on Mon Nov 18, 2019 5:32 pm

    Dale Stevens wrote:B&W, hold on; Be sure you read the sticky here on SAFETY.

    Bob, why not measure the bias res from the top without opening the amp??
           Just go from pin 8 on each power tube to ground?  Dale

    I have had people measure the wrong pin before to chassis ground from the top of the amp. Actually > you can measure the 10 ohms to chassis ground at each bias measuring point. Just have your meter set on ohms instead of DC volts and have the amp OFF.

    Bob
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    B&WTube

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    Post by B&WTube on Tue Nov 19, 2019 12:01 am

    Alright, so all sockets are measuring .5 on the 1/8 pin, including my problem child rear right. Any other ideas?
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    B&WTube

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    Post by B&WTube on Tue Nov 19, 2019 12:42 am

    Could it be something to do with pin6 of V7, where it connects to the VTA board or the other wires off the VTA? My triode wires are correct there is a place where the insulated wire touches a solder on the neighboring pin.
    Bob Latino
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    Post by Bob Latino on Tue Nov 19, 2019 9:21 am

    B&WTube wrote:Could it be something to do with pin6 of V7, where it connects to the VTA board or the other wires off the VTA?  My triode wires are correct there is a place where the insulated wire touches a solder on the neighboring pin.

    On the VTA amp output tube sockets, pin 6 is where the bias control for that tube socket comes in from the driver board. That pin should not touch any other pin. No pin on that tube socket should touch another pin EXCEPT that pins 1 and 8 are purposely wired together. Check out the wiring of that tube socket carefully.

    Bob
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    B&WTube

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    Post by B&WTube on Tue Nov 19, 2019 10:44 am

    Bias not stopping to zero  75a5de10

    Here is that socket. There is another resistor besides the 1/8 that bridges on all of my sockets. (Please ignore the red/yellow wire- I used the excess wire from the transformer, in place of the red wire)
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    rjpjnk

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    Post by rjpjnk on Tue Nov 19, 2019 6:55 pm

    Is the tube in question lighting up? Might just be an illusion but it looks like the keyway might be between pins 4 and 5 in this pic. Hard to see.
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    B&WTube

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    Post by B&WTube on Tue Nov 19, 2019 7:57 pm

    Yes, it does light up, and it does fit flush and level.
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    B&WTube

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    Post by B&WTube on Wed Nov 20, 2019 9:57 am

    Any other ideas???

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