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    Possible bad bias pot?

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    hwaitung


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    Join date : 2018-05-05

    Possible bad bias pot? Empty Possible bad bias pot?

    Post by hwaitung Mon Jan 02, 2023 4:21 am

    My VTA-ST70 is developing some random pops on left channel recently (It doesn't go away if I put in another power tube). It also produce a pop one or twice when I was making adjustment with front left trim pot. Today I found that the front left power tube has no bias value (0v) regardless of the adjustments. The amp is producing sound, even though I got no bias value. Is that a bad trim pot or I should look deeper for some other troubles?

    Any advice will be appreciated!
    Bob Latino
    Bob Latino
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    Possible bad bias pot? Empty Re: Possible bad bias pot?

    Post by Bob Latino Mon Jan 02, 2023 8:49 am

    hwaitung wrote:My VTA-ST70 is developing some random pops on left channel recently (It doesn't go away if I put in another power tube). It also produce a pop one or twice when I was making adjustment with front left trim pot. Today I found that the front left power tube has no bias value (0v) regardless of the adjustments. The amp is producing sound, even though I got no bias value. Is that a bad trim pot or I should look deeper for some other troubles?

    Any advice will be appreciated!

    With the amp OFF, check that the 10 ohm resistor that goes from pins 1 and 8 to chassis ground on that tube socket does measure about 10 ohms ?

    Bob
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    hwaitung


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    Possible bad bias pot? Empty Re: Possible bad bias pot?

    Post by hwaitung Tue Jan 03, 2023 9:18 am

    That resistor does have a value around 10 ohm and I don't see any unusual burn marks around that as well.
    Bob Latino
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    Post by Bob Latino Tue Jan 03, 2023 10:06 am

    hwaitung wrote:That resistor does have a value around 10 ohm and I don't see any unusual burn marks around that as well.

    OK good - next with the amp ON but no rectifier in the amp and the bias pot for that tube socket centered > Check the negative DC voltage to chassis ground on pins 5 and 6 on that socket. Black probe on the chassis and red probe on pin 5 and then on pin 6. You should get about -25 to -45 VDC on both pins 5 and 6. If you do, then the bias system is working for that tube. If you don't get any voltage or some other strange voltage, then the bias system (on the driver board) has a problem.

    Bob

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    hwaitung


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    Possible bad bias pot? Empty Re: Possible bad bias pot?

    Post by hwaitung Tue Jan 03, 2023 11:13 am

    Bob Latino wrote:
    hwaitung wrote:That resistor does have a value around 10 ohm and I don't see any unusual burn marks around that as well.

    OK good - next with the amp ON but no rectifier in the amp and the bias pot for that tube socket centered > Check the negative DC voltage to chassis ground on pins 5 and 6 on that socket. Black probe on the chassis and red probe on pin 5 and then on pin 6. You should get about -25 to -45 VDC on both pins 5 and 6. If you do, then the bias system is working for that tube. If you don't get any voltage or some other strange voltage, then the bias system (on the driver board) has a problem.

    Bob
    [/size]

    I guess there is no need to connect the speaker when doing this test?
    Bob Latino
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    Possible bad bias pot? Empty Re: Possible bad bias pot?

    Post by Bob Latino Tue Jan 03, 2023 11:17 am

    hwaitung wrote:
    Bob Latino wrote:
    hwaitung wrote:That resistor does have a value around 10 ohm and I don't see any unusual burn marks around that as well.

    OK good - next with the amp ON but no rectifier in the amp and the bias pot for that tube socket centered > Check the negative DC voltage to chassis ground on pins 5 and 6 on that socket. Black probe on the chassis and red probe on pin 5 and then on pin 6. You should get about -25 to -45 VDC on both pins 5 and 6. If you do, then the bias system is working for that tube. If you don't get any voltage or some other strange voltage, then the bias system (on the driver board) has a problem.

    Bob
    [/size]

    I guess there is no need to connect the speaker when doing this test?

    NO - You only need to connect the speaker if the RECTIFIER is plugged in.

    Bob
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    hwaitung


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    Post by hwaitung Wed Jan 04, 2023 9:04 am

    Bob Latino wrote:
    OK good - next with the amp ON but no rectifier in the amp and the bias pot for that tube socket centered > Check the negative DC voltage to chassis ground on pins 5 and 6 on that socket. Black probe on the chassis and red probe on pin 5 and then on pin 6. You should get about -25 to -45 VDC on both pins 5 and 6. If you do, then the bias system is working for that tube. If you don't get any voltage or some other strange voltage, then the bias system (on the driver board) has a problem.
    Bob
    [/size]

    I did that on all 4 sockets. They all have values around -35 VDC.
    Bob Latino
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    Post by Bob Latino Wed Jan 04, 2023 10:08 am

    hwaitung wrote:

    I did that on all 4 sockets. They all have values around -35 VDC.

    That reading of -35 VDC on all four output tube sockets with the bias pots centered is right on ... The bias system is working properly. There has to be something wrong on that tube socket. Either ..

    1. A bad solder connection - Try resoldering all 8 socket pins

    or (more likely)

    2. Bad contact between the tube's pins with the socket pins.

    Try cleaning and then "retensioning" the tube socket pins. To retension the socket pins you take a small flat head screwdriver and push each pin opening to make the opening in the socket pins a little smaller. Clean any dust off the socket pins first.

    Bob
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    hwaitung


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    Possible bad bias pot? Empty Re: Possible bad bias pot?

    Post by hwaitung Wed Jan 04, 2023 10:14 am

    Bob Latino wrote:
    hwaitung wrote:

    I did that on all 4 sockets. They all have values around -35 VDC.

    That reading of -35 VDC on all four output tube sockets with the bias pots centered is right on ... The bias system is working properly. There has to be something wrong on that tube socket. Either ..

    1. A bad solder connection - Try resoldering all 8 socket pins

    or (more likely)

    2. Bad contact between the tube's pins with the socket pins.

    Try cleaning and then "retensioning" the tube socket pins. To retension the socket pins you take a small flat head screwdriver and push each pin opening to make the opening in the socket pins a little smaller. Clean any dust off the socket pins first.

    Bob
    [/size]

    Thank you so much for the explanations! Let me try out your suggestions about the tube socket.
    Seamus
    Seamus


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    Possible bad bias pot? Empty Retention

    Post by Seamus Mon Jan 09, 2023 1:46 pm

    IMO:

    Octal KT88 / 6550 / 6L6 style tubes have pins about 0.095" diameter. A 3/32" drill is 0.09375" diameter, the perfect 'go-nogo' checker. If the shank end of a 3/32" drill will not slide easily into the pin socket, the socket does not need retentioning.

    If the socket does need retentioning, it may need replacing. Sleeve sockets should probably NOT be retentioned as the socket is flared and bending in the top will only provide minimal contact.


    Last edited by Seamus on Mon Jan 09, 2023 1:51 pm; edited 1 time in total (Reason for editing : Can't display image)

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