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Thread: Marshall DSL40C - New tubes are hotter than old ones - bias pots all way down - 41mA

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    Marshall DSL40C - New tubes are hotter than old ones - bias pots all way down - 41mA

    hello, me again with another marshall. This time just a simple tube change, just too old tubes (like 6 years and some noises).

    With the old tubes I can go from 16mA to like 60mA using the bias pots (x2). With the new tubes, both bias pots all way down, I get 38/41 mA and I want to be able to set the bias colder. By the way, the amp was set at 40mA with the old tubes (factory, no service done) and I wanted to set my new tubes at 36mA but couldn't.

    I have noticed the old tubes have stickers that read 46mA, and my new tubes have the same sticker, but it read 70mA, so I asume the new tubes are way hotter or draw more current, and to put them in a safe range I must change or add some resistors in the bias circuit, am I correct?

    Taking a look at the schematic, is it ok If I put a resistor in parallel with R106 to compensate for those new hotter tubes and not taking the pcb out? What would be a recommended value?

    These tubes were brought from the USA and can't be sent back, I have to work with them.

    Click image for larger version. 

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    Schematic: DSL40-60-02-v02.pdf


    EDIT: New info

    I was making all the measurements at the test points on the pcb, 1 ohm resistor method.
    Now I have plugged in my bias probe (plate current) and both new tubes read 36mA. At the same time I tested at the pcb bias test point and one reads 37mA, the other one reads 40mA.

    My plate voltage is 456VDC

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    Last edited by spunko; 11-13-2019 at 09:18 PM.

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    I'd try a 47K in parallel with R106 and see what kind of range that gives you.

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    Maybe the board 1 ohm cathode current sensing resistor/s have drifted up in value a bit. Perhaps following a tube short incident.

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    Quote Originally Posted by g1 View Post
    I'd try a 47K in parallel with R106 and see what kind of range that gives you.
    I found a 40K 1/2 watt resistor, place it in parallel with R106 and it droped to 22mA with both bias pot all the way down, so it worked. Thank you very much.

    Quote Originally Posted by pdf64 View Post
    Maybe the board 1 ohm cathode current sensing resistor/s have drifted up in value a bit. Perhaps following a tube short incident.
    The amp has been working great since new, no shorts, no repairs, just really old tubes making noises. But maybe those resistors drifted a bit.



    The strange thing is that the pcb test point always gives 2mA or 3mA more that what gives the bias probe.

    I have it now at 34mA with the bias probe, and the pcb TP gives 36mA. Is it safe to take it to 37mA, no matter if the pcb TP reads 40mA or so?

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    Now I have plugged in my bias probe (plate current)...
    You say your bias probe measures plate current (some do - most measure cathode current). Plate current is always lower than cathode current (with pentodes) as cathode current = plate current + screen current. The PCB testpoint would show cathode current.

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    Last edited by Helmholtz; 11-14-2019 at 03:22 PM.
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