DSL40C

Danny1969Danny1969 Frets: 3000
Just had a DSL40C come in which was making a terrible noise with nothing plugged in. New valves and a bias have sorted it out and it sounds nice ... it's just there's a bit of mains hum which doesn't get any louder when you turn up the amp and onstage you probably would't notice it but on a quiet front room it's annoying 

Any words of wisdom of what I should be looking for on this amp or is this normal for this model ?
www.2020studios.co.uk 
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  • DJH83004DJH83004 Frets: 130
    Does changing over the triode / pentode switch make any appreciable difference?
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  • Danny1969Danny1969 Frets: 3000
    Yes quieter in full power mode 
    www.2020studios.co.uk 
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  • I tried a brand new one in a shop and it hummed like a shop door tramp at any volume. I did not buy it. 

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  • DJH83004DJH83004 Frets: 130
    Danny1969 said:
    Yes quieter in full power mode 
    Checking back on my notes, on the last one I serviced, yours sounds typical, it is a very high gain amp, and not the best layout, I did think it would also benefit from from valve shields on the preamp valves, but alas not fitted.  
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  • Danny1969Danny1969 Frets: 3000
    DJH83004 said:
    Danny1969 said:
    Yes quieter in full power mode 
    Checking back on my notes, on the last one I serviced, yours sounds typical, it is a very high gain amp, and not the best layout, I did think it would also benefit from from valve shields on the preamp valves, but alas not fitted.  
    Thanks for that, one more thing a quick google suggested about 38mv for bias would you agree with that ?
    www.2020studios.co.uk 
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  • DJH83004DJH83004 Frets: 130
    edited November 10
    Again according to my notes I set it at 37.6mA @ 400Va which equates to around 60% max dissipation, which is what the customer had asked for.
    Edit: which across the 1 ohm cathode resistor, is about where you are.
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  • ICBMICBM Frets: 26818
    Hum in triode mode is normal.

    I don't understand the logic of using it to be honest - it always hums at least a bit, it sounds shit and it doesn't give a useful volume reduction. The valves are pentodes and don't truly operate as triodes with the screen connected to the plate anyway, it's basically a bodge.

    Apart from a couple of blown valves I've seen no faults with the new-model DSL40C (as opposed to DSL401) at all.
    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone."
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  • DJH83004DJH83004 Frets: 130
    I fairness IC, this one just came just came in for a service and an output valve swap, so no issues, just a bit on the noisy side (preamp / gain wise). I did note that they only fed the heater of V1 with DC, putting a lot less stress on the bridge rectifiers.      
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  • ICBMICBM Frets: 26818
    DJH83004 said:
    I fairness IC, this one just came just came in for a service and an output valve swap, so no issues, just a bit on the noisy side (preamp / gain wise). I did note that they only fed the heater of V1 with DC, putting a lot less stress on the bridge rectifiers.      
    That's sensible.

    What's so stupid about the fault on the old model is that they could have fixed it very simply by adding a cheap heatsink - but they never did despite the problem being well-known for years.
    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone."
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