Fender Twin vibrato experiments (geekery warning)
Posted: Mon Feb 18, 2008 2:09 am
I worked on a friend's silverface Twin a while back and liked the Vibrato on his better than the effect on mine. Mine was decent, but I wanted more "chop" out of it with the Intensity turned up all the way.
Long story short, after reading up on Fender vibratos and studying the diagram for my amp, I thought I saw a quick fix. Basically, the vib circuit provides the guitar signal with 2 parallel paths to ground: one through the vib photoresistor and the other through the "low" end of the Intensity pot. All I did was disconnect the grounded end of the Intensity pot.
Now the only vib-related path to ground is through the photoresistor, and it has a slightly greater effect on the signal. The difference between full-on and full-off is greater. This could be because the "on" portion of each vib cycle is louder than it used to be, instead of the "off" portion being quieter. I'm not sure.
As a side benefit, this mod boosts the amp's overall signal a tiny bit, because none is leaking to ground through the Intensity pot anymore. Another side benefit is that the Intensity knob is useful through more of its sweep, with the effect noticeable down to about 2. I can still hear the amp's circuit noise being modulated when the effect is on 0. I just stomp on the footswitch to remove the effect entirely.
I also ordered a new vibrato "roach" and a new cap for one of the tubes in the vibrato circuit. Both were original to the amp, and I figured replacements might improve things further. The "roach" is a neon lamp with a photoresistor shrink-tubed to it. Lamp leads stick out one end, cell leads stick out the other, and it looks like a four-legged bug.
Old on the left, new on the right.
The vibrato oscillator circuit makes the lamp flash, which is mildly cool to see when the amp's open. The photoresistor responds to the flashing lamp by leaking various amounts of precious mojo away to ground depending on the brightness of the light, and you've got your vibrato effect.
Here are the new "roach" and new capacitor soldered in place. For reference, those input jacks are for the Vibrato channel. (I reconnected the Intensity control to ground also.)
The new "roach" has about as much "chop" as my mod did. However, it also ticks loudly -- a common problem with this circuit. Also, it's only effective between about 5 and 10 on the Intensity knob. Below 5 the vibrato is so faint it's virtually gone. I did a few things to deal with the ticking, but those weren't effective.
I did some quickee recordings while working, and overall I liked my modded version better than unmodded-with-new-bug. So I removed the new insect, put the old one back in, and disconnected the Intensity pot from ground again. Live-n-learn.
My nifty new Fender reverb/vibrato footswitch looks lots better than the Peavey I was using, but even though it has the correct cable it didn't correct the loud POP I get when the reverb's on and I turn on the vibrato too. Basically when this happens, the vib switch is connecting about -50 volts (negative) to ground, and the sensitive reverb circuit "hears" this temporary rush of current in the footswitch cable. That's what I think is happening anyway.
I tried putting a few low-value capacitors across the vib footswitch contacts. These took a little edge off the pop, but just barely. I noticed that if I hold my thumb firmly against both switch contacts, it softened the pop sound better than any of the caps I'd tried. The human bod is essentially a big capacitor with personality problems, sooo....
Enter the nasty old 5uF 50Volt electrolytic capacitor I'd just replaced in the vibrato circuit. I clipped that in place of my thumb, and it takes the footswitch pop down quite a lot. Enough so that it's barely noticeable if I'm playing at the same time. Time to solder it in there.
Works great so far. A side effect is that the vibrato effect lingers for a second or so after I turn it =off=. That makes sense, because the cap takes a while to recharge itself when that happens. I can live with that, though. WAY better than that gritty CLICK the footswitch was making before.
Long story short, after reading up on Fender vibratos and studying the diagram for my amp, I thought I saw a quick fix. Basically, the vib circuit provides the guitar signal with 2 parallel paths to ground: one through the vib photoresistor and the other through the "low" end of the Intensity pot. All I did was disconnect the grounded end of the Intensity pot.
Now the only vib-related path to ground is through the photoresistor, and it has a slightly greater effect on the signal. The difference between full-on and full-off is greater. This could be because the "on" portion of each vib cycle is louder than it used to be, instead of the "off" portion being quieter. I'm not sure.
As a side benefit, this mod boosts the amp's overall signal a tiny bit, because none is leaking to ground through the Intensity pot anymore. Another side benefit is that the Intensity knob is useful through more of its sweep, with the effect noticeable down to about 2. I can still hear the amp's circuit noise being modulated when the effect is on 0. I just stomp on the footswitch to remove the effect entirely.
I also ordered a new vibrato "roach" and a new cap for one of the tubes in the vibrato circuit. Both were original to the amp, and I figured replacements might improve things further. The "roach" is a neon lamp with a photoresistor shrink-tubed to it. Lamp leads stick out one end, cell leads stick out the other, and it looks like a four-legged bug.
Old on the left, new on the right.
The vibrato oscillator circuit makes the lamp flash, which is mildly cool to see when the amp's open. The photoresistor responds to the flashing lamp by leaking various amounts of precious mojo away to ground depending on the brightness of the light, and you've got your vibrato effect.
Here are the new "roach" and new capacitor soldered in place. For reference, those input jacks are for the Vibrato channel. (I reconnected the Intensity control to ground also.)
The new "roach" has about as much "chop" as my mod did. However, it also ticks loudly -- a common problem with this circuit. Also, it's only effective between about 5 and 10 on the Intensity knob. Below 5 the vibrato is so faint it's virtually gone. I did a few things to deal with the ticking, but those weren't effective.
I did some quickee recordings while working, and overall I liked my modded version better than unmodded-with-new-bug. So I removed the new insect, put the old one back in, and disconnected the Intensity pot from ground again. Live-n-learn.
My nifty new Fender reverb/vibrato footswitch looks lots better than the Peavey I was using, but even though it has the correct cable it didn't correct the loud POP I get when the reverb's on and I turn on the vibrato too. Basically when this happens, the vib switch is connecting about -50 volts (negative) to ground, and the sensitive reverb circuit "hears" this temporary rush of current in the footswitch cable. That's what I think is happening anyway.
I tried putting a few low-value capacitors across the vib footswitch contacts. These took a little edge off the pop, but just barely. I noticed that if I hold my thumb firmly against both switch contacts, it softened the pop sound better than any of the caps I'd tried. The human bod is essentially a big capacitor with personality problems, sooo....
Enter the nasty old 5uF 50Volt electrolytic capacitor I'd just replaced in the vibrato circuit. I clipped that in place of my thumb, and it takes the footswitch pop down quite a lot. Enough so that it's barely noticeable if I'm playing at the same time. Time to solder it in there.
Works great so far. A side effect is that the vibrato effect lingers for a second or so after I turn it =off=. That makes sense, because the cap takes a while to recharge itself when that happens. I can live with that, though. WAY better than that gritty CLICK the footswitch was making before.
