Adding switchable negative feedback to an amp that don't got none

smurfco

smurfco

Meatus McPrepuce
Hey @Wyatt or anyone else who may know...

I recently got a 68 Custom Vibrolux Reverb and it is a cool sounding amp. It does not have a negative feedback loop, which allows for early breakup and a spongier feel, or something. And I really like the way it sounds.

But...

I have read that it would be possible to add a negative feedback loop into the amp and have it be switchable. So if I wanted to tighten up the bass and raise the headroom some I could flip a switch and have the negative feedback loop come into play.

I am having a hard time finding a schematic for the 68 Custom Vibrolux Reverb (which is different from the "Custom Vibrolux Reverb" from a few years ago) but conceptually is this a simple mod? Would it be worthwhile?
 
The '68 CVR (the Silverface one) does have negative feedback loop, but they lowered the amount of negative feedback. If they did the same with the Vibrolux that the did with the '68 Custom Deluxe Reverb, then they increased the series feedback resistor from 820R (820 ohms) to 1K5 (1,500 ohms).

Somewhere on the PCB is a 1K5 resistor (on the CDR, it's R64). If you install a switch to add a second 1K5 resistor in parallel to the first, you get closer stock. Or you can add a switch to give 1K5 or 820R or leave the circuit open (no negative feedback).

It's the Brown Custom Vibrolux Reverb from the '90's and 00's that has no negative feedback loop at all (it was a Zinky amp design).
 
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Wyatt said:
The '68 CVR (the Silverface one) does have negative feedback loop, but they lowered the amount of negative feedback. If they did the same with the Vibrolux that the did with the 668 Custom Deluxe Reverb, then they increased the series feedback resistor from 820R (820 ohms) to 1K5 (1,500 ohms).

Somewhere on the PCB is a 1K5 resistor (on the CDR, it's R64). If you install a switch to add a second 1K5 resistor in parallel to the first, you get closer stock. Or you can add a switch to give 1K5 or 820R or leave the circuit open (no negative feedback).
Click to expand...

That's the amp. Would it be possible to add a pot to dial in the amount of negative feedback? That would be cool.
 
Wyatt said:
The '68 CVR (the Silverface one) does have negative feedback loop, but they lowered the amount of negative feedback. If they did the same with the Vibrolux that the did with the '68 Custom Deluxe Reverb, then they increased the series feedback resistor from 820R (820 ohms) to 1K5 (1,500 ohms).

Somewhere on the PCB is a 1K5 resistor (on the CDR, it's R64). If you install a switch to add a second 1K5 resistor in parallel to the first, you get closer stock. Or you can add a switch to give 1K5 or 820R or leave the circuit open (no negative feedback).

It's the Brown Custom Vibrolux Reverb from the '90's and 00's that has no negative feedback loop at all (it was a Zinky amp design).
Click to expand...
Oh cool! that sounds fairly straightforward...
 
Help!I'maRock! said:
That's the amp. Would it be possible to add a pot to dial in the amount of negative feedback? That would be cool.
Click to expand...

Two places you can add a pot.
  1. You can put a 820Ω resistor in series with a 10KΩ pot at the series feedback resistor (R64 on the CDR/DRRI). You can play with that 820R value, but that would give you the stock BF/SF NFB value when the pot is zeroed out, or...
  2. You could swap out the shunt/tail resistor — 47Ω on CDR/DRRI (R53), 100Ω on the Vibrolux — with a pot...if you add a cap as a bandpass filter, you create a Presence control
The amount of negative feedback is controlled by the ratio of the series feedback resistor and the shunt resistor, so you can adjust either one to change the ratio.
 
smurfco said:
Now alls I need to do is find a safe way to drain the caps so I don't zap myself.
Click to expand...
When I replaced the electrolytics on my Twin, I too was worried about this. I found that if I unplugged it from the wall, turned on the power switch and let it sit for a bit (20 seconds), the caps had no charge on them. I shorted them out with a well insulated screwdriver to be sure, and there were no sparks when I did so. I'd recommend checking the voltage across the caps with a voltmeter before touching anything. That's what I did and I'm still alive. :shrug:
 
Hey @Wyatt I got Fender to send me the schematic for the 68 Custom Vibrolux Reverb - will the mod above still work?
 

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shoeless said:
When I replaced the electrolytics on my Twin, I too was worried about this. I found that if I unplugged it from the wall, turned on the power switch and let it sit for a bit (20 seconds), the caps had no charge on them. I shorted them out with a well insulated screwdriver to be sure, and there were no sparks when I did so. I'd recommend checking the voltage across the caps with a voltmeter before touching anything. That's what I did and I'm still alive. :shrug:
Click to expand...

Way back when, when I built and amp and replaced the caps on a couple of SilverFaced Fenders, I turned the amp on, and pulled the power cord from the wall. I left it on and didn't open it up for a day or 2. I'm sure that's not the right way to do it, but I'm still here.
 
Interesting... R64 in the Vibrolux is already 820. On the Custom Deluxe Reverb schematic I found it's 1.5K. Typo on the Vibrolux schem?

I guess I will be able to tell for sure when I open the amp and see what value the stock resistor is. If it is 820 I will be surprised and enraged.
 
smurfco said:
Interesting... R64 in the Vibrolux is already 820. On the Custom Deluxe Reverb schematic I found it's 1.5K. Typo on the Vibrolux schem?

I guess I will be able to tell for sure when I open the amp and see what value the stock resistor is. If it is 820 I will be surprised and enraged.
Click to expand...

Interesting.

Historically, the "8 ohm" Fenders (Deluxe , Vibroverb, Concert, etc.) used a 820Ω series resistor and a 47Ω shunt resistor. The 4-ohm and 2-ohm Fenders (Vibrolux , Super, Twin, Pro) used a 820Ω series resistor and a 100Ω shunt resistor. This is because the lower output impedance feeds less voltage into the negative feedback loop.

  • With the '68 Custom Deluxe Reverb, they reduced the NFB by doubling the series resistor (R64) from 820Ω to 1500Ω
  • With the '68 Custom Vibrolux Reverb, they reduced the NFB by halving the shunt resistor (R53) from 100Ω to 47Ω
Both ways give identical outcomes since the feedback circuit is all about the ratio between those two resistors. Doubling one resistor is the same as halving the other, and vice-versa.
 
Wyatt said:
Interesting.

Historically, the "8 ohm" Fenders (Deluxe , Vibroverb, Concert, etc.) used a 820Ω series resistor and a 47Ω shunt resistor. The 4-ohm and 2-ohm Fenders (Vibrolux , Super, Twin, Pro) used a 820Ω series resistor and a 100Ω shunt resistor. This is because the lower output impedance feeds less voltage into the negative feedback loop.

  • With the '68 Custom Deluxe Reverb, they reduced the NFB by doubling the series resistor (R64) from 820Ω to 1500Ω
  • With the '68 Custom Vibrolux Reverb, they reduced the NFB