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    Bias low on MKIII

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    brud

    Posts : 3
    Join date : 2017-01-31

    Bias low on MKIII

    Post by brud on Sun Mar 18, 2018 3:26 am

    Hello all

    I have just build my first Dynaco Mk III ( my second currently in parts !) . Let me outline what mods I have made to the original design.

    1. I have assembled and installed the SDS Quad Cap Replacement Board.
    2. I have assembled and installed the Triode PC Board for Dynaco Mk 3 ( and 2 amps).
    3. I have installed a 240 VAC Transformer from Dynakit.
    4. The bias kit from Dynakit, replacing Selenium Rectifier.

    This kit was originally unassembled when I purchased from ebay.

    Valves I am using :

    1. Sovtek 5AR4
    2. Matched pair Golden Lion KT88's
    3. Genalex 12AU7 (ECC82/B749)

    Have carefully assembled unit over 3 months ( slow I know !! Careful worker ), following the book and Mark Walkers (Blueglow Electronics Youtube videos - thanks Mark !)

    "Checked twice cut once" before turning on. Used Variac to turn on slowly - measured voltages -

    B+ = 765vac
    6.3 heater voltage = 6.0vac
    5.0 heater voltage = 4.7vac

    No sparks or smoke - transformers seemed to run cool !

    Placed in all valves and tested again - brought up slowly on variac - no sparks/smoke . Had DVM ready to read Bias Voltage. Initially zero as tubes warmed up rose to about 300mV dc - attempted to adjust pot and could only get up to 650mV max.

    I have checked the resistors in the bias cct , both 18K and 4.7K checked in tolerance.

    I notice that there is a pot on the Triode amp pcb labeled RX1, ? setting procedure ? relevant in my issues.

    I wonder if I could ask the experts in the group how I should start trouble shooting this amp.

    Is it ok to leave on and check out voltages ??

    Any help or guidance would really be appreciated !!

    I feel I am nearly there just need some troubleshooting tips !

    Cheers
    Bruce
    Sydney , Oz
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    peterh

    Posts : 967
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Bias low on MKIII

    Post by peterh on Sun Mar 18, 2018 4:50 am

    brud wrote:Hello all

    I have just build my first Dynaco Mk III ( my second currently in parts !) . Let me outline what mods I have made to the original design.

    1. I have assembled and installed the SDS Quad Cap Replacement Board.
    2. I have assembled and installed the Triode PC Board for Dynaco Mk 3 ( and 2 amps).
    3. I have installed a 240 VAC Transformer from Dynakit.
    4. The bias kit from Dynakit, replacing Selenium Rectifier.

    This kit was originally unassembled when I purchased from ebay.

    Valves I am using :

    1. Sovtek 5AR4
    2. Matched pair Golden Lion KT88's
    3. Genalex 12AU7 (ECC82/B749)

    Have carefully assembled unit over 3 months ( slow I know !! Careful worker ), following the book and Mark Walkers (Blueglow Electronics Youtube videos - thanks Mark !)

    "Checked twice cut once" before turning on. Used Variac to turn on slowly - measured voltages -

    B+ = 765vac
    6.3 heater voltage = 6.0vac
    5.0 heater voltage = 4.7vac

    No sparks or smoke - transformers seemed to run cool !

    Placed in all valves and tested again - brought up slowly on variac - no sparks/smoke . Had DVM ready to read Bias Voltage. Initially zero as tubes warmed up  rose to about 300mV dc - attempted to adjust pot and could only get up to 650mV max.

    I have checked the resistors in the bias cct , both 18K and 4.7K checked  in tolerance.

    I notice that there is a pot on the Triode amp pcb labeled RX1, ? setting procedure ? relevant in my issues.

    I wonder if I could ask the experts in the group how I should start trouble shooting this amp.

    Is it ok to leave on and check out voltages ??

    Any help or guidance would really be appreciated !!

    I feel I am nearly there just need some troubleshooting tips !

    Cheers
    Bruce
    Sydney , Oz

    If documentation and function arn't right using the triode board, i suggest you
    purchase original boards from dynakitparts and start with something that
    is a dynaco MkIII.

    Do you have any means of using the KT88 and verify their function in another amp ?

    B+ seems alarmingly high, that can cap is at risk here !, the filament(s) are on the low side. It
    might be that the bias is to high ( too much negative) but i'd anyway not have this unit
    powered on for any length of time.

    Thus, remove all non-documented stuff, replace with things that is documented and start over.
    avatar
    WLT

    Posts : 22
    Join date : 2013-07-13
    Location : Rochester NY

    B+

    Post by WLT on Sun Mar 18, 2018 9:35 am

    The B+ may actually be OK. Please correct but you were measuring at each of the red leg voltages - correct. Not red leg to ground. The important voltage would be the actual DC voltage after the 5AR4. Is it anywhere near 500DC? If so you are OK

    To answer one of your questions. With the bias reading low it is OK to leave it on. The output tubes would be running a lower current thru them than usual which is a safer position than the other way around.

    My MK III are stock so I can not help with the mods you did. If you measure the DC voltages at the bias pot I can compare with the stock values and then offer some opinions. Per Peterh it may be best to try more stock arrangement first. Do you have the original PC boards?
    avatar
    DynakitParts
    Admin

    Posts : 195
    Join date : 2008-11-30

    MK III low bias

    Post by DynakitParts on Sun Mar 18, 2018 1:10 pm

    I suspect that you may be measuring the high voltage secondary's (RED --RED) like WLT stated.....

    Assuming this is correct and your B + is around 500 VDC....then what you may need to do is to change the bias supply 4.7K resistor value to better suit the operating parameters of your Gold Lion KT 88 output tubes. This is a bit trail & error but in combination with the 10K bias pot you should be able to achieve a suitable bias setting. Try replacing the 4.7K resistor with a 1K 1/2 or 1 watt resistor....metal film, carbon comp, metal oxide, wire wound...whatever you have on hand. See if this resistor value change allows you to now set the bias....let us know.

    I'm not familiar with Triode's MK III PC board or the adjustment pot you are referring to.......Is Triode still in business?

    Kevin

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