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  • simms watt PA150 bias issue

    Good morning

    I have a simms watt PA150 in. When the customer brought it ..it lasted about 30mins and then died.
    He popped it too me with new Golden lion KT88 and 4 x new grid stoppers 1k 5 watt resistors. supplied by the previous owner .
    I fitted the resistors and the put tubes in with a bias probe. When i turned on i can see the bias was going up over 70ma pt and rising . so i switched off and turned both bias pots to full. I turned on the bias was at 37-42ma but this is at bias voltage of -82 v
    I changed all the caps and resistors / pots in the bias circuit ( Original ) still the same
    The HT caps had all been changed before 4 x 470UF 450v with 100k resistors

    Tried 2 x used quad sets of 6550's but still the same
    I ordered some new JJ KT88. Fitted bias slightly lower at 35-38ma, but still at -82v

    while i was checking things out i accidentally busted a pin on the tube base. So i decided to replaced all tube bases fitting 1ohm 2 watt resistors for bias monitoring

    Power tube base voltages

    Heater = 6.3v ac
    pin 2 = 661v
    Pin 3 = 656v
    Pin 4 = -82v
    Pin 6 = 663v

    Now too me there is something not right here. All the 150-200 watt 6550s/kt88 amps i do the bias is at -45-50v . I have hunted for the schematic but no one has it

    The owner is going to use the amp on a uk Tour this year, I need it to be right.

    Thank you for reading .. any thought's

    BBB

  • #2
    Originally posted by blindboybenton View Post
    ...Power tube base voltages

    Heater = 6.3v ac
    pin 2 = 661v
    Pin 3 = 656v
    Pin 4 = -82v
    Pin 6 = 663v

    Now too me there is something not right here. All the 150-200 watt 6550s/kt88 amps i do the bias is at -45-50v...
    What were their g2 voltages? Many high voltage/power designs operate g2 on a separate, lower HT than the main plate supply.
    As Vg2 increases in magnitude, so must Vg1; it may be that with this amp, the very high Vg2 require a lot of bias on g1 to keep plate current in check.
    See http://www.mif.pg.gda.pl/homepages/f...084/k/KT88.pdf
    With HT of 560V, g1 needs 80V bias.
    So noting the +/-25% for bias and the higher HT of your amp, you should have a max available bias of perhaps -120V.
    That may require a voltage doubler for the bias supply, which is common with many vintage UK amps.
    My band:- http://www.youtube.com/user/RedwingBand

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    • #3
      I fitted a Blankenship amp with 4xKT88 Golden Lion tubes and this had a bias voltage of -78v. Nowhere near your B+ and I think it was about 520v.

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      • #4
        many thanks Chaps. Ive learnt something today. I will do a voltage doubler circuit later .

        BBB

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