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The Dynaco Tube Audio Forum

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


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    MKIII Bias Not Controllable

    snip73
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    snip73


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    • Post n°1

    MKIII Bias Not Controllable Empty MKIII Bias Not Controllable

    Post by snip73 Wed May 10, 2023 11:47 am

    I have a pair of MKIII's that have VTA (Roy's) latest revision board installed. The bias on one drifts up to the point where the tubes cherry plate. No issues with sound.

    I have followed the Curio troubleshooting guide linked in another post. So far:

    • The 10K resistor measures correctly
    • Pin 3 and Pin 4 both read 590vDC
    • With the output tubes removed, rectifier installed - I meausre -39.5vDC to -77.2vDC as the range when moving the bias pot(s) full clockwise to full counter-clockwise
    • I have swapped in other output tubes - no change
    • These measurements closely match my other amp



    What should I consider next?
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    Bob Latino
    Bob Latino
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    • Post n°2

    MKIII Bias Not Controllable Empty Re: MKIII Bias Not Controllable

    Post by Bob Latino Wed May 10, 2023 12:30 pm

    snip73 wrote:I have a pair of MKIII's that have VTA (Roy's) latest revision board installed. The bias on one drifts up to the point where the tubes cherry plate. No issues with sound.

    I have followed the Curio troubleshooting guide linked in another post. So far:

    • The 10K resistor measures correctly
    • Pin 3 and Pin 4 both read 590vDC
    • With the output tubes removed, rectifier installed - I meausre -39.5vDC to -77.2vDC as the range when moving the bias pot(s) full clockwise to full counter-clockwise
    • I have swapped in other output tubes - no change
    • These measurements closely match my other amp



    What should I consider next?

    Pin 3 and pin 4 measure about 100 volts over spec ! A Mark III should be at about 475 - 500 VDC on pins 3 and 4. Check your line voltage. It is probably higher than 120 VAC ? If your line voltage is at or over 122 VAC, run the two amps off a variac set to 118 VAC. As to the gradual updrift of the bias > maybe bad rectifier tube or possibly a bad quad cap ? Swap rectifier tube between the two amps and see if the condition moves over to the other amp ? If it does, you need a new rectifier for that amp. If a known to be good rectifier does not help, I am thinking that the quad cap may be leaking

    Bob

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    snip73
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    snip73


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    • Post n°3

    MKIII Bias Not Controllable Empty Re: MKIII Bias Not Controllable

    Post by snip73 Wed May 10, 2023 1:14 pm

    Thank you Bob.

    Line voltage is 122.1

    Pin 3 and 4 read 590 on the other amp as well

    I switched rectifiers. The "good" amp with the "bad" amp's rectifier is reading roughly .5vDC bias

    I rebuilt these with 4 separate caps (three below the chassis, one up top) - not a quad cap.
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    peterh
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    • Post n°4

    MKIII Bias Not Controllable Empty Re: MKIII Bias Not Controllable

    Post by peterh Wed May 10, 2023 2:02 pm

    snip73 wrote:I have a pair of MKIII's that have VTA (Roy's) latest revision board installed. The bias on one drifts up to the point where the tubes cherry plate. No issues with sound.

    I have followed the Curio troubleshooting guide linked in another post. So far:

    • The 10K resistor measures correctly
    • Pin 3 and Pin 4 both read 590vDC
    • With the output tubes removed, rectifier installed - I meausre -39.5vDC to -77.2vDC as the range when moving the bias pot(s) full clockwise to full counter-clockwise
    • I have swapped in other output tubes - no change
    • These measurements closely match my other amp



    What should I consider next?
    590 Volt could be possible if you removed the power tubes and measure in the socket. Don't do this,
    have the tubes installed and speakers or a power resistor as load while measuring coltages.
    I suspect you will find app 500Volt.

    What could cause your problems is that the g1 resistor ( between grid and bias pot) should be < 51k if using 6550
    ( dynaco used 100k here which is a little high. If using KT88 100k is ok.) Check your board for these.

    Also the bias circuit if it is different from Dynaco, it must be "stiff enough" to not drift significantly.

    Another question : is this a Dynaco MkIII or is it a different type ? If so, please supply schematics
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    snip73
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    • Post n°5

    MKIII Bias Not Controllable Empty Re: MKIII Bias Not Controllable

    Post by snip73 Wed May 10, 2023 4:23 pm

    "I suspect you will find app 500Volt"...

    Yes! I knew this, but forgot. 480vDC now.

    This is a Dynaco MKIII modded with the boards from Tubes4hifii. Links to a drawing and schematic:

    https://www.dropbox.com/s/l3735qfcovmhs92/Page%206.pdf?dl=0


    https://www.dropbox.com/s/6azpibph7h15353/Page%208.pdf?dl=0
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    peterh
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    • Post n°6

    MKIII Bias Not Controllable Empty Re: MKIII Bias Not Controllable

    Post by peterh Thu May 11, 2023 12:47 am

    C6 and C8 must be 630V as they have > 500Volt across.
    grid resistors 100k is ok using KT88, some 6550 might have problems.
    If your tubes has been redplated they should be replaced with new as their innards
    cannot be trusted.
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    snip73
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    • Post n°7

    MKIII Bias Not Controllable Empty Re: MKIII Bias Not Controllable

    Post by snip73 Sat May 13, 2023 2:02 pm

    I have swapped in good KT88 tubes. No change in the bias voltages.

    Checking C6 and C8: 260v on each. The good amp measures 400vDC on each of them.

    I also removed the 4 capacitors that are wired in series to replace the quad cap. Each measures rated capacitance, within 10%.

    peterh wrote:C6 and C8 must be 630V as they have > 500Volt across.
    grid resistors 100k is ok using KT88, some 6550 might have problems.
    If your tubes has been redplated they should be replaced with new as their innards
    cannot be trusted.
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    snip73
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    • Post n°8

    MKIII Bias Not Controllable Empty Re: MKIII Bias Not Controllable

    Post by snip73 Mon May 15, 2023 7:43 am

    I am now somewhat convinced it all boils down to a tube socket. After removing the rectifier and output tubes, and trying to tighten all the pins - the bias is now holding at about .45vDC for two+ hours.
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    Seamus
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    • Post n°9

    MKIII Bias Not Controllable Empty Check Pins Every Time a Tube is Removed

    Post by Seamus Mon May 22, 2023 12:43 pm


    Always check tube sockets any time you pull a tube. A 3/32" drill works a treat for octal sockets.

    Or buy a couple of gage pins for the tube types used.

    !!_ ALWAYS VERIFY 0vdc ON TUBE PINS BEFORE GAGING _!!
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    MKIII Bias Not Controllable Empty Re: MKIII Bias Not Controllable

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