SR RI Disable Vibrato

[stratomaster]

Hello,
I am new to the forum and have learned so much in just the few hours I have been browsing. I hear that disabling the vibrato circuit from Fender amps has the effect of 'waking up' the sound. Is this true? And if so, is it easy to do? Most of my experience with electronics comes from working with effects and I am quite familiar with electronics; I just have never worked on an amp before (and yes, I do understand how and why to drain the electrolytic caps in the amp prior to working on them). Can someone shed some light on this? Thanks in advance for your help.

[Bruce / Mission Amps]

Well, disabling the vibrato won't but unloading the intensity pot from ground will increase the signal from the preamp to the phase inverter by quite a bit.
Is that what you are referring to?
Most classic Fender amp lovers do not really like the way the amp sounds after doing that... however here is a trick I do to a few of my customers who are looking for a little more gas....
you can lift the intensity pot's ground point and insert some resistance in series with the bottom of the pot to ground.
Try 27K to 68K, or a small 100K linear trim pot wired as a variable resistor (using the middle lug and one outside lug) and tweak the trim pot to taste.
You'll probably find it doesn't take very much additional resistance to increase the available gain to the phase inverter.
Regardless, when you find what setting gives you a sound you like, measure the resistance of the pot and use a fixed resistor.

[chipaudette]

Bruce,

I've got a Fender DRRI and have been eyeing that intensity pot for a while. I always thought that 50k to ground was quite low compared to all the other resistances to ground within the amp. I always wondered if that was considered a "low" impedence to be driven by the tube stage just before it. I've always wondered if, by drawing out so much current, if it changed the sound produced by that stage compared to a more typically loaded stage that is followed by 1M to ground.

To test this, I have often considered lifting the pot's ground to do a test. But, won't lifting the ground on the 50k pot cause a huge change in the cutoff frequency of the coupling cap just before it? It's a 0.1 uF cap preceded by a 100k plate resistor to B+, and followed by that 50K pot to ground as well as some equivalent resistance much greater than 50K that heads off to the PI. I assume that changing that cutoff frequency would have a reasonably big impact on the sound.

I'm kinda bad at computing cutoff frequencies in these multi-branched circuits. What is the cutoff frequency of the stock config versus the cutoff with the 50K lifted? If I wanted to keep the cutoff the same, what value would I have to change the cap to?

Thanks,

Chip

[Rick Erickson]

But, won't lifting the ground on the 50k pot cause a huge change in the cutoff frequency of the coupling cap just before it?

Yes - the Bass response will increase - not necesarily a good thing...

I assume that changing that cutoff frequency would have a reasonably big impact on the sound.

You assume correctly.

If I wanted to keep the cutoff the same, what value would I have to change the cap to?

Best to use your ears here and select the value that sounds good to you. Or better yet - leave the amp alone. Fender amps - particularly the BF era circuits sound great in their stock form. If you can't get a good tone from one then you need to look beyond the circuit. Is my amp working in top form? Are the speakers a good match for the amp? Does my guitar/pickups/pedals/cables etc. suck? Do I have the playing skills to extract the tones I desire from an amp?
All these things add up to a killer tone.
fwiw - I've heard some killer players using the DR RI and getting great tone. The amp does not need circuit modification to sound good.

RE

[chipaudette]

I don't deny that the DRRI has a pretty darned good clean (and lightly gritty) sound. No doubt. But, my thoughts on exploring the lifting of the Intensity pot are not really motivated by a desire to improve the amp's tone...no. Instead, I'm using these kinds of mods as a method of exploring amp design to learn about my instrument and to see what affects tone. When I want to go and play my amp clean for real, I always have switches that bring me back to the stock amp.

But, I'm not playing clean (or even lightly gritty) all the time. Sometimes I need a little more kick. And, for the DRRI at least, I think that there are some mods that can make an amp much more friendly to distortion/overdrive pedals.

