5F6a Internal jumper - before or after 68k resistors?

Discussion in 'Glowing Bottle Tube Amp Forum' started by Antoon, May 26, 2020.

  1. Antoon

    Antoon Tele-Afflicted

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    I have a Bassman build to which I added a switchable internal channel jumper. Each channel (bright and normal) has only one input in my amp (high gain only, 68k's paralleled for 34k). When the jumper is engaged it also disconnects the shunt of the other (non-used) input from ground. So during jumpering the signal is connected to ground by only 500k (2 x 1M in parallel on the inputs) and not also through the shunt. It is basically a direct connection between pin2 and pin7 of V1.

    When I jumper the channels as described and play it with my Tele (both volumes at, or above 9:00h) I think I am getting some sort of mild distortion in the mids/lows of which I am not sure that I like it. I do not hear this distortion when I only use one channel without the jumper (at similar playing volume). When I look at other schematics, most people (including Rob Robinette for instance) put the jumper before (rather than after) the 68k input resistors. So the jumper connection is right at the inputs. This may be out of practical reasons, or is this perhaps a better way to do it? Perhaps to avoid the type of distortion that I am getting? I sometimes think I hear the same type of distortion when I listen to online demo's of channel jumpering on 5F6a's, so I also not sure if the way I connected the channels is the cause. Would increasing the mixer resistors to 470k improve the sound in the jumpered mode?
     
  2. Antoon

    Antoon Tele-Afflicted

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    I changed something on the speaker baffle and the problem seems to have been solved. Apparently there was some kind of resonance somewhere that was triggered by the heavier low-mids in the jumpered mode. I can now recommend the setup I used (connection between pin2 and 7 and simultaneously disconnecting the shunt of the unused input). I used a 0.005uF cap for coupling in the normal channel (bright channel retains the 0.02uF) to avoid the bass getting too heavy in the jumpered mode. Seems to work, when I switch from bright channel only, to jumpered I am not getting an overwhelming bass response (a constant struggle in a 5F6a IMO). The tone just gets thicker and somewhat more complex. Just what I want :)
     
    Last edited: May 27, 2020
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