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    Crossover Distortion in One Channel of VTA70. Causes? (FIXED: False alarm)

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    rjpjnk

    Posts : 59
    Join date : 2018-07-18

    Crossover Distortion in One Channel of VTA70. Causes? (FIXED: False alarm)

    Post by rjpjnk on Sat Jul 21, 2018 10:29 am

    I put my VTA70 on the bench for some testing last night and discovered that the right channel is not working properly.

    I see significant crossover distortion in the output waveform like the picture below. The left channel appears perfect. Switching tubes made no difference. In addition to the crossover distortion the overall amplitude of the sine wave is lower than the left channel. But this is probably to be expected as they likely have the same cause. The bias of all 4 tubes is hanging nicely at 40.

    I plan to open it up and start troubleshooting today but was wondering if anyone might have any hits for what might have failed.

    I purchased this amp in May of this year. It was so much fun to build, and and it has been a real delight in every way. A truly stellar performer on the bench as well as in the living room. I last had the amp on the bench for measurements about a month ago, so some time since then the problem began.

    I will update this thread as I troubleshoot this weekend. Any help appreciated!  

    Thanks

    NOTE: I tried to post a picture of the scope but as a new member I cannot for 7 days it seems Sad


    Last edited by rjpjnk on Tue Jul 31, 2018 8:31 pm; edited 1 time in total (Reason for editing : update)

    rjpjnk

    Posts : 59
    Join date : 2018-07-18

    Re: Crossover Distortion in One Channel of VTA70. Causes? (FIXED: False alarm)

    Post by rjpjnk on Sat Jul 21, 2018 12:16 pm

    FALSE ALARM. Turns out is was my own dumb error. Everything is ok now.

    I knew we needed a 40mA bias current, and I forgot the measurement resistor on the cathode was 10 Ohms rather than 1 Ohm, and so I mistakenly set the bias voltage on one channel to 40 mV rather than 400 mV.

    All is well now.
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    solderblob

    Posts : 29
    Join date : 2018-05-20
    Location : Placerville, California

    Re: Crossover Distortion in One Channel of VTA70. Causes? (FIXED: False alarm)

    Post by solderblob on Sun Jul 22, 2018 11:32 am

    So you're back in the good graces of Mr. Ohm.  Good catch! Like a Star @ heaven
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    Peter W.

    Posts : 831
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Crossover Distortion in One Channel of VTA70. Causes? (FIXED: False alarm)

    Post by Peter W. on Mon Jul 23, 2018 9:08 am

    rjpjnk wrote:FALSE ALARM. Turns out is was my own dumb error. Everything is ok now.

    I knew we needed a 40mA bias current, and I forgot the measurement resistor on the cathode was 10 Ohms rather than 1 Ohm, and so I mistakenly set the bias voltage on one channel to 40 mV rather than 400 mV.

    All is well now.

    Happens to the best of us!

    For my sins, I also restore vintage radios. One day, I had a Zenith B600 chassis on the bench, my first 600-type - dead silent. Turns out that the headphone jack (stays in the case) is a shorting-jack on these beasts. Oops! Three hours, one jumper later, I figured it out.

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    Re: Crossover Distortion in One Channel of VTA70. Causes? (FIXED: False alarm)

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