Some people think that the stock DRRI works well with pedals. Great. But I don't. I think that it's got way way way too much high end...especially in the highest frequencies...the sizzle. Way too much. I've tried a lot of distortion pedals with this thing (both the old classics and new ones) and I think that it makes all of them sound way too fizzy.

To tame the fizz on a DRRI, there are several mods one can do. I think that the first step is to put the built-in bright cap on a switch. That's an easy one. As a second step, I would suggest that one consider putting the 10pF cap that bypasses the reverb's 3M resistor on a switch as well. If you want to do both mods, you can even gang them together via single DPDT switch. That way, you can easily switch IN the elements for the classic clean sound and then easily switch OUT those elements to make your gain pedals sound more natural (less fizzy).

Another area of possibly useful mods is in bass management. One traditional (and fun) way to use tube amps (not that I need to tell you) is by turning the amp up past 5 so that it can get really overdriven if the guitar is opened up. Turning the guitar volume knob back a bit then acts to clean up the sound. I love that kind of controllability. It's really a sweet way to play.

But, when heavily overdriven, I feel that Fender amps need to have their bass dialed back (say, 0-2 on the bass dial) to keep the distortion from being flabby and unfocused. OK, so I have to set the bass low. Fine. But then, when I dial the guitar back to get it cleaner, I feel like I want to get that bass back (say, 4-6 on the dial). What am I to do? Well, I can easily mod the tone stack to put the bass-cut setting on a switch. Toggle up, original bass...toggle down, less bass. Done.

I admit that using this switch in the middle of playing is a little tricky...but I'm pretty adept at flipping my switches quickly with my shoe...it looks like I'm kicking my amp...which my friends are highly amused by. If you're really a hotshot modder, you could footswitch-activate these features, but I'm not that good yet.

So, overall, for clean and lightly gritty sounds we agree...there's no real need for mods on the DRRI. For pedals and heavier overdrive, though, I think that there's room for fiddling and twiddling if one is so inclined.

Chip

[stratomaster]

I had heard that the opto-coupler was a drain on the signal, and if the vibrato circuit was disabled, then more of the original guitar signal could make its way into the amplifier. I don't think I need to do this, but you never know until you try. You can always go back, right? (barring FUBARs)

[Bruce / Mission Amps]

The website site screwed up and double posted

[Bruce / Mission Amps]

I had heard that the opto-coupler was a drain on the signal, and if the vibrato circuit was disabled, then more of the original guitar signal could make its way into the amplifier. I don't think I need to do this, but you never know until you try. You can always go back, right? (barring FUBARs)

Well if the cell is good, certainly not very much of drain when the circuit is off.
And Opto-coupler? I've seen others use that term too but the Fender amps uses a made up circuit. A flashing neon bulb with an LDR, a light dependent resistor, sometimes called a photocell ... etc. An Opto-coupler is a different device. I'm sure it can be used in some way with a vibrato circuit but it's not in the Fender amps using the LDR.
Anyhow, () the actual variable resistance of the photo cell is connected to the wiper of the 50K pot, so look at what is in total parallel with the LDR with the wiper set to it's maximum intensity, ...the 50K pot and both are exposed to the entire audio signal at that stage.
When the cell goes to extremely low resistance during the oscillation, a HUGE portion of the audio signal is simply grounded out!!
Now look at what happens when the wiper is turned all the way down to minimum intensity.... the cell is grounded on both ends and it's parallel resistance can't effect the audio path... only the 50K pot does.

A photo cell in the LDR device, at max resistance, is hundreds of thousands of ohms, much higher then the 50K pot. At minimum resistance, it is only a few tens of ohms... compare that to the 50K pot. The LDR photo cell swamps it out when go low in resistance.
By turning the wiper up or down, you can set the over all effect of the photo cell by having both ends grounded and having virtually no effect on preamp volume, to turning the pot up all the way and exposing the preamp audio signal to the photo cell when it is at a very low resistance, which kills the audio signal.
Of course, having the photo cell oscillating in resistance by the neon light blinking causes "tremolo/vibrato".
At that point it is the same as turning a volume pot up and down real fast at the same location.