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Alt Channel Link Mod - BF/SF Fenders

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  • Alt Channel Link Mod - BF/SF Fenders

    It's been noted that that regular 1 wire channel link 'Torres' mod for 2 channel BF/SF Fenders makes the normal channel bassier than the vibrato channel.
    I wondered how that could be, given that the coupling caps, 0.047 and 0.022uF, driving into the 3M3+ load of the reverb mixer, has a sub sonic low frequency -3dB breakpoint.
    I scoped it out on the bench and found that it was true - feed the same signal into those 2nd stage grids and when they're mixed together, measured at the 3rd stage mixer plate, at 80Hz the normal channel is boosted about 2.5dB, the vibrato channel cut 3dB (compared to mid level 400-1kHz).
    I've reasoned that this is due to, rather than the 3M3 mixer being the load, the load for one channel is actually the plate resistance of the other channel, + the impedance of the 2 coupling caps. Effectively a capacitive potential divider with a resistive load, in the form of the other channel's plate resistance. That's why the channel with the bigger cap gets a low boost, while the channel with the smaller cap actual gets a low cut.
    A frequency dependant mixer has been created, due to the 2 caps being different values.
    If that's what you want then fine, but I've been thinking about how this artificial cut/boost can be avoided. 1 way would be to fit equal value capacitors, however, a simpler and cleaner solution is to directly link the plates of the 2 stages to be mixed, rather than mix them via coupling caps.
    Therefore the alternative channel link mod is to remove the 0.047uF normal channel coupling cap, and to fit a short wire link between terminal 6 of V1 and V2, either on the tube socket lugs, or on the eyelet board.
    See
    http://i963.photobucket.com/albums/a...nelLinkMod.jpg
    The frequency dependance of the channel mixing is removed, both channels are mixed evenly across the audio band.
    I felt uneasy about dc linking the plates, however, I reasoned that the cathodes are dc coupled already, some designs use the 2 sections in V1 linked together, and power tube plates get linked (in parallel and parallel push/pull output tube arrangements) with no ill effect.
    I've tested the mod now on a couple of amps and can report that it works great. I arranged a switch in order to flip between the 2 mixing methods (with standby engaged), and the direct plate link seems to sound warmer and more open, the 'Torres' method sounding constricted and less natural in comparisson.
    Maybe it's because the coupling cap isn't getting loaded so heavily, the 0.022uF is now having a easy life driving the 3M3 mixer load, rather than the ~40k of the other plate.
    I think that it may be preferable for both V1 and V2 to be similar 12AX7 types - if there's a mix of a 12AY7 and 12AX7, or 12AU7 and 12AX7, then the widely differng gains, plate resistances and current draws might cause operating issues. However, I can confirm that 12AX7 and 5751 are fine.
    I'd be interested to hear what anyone else makes of this method. Apologies if I've re-invented the wheel and all the above is common knowledge, it's been new to me at least! Peter.
    My band:- http://www.youtube.com/user/RedwingBand

  • #2
    Very nice solution---I have noticed this in a few of my amps after the normal channel mods and I will give it a go. Thanks.

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    • #3
      Thanks ES350 - please report back with your findings.
      My band:- http://www.youtube.com/user/RedwingBand

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      • #4
        Yeah, a good and simple solution, thanks a lot. I tried it in a SF DeluxeRev.for some weeks with good results and no ill side effects up to now.It sounds warmer and more natural indeed.
        Zouto

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        • #5
          Originally posted by pdf64 View Post
          ... Apologies if I've re-invented the wheel and all the above is common knowledge, it's been new to me at least! Peter.
          OK you're forgiven... it's a well known secret Fender Reverb amp mod from the mid-late 70's on until revealed on the NET 6 or 7 years ago.
          Bruce

          Mission Amps
          Denver, CO. 80022
          www.missionamps.com
          303-955-2412

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          • #6
            What is the goal of this mod? Is it to get reverb on the normal channel, or is it to get a better tone than going through the 220K mixing resistors?

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            • #7
              It's to get the effects on both channels, and also to stop one being inverted compared to the other, allowing the channels to be mixed without cancellation.
              With the mod as described above, the signal still goes through one of the 220k mixer resistors (in front of the phase splitter). There's no longer a 6dB loss through them, as the normal channel 220k mixer is now open circuit, but a 6dB loss has been introduced at the 2nd section plates of V1 and V2.
              My band:- http://www.youtube.com/user/RedwingBand

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              • #8
                It seems like you could also reduce (or remove?) the other 220k summing resistor with this mod, or would that mess with the vibrato?
                Last edited by m-fine; 03-24-2010, 03:02 PM.

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                • #9
                  I can't hear a difference between when the resistor in circuit or shorted out.
                  In circuit, it may serve to limit blocking distortion when the phase splitter gets overdriven.
                  In circuit there's insignificant signal drop across it.
                  In conjunction with the input capacitence of the phase splitter, it will act as a low pass filter, but it's a sufficiently high frequency that it's not noticeable.

                  If you want more gain, then open the negative feedback loop on the power amp, and replace the trem intensity pot with a 100k linear type.
                  My band:- http://www.youtube.com/user/RedwingBand

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                  • #10
                    Thanks for the reply. I figured the signal loss might be higher than it probably is. I do like the idea of minimizing components in the signal path, so if I do this I might try removing the resistor anyway.

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                    • #11
                      Originally posted by pdf64 View Post
                      I think that it may be preferable for both V1 and V2 to be similar 12AX7 types - if there's a mix of a 12AY7 and 12AX7, or 12AU7 and 12AX7, then the widely differng gains, plate resistances and current draws might cause operating issues. However, I can confirm that 12AX7 and 5751 are fine.
                      I'd be interested to hear what anyone else makes of this method. Apologies if I've re-invented the wheel and all the above is common knowledge, it's been new to me at least! Peter.
                      I've been using this system for 12 years or so... Using 12AX7 and 12AY7 sometimes there are problems. With 12AX7 and GE/Philips 5751 I have never found one.
                      I make the connection directly between sockets and remove the wire, 100K plate resistor, 47n capacitor, and 220K mixing resistor. I use a common 47K/2W for both plates.
                      Regards

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                      • #12
                        Doesn't removing the 100k plate resister for plate 2 effect the plate voltage going to plate 1 because the voltage divider is gone?

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                        • #13
                          No because Pedro has replaced the (now) parallel pair of 100k plate resistors with a single 47k.
                          By the way, thanks Pedro for reporting your experience with this method.
                          My band:- http://www.youtube.com/user/RedwingBand

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                          • #14
                            Thanks for the quick response...how are the (2) 100k plate resisters now parallel if they are replaced wirth 1 common 47k for both plates?

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                            • #15
                              The mod links the plates together. This puts their associated plate resistors in parallel. Therefore the 2 x 100k plate resistors could be removed and replaced with a single 47k plate resistor (feeding both plates).
                              Sorry if that wasn't clear.
                              My band:- http://www.youtube.com/user/RedwingBand

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