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Thread: Cap in early bassman layout

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    Cap in early bassman layout

    Hi All

    One of the facebook tube amp groups, someone posted a layout clip to an early bassman ive never seen before. The question is what the 0.1uf cap between the plate on one tube and the HV leading to the OT was for.


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    ive seen a few schematics with small caps from plate to ground, to reduce oscillation, but this doesn't quite look like that. It looks like it would "short" AC (audio) but I could not figure out what frequency since its not one of the two low pass/high pass basic circuits I know about (one resistor one cap).

    And, to me anyway, its most odd because it is only on one plate, why didn't they put two caps, from each plate?

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    Senior Member Pedro Vecino's Avatar
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    It must be a mistake. In the schematic of course it does not appear.

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    The cap is wired across one primary half. By transformer coupling it gets reflected and effects the other half as well.
    It constitutes a LP filter together with the reflected load resistance as seen by one tube. This is one forth of total primary impedance. I would expect a cutoff frequency of 1kHz to 1.5KHz depending on OT. Well, it's a bass amp, isn't it?

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    Way cool, thanks. yeah, Pedro, the giuy who posted thought it was an error also.

    Helmholtz, thanks, I thought it was a crude filter, but how did you calculate the knee? Its LC filter? L across the cap? I could not imagine why they would filter unless it was a last minute "Geez this sounds terrible, maybe this will help" thing. I found a great doc, something like history of the bassman, they said only a few of these exist, but no known schematics for the very first ones. This must be a second or third gen?

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    Likely a simple way to use the same circuit for both guitar amp and bass amp. Just add the cap for a bass amp.

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    but how did you calculate the knee?
    It's essentially an RC filter. The R value is the reflected load resistance at one half of the primary. The rated secondary load reflects to one forth of the total primary impedance across each primary half. So an OT with a primary impedance of 4k will give a load of 1k at each tube.

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    Quote Originally Posted by Helmholtz View Post
    It's essentially an RC filter. The R value is the reflected load resistance at one half of the primary. The rated secondary load reflects to one forth of the total primary impedance across each primary half. So an OT with a primary impedance of 4k will give a load of 1k at each tube.
    Cool, thanks.

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    I missed that the amp (seems to be BM version 5B6) has global NFB. This will mitigate the HF roll off and I can't exclude that the cap was considered necessary to prevent oscillation.

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    If compare schematic for 5b6 and 5e6, you can see that is error when drawing layout for 5b6.

    https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_5b6_schem.pdf
    https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_5e6.pdf

    Capacitor .1/600V instead on pin3 5881 should be connect on pins1/2 (gnd) 5881 (in parallel with 2x16/450V capacitor).
    Capacitor .1/600V serves to reduce self-oscillation (whistling) if 2x16/450V capacitors over time dry or change their value.
    Already seen at Early Fender and Marshall amps ... ...

    https://drtube.com/schematics/marshall/1959t-66.gif

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