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Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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

    Any other ideas???
    MechEngVic
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    Post by MechEngVic on Tue Dec 24, 2019 1:41 am

    Did you try moving the tubes around?
    tubes4hifi
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    Post by tubes4hifi on Fri Dec 27, 2019 10:07 pm

    going all the way back to post #1
    1 - with power off, check each 10 ohm resistor on each output tube to ground, all should be within 9.5 - 10.5 ohms, if not, replace
    2 - try swapping tubes around to see if the bias problem follows the tube or stays at that socket. If it follows the tube, the tube is bad.
    3 - with ALL tubes out of the amp, with each bias pot at center position, you should measure between 35-40vdc from each output tube socket pin 6 to ground.
    If ONE of them measures different, maybe that bias pot needs to be replaced. They are 3386P type, and must be 50K ohms.
    4 - with the power off, measure from the top of R29, top of R30, top of R31, and top of R32 to ground.
    With the bias pots set at center position, you should get a reading of 30.5K (from 30K to 31K)from each of those points.
    (You'll need to wait about ten seconds for the reading to settle due to C17 in the circuit) If one is way different, you've got a bad bias pot.
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    Post by peterh on Sat Dec 28, 2019 4:34 am

    tubes4hifi wrote:going all the way back to post #1
    1 - with power off, check each 10 ohm resistor on each output tube to ground, all should be within 9.5 - 10.5 ohms, if not, replace
    2 - try swapping tubes around to see if the bias problem follows the tube or stays at that socket.   If it follows the tube, the tube is bad.
    3 - with ALL tubes out of the amp, with each bias pot at center position, you should measure between 35-40vdc from each output tube socket pin 6 to ground.
         If ONE of them measures different, maybe that bias pot needs to be replaced.   They are 3386P type, and must be 50K ohms.
    4 - with the power off, measure from the top of R29, top of R30, top of R31, and top of R32 to ground.
        With the bias pots set at center position, you should get a reading of 30.5K (from 30K to 31K)from each of those points.  
         (You'll need to wait about ten seconds for the reading to settle due to C17 in the circuit)   If one is way different, you've got a bad bias pot.
    Point 3 above : pin 6 should be pin 5 !
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    Post by tubes4hifi on Sat Dec 28, 2019 2:21 pm

    peterh
    pin 5 or pin 6 works, there should be a 1K resistor between the two, with very little current thru it, either way you'll get the same voltage reading
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    Post by B&WTube on Mon Jan 27, 2020 1:04 pm

    tubes4hifi wrote:going all the way back to post #1
    1 - with power off, check each 10 ohm resistor on each output tube to ground, all should be within 9.5 - 10.5 ohms, if not, replace
    2 - try swapping tubes around to see if the bias problem follows the tube or stays at that socket.   If it follows the tube, the tube is bad.
    3 - with ALL tubes out of the amp, with each bias pot at center position, you should measure between 35-40vdc from each output tube socket pin 6 to ground.
         If ONE of them measures different, maybe that bias pot needs to be replaced.   They are 3386P type, and must be 50K ohms.
    4 - with the power off, measure from the top of R29, top of R30, top of R31, and top of R32 to ground.
        With the bias pots set at center position, you should get a reading of 30.5K (from 30K to 31K)from each of those points.  
         (You'll need to wait about ten seconds for the reading to settle due to C17 in the circuit)   If one is way different, you've got a bad bias pot.

    I thought the problem was just a bad tube, last time, BUT just had another tube in this socket go go red on me. So, I am back to the steps you have outlined. The 10ohm 2 watt resistors on 1/8 pins on all sockets are measuring the same, at .5. I have a couple of questions on the other steps.

    -I am going to start measuring the bias pots. When you say "Center position", do you mean in roughly the middle of the 270' rotation that the screw can rotate?
    -Step 3 is with the amp 'ON', correct?
    -In Step 4, I don't understand what exactly I am supposed to be testing. Can you elaborate?

    I am sorry for my lack of knowledge on this, but am trying to learn, and greatly appreciate the help!
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    Post by tubes4hifi on Mon Jan 27, 2020 2:07 pm

    step 1 insures that you have a ten ohm resistor connected from pins 1 and 8 (tied together) to ground on each output tube, with good solder connections on each end.
    step 2 is to see if the problem is a particular tube
    step 3 is to insure that all four legs of the bias circuit are working correctly (it should be about NEGATIVE 40vdc from pin 5 to ground on each output socket)
    step 4 is to find out (if step 3 didn't confirm) that possibly you have one bad BIAS POT. With the amp OFF, you are taking an OHM measurement, thru the entire bias circuit.
    You could also of course measure across each bias pot (50K) and from center of bias pot to either end (about 25K) and making sure it is adjustable.
    This could also be a part of step 3 (no tubes, power ON), on the socket you are having problems with, the voltage should be adjustable from about negative 25vdc to negative 50vdc
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    Post by tubes4hifi on Mon Jan 27, 2020 2:34 pm

    here's a combination schematic and pictorial diagram of one-half of an ST70 / ST120 tube amp

    Bias not stopping to zero  VTA bias2
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    B&WTube

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    Post by B&WTube on Mon Jan 27, 2020 3:58 pm

    Roy, thank you so much for your posts. One more clarification on step 3- I'm not going to touch something that kills me with the amp open and upside down, with the power on- right (my wife and kids still need me Smile

    Also, I don't know if this means anything, but the tube that is going bad has silver/mirror finish on the glass on the rear of the tube (rear side, when tube is installed in amp). Is that indicative of anything?
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    Post by B&WTube on Tue Jan 28, 2020 12:15 am

    OK, so everything so far is checking out:
    -10ohm resistors all showing 10.2-10.5
    -With all biases set in the middle, all tubes (including rectifier out), and amp ON- Pin 6 is reading about -31.5 on all sockets
    -Resistor between pins 5&6 on all sockets is about 985
    -On my problem socket (V7), bias range is -43.4 to -22.6 ( v6 measured similar)

    So, it all seems to my eyes to be normal. So, I would think I am good, BUT I have burned out 2 Gold Lion KT77’s in this socket in 250 hours. Is there anything else I can test- could it be attenuator related? What do you think? Thanks so much again for your help, this noob greatly appreciates it!
    -
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    Post by tubes4hifi on Tue Jan 28, 2020 1:49 am

    so it seems that socket and the bias are all OK.
    If you've had a couple of tubes go bad in that particular socket, I would just make sure to re-do all the solder connections on that socket.
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    Post by B&WTube on Wed Apr 22, 2020 2:15 pm

    I hate to bring it up again, but I am having issues with the same back right socket again. I was quietly listening to music, and heard heavy distortion and the tube was glowing red hot. I cut the amp off for a while, turned the bias all of the way down, and then fired it back up. No problems after 30 minutes. So I added the bias, but it will not go high enough- with the highest bias pot position i can only get to .240 Other GL kt77's are at .450).

    After the last round of issues put in a new tube, tested all the resistors around this socket as well as the ground- everything was within spec. I also re-soldered everything related to the socket. Life has been good for the last almost 3 months. Amp had been on for 2 hours before this. Anyone have any advice?

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