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    B+ is running high in one of my MKIV's

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    B+ is running high in one of my MKIV's

    Post by Guest on Tue Mar 13, 2012 12:16 pm

    I am not sure how this may relate to the problem I am having biasing the amp. I checked the B+ at the VTA board, and it is reading 483 VDC, and that is with the 6.8K in place at the quad cap. The amp that is biasing properly, is reading 426 VDC, which is right it the middle of the 400-440 VDC recommended in the VTA board install manual. PT are not really suppose to have many problems. Do you have any suggestions as to how to further explore this problem, and what might be the source of the problem: PT, Quad cap, I checked the resistor, and it's fine. How would the higher voltage effect the biasing problem I am having? ?

    DynakitParts
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    High Voltage

    Post by DynakitParts on Tue Mar 13, 2012 9:29 pm


    Mike,
    Remove the output tubes & rectifier tube on both amps and measure the high
    voltage secondary (Red) (Red) leads connected to rectifier Socket (V1). Measure each
    lead to Ground. Compare readings to each amplifier. Let us know what you find.

    Have you tried swapping out rectifier tubes?

    Kevin

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    Re: B+ is running high in one of my MKIV's

    Post by Guest on Tue Mar 13, 2012 9:41 pm

    Kevin: Thanks for the suggestion. Yes, I have had 3 pairs of tube, and 3 different rectifiers in each amp. I will measure the PT red leads and get back with the results. Thanks Michael

    Guest
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    Re: B+ is running high in one of my MKIV's

    Post by Guest on Wed Mar 14, 2012 10:49 am

    Kevin: Although I did not do exactly what you suggested, it gave me an idea. I decided to load all the tubes in the amps and trace the voltage from where it entered the rectifier, all the way to where the B+ attaches to the VTA board. Below are the results:

    Amp 1 (the amp with the high B+) B+ 435 VDC at VTA board, PT voltage input at rectifier 176 -176, output at rectifier -481 VDC, output at quad cap -416 VDC

    Amp 2 (the amp with low bias) B+ 380 VDC at VTA board, PT voltage input at rectifier 181 -181 output at rectifiier -418 VDC, output of quad cap -382 VDC

    As you can see, it drops off at the cap, which is weak, or on the way out; my guess any way. I was told by a guy at Triode Electronics, that the chokes rarely fail, but I checked them and they appear OK. Thanks for your help, I not sure it would have occurred to me to this, had you not mentioned checking the PT output at the rectifier.

    Michael

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    Guest

    Re: B+ is running high in one of my MKIV's

    Post by Guest on Wed Mar 14, 2012 2:48 pm

    There is concern I wired the triode/UL switch wrong. Here is a photo. https://i45.servimg.com/u/f45/17/35/86/89/dynaco22.jpg

    Guest
    Guest

    Re: B+ is running high in one of my MKIV's

    Post by Guest on Thu Mar 15, 2012 1:53 am

    Kevin: I pulled all the wires, and the resistor off the quad cap, made sure it was discharged, and measured it. Turns out the 80uF section is dead. Look forward to getting the new caps. Thanks again!

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