NAD + Q's on my 5F1-ish circuit mods

Discussion in 'Shock Brother's DIY Amps' started by timfred, Feb 10, 2019.

  1. timfred

    timfred TDPRI Member

    Mar 14, 2016
    Northern CA
    So about a week ago I picked up this super cool-looking MOTS-covered circa late 1950's Magnatone 108:


    The original circuit is here (thanks to the Magnatone amps pages). It sounded nice, but with the tone control and heavy NFB it was way too clean. I mean, who needs a small clean amp?

    So my apologies to Mangatone amp lovers, but I started modding it. Nothing I've done is irreversible, so it can always be returned to original form (except for solder).

    Here's the resulting circuit.


    It has a more overdrive than before - it starts breaking up around halfway. Compared to other similar amps the distortion balance seems shifted more towards pre-amp than power amp. It's nice.

    This is the first time I've played with this circuit. For those of you who have played with this more than me, here are some questions:
    1. Any worries or tips? What would you tweak?
    2. I think the 6V6 is dissipating about 17W, which seems super high to me but Google tells me that lots of Tweed Champs are in that range. Should I think about biasing it cooler? (original black plate RCA)
    3. two-node vs. three-node power supply. I increased the first stage to 32uF, but looking at other similar amps from Fender, Gibson, Supro, etc they all have a three-node PS. Should I think about adding that? Why?
    4. Perhaps related to #3 - does the power amp screen voltage seem low to you? What does this do to the sound or reliability of the 6V6?
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