Twin master volume, boost switch

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Cejay825, Jan 30, 2012.

  1. Cejay825

    Cejay825 Member

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    Posted this in the amp section last week, somebody siad I might have better luck here.....
    Maybe someone has done this before......I play my 78 SF twin with the master volume all the way up and use only the channel volume to adjust. When running like this, when you pull the master volume knob the boost sound is awesome. Yes it sounds like crap with the channel volume at 10 and gradually raise the master volume, but not the other way around,,,at least not on this Twin, the SFT 78-81 pull boost knobs seem to be very different sounding than the earlier ones. Okay, my question, I would like to mount a 1/4" jack (or equivilant) in the back of the chassis and wire the pull/push switch portion of the master volume to the jack and operate it with a foot switch. Anyone ever done this? or is it possible ?
     
  2. zenas

    zenas Member

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    I haven't work on one of those in a good ten years. But the push pull is just a switch so it should work. A sheilded cable might be a good idea to keep the noise down.
    Would make it alot more useful.
     
  3. pdf64

    pdf64 Member

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    Yes, that's an example of the difference between overdriving the TR's power amp and overdriving the TR's pre-amp.
    To replicate the front panel boost switch remotely, 2 signal conductors would be needed; a signal feed and a switched signal return. It's at a fairly low impedance, so a stereo jack socket and cable made from a balanced mic lead may work ok. The remote switch just connects the 2 signal conductors together (or not); it doesn't short to ground.
    The front panel boost switch can be left wired in place, just left switched off. Or a switching socket for the footswitch could be arranged so as to override the front panel boost switch.
    I can't see any significant difference between the boost circuit of the 2 SF MV TR schematics at schematic heaven
    http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/fenderamps/twin_reverb_sf_135_schem.pdf
    http://www.webphix.com/schematic he...n.com/fenderamps/twin_reverb_sf_100_schem.pdf
    Maybe it's just that the later UL power amp overdrives with a different tonal quality?
    Pete.
     
  4. Cejay825

    Cejay825 Member

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    Thanks Pete, I don't know why it sounds different, I've been around alot of Twins and have never heard one like this. I'm going to take the schematics and look inside to see if anything has been changed. You're idea seems easy enough but I think a relay will be needed to cut the noise down, other than that it seems pretty straight forward
     
  5. pdf64

    pdf64 Member

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    That's what I thought at first, however the signal level is quite high and the impedance is low, 1k max. And there's only 1 gain stage following it before the power amp. So the potential for noise getting in is low.
    Does the present Reverb footswitch add an unacceptable amount of noise? There's 2 cascading gain stages between that and the power amp, at a much higher impedance level.
    So if that's ok, noise from a boost footswitch described will be insignificant.
    Pete.
     

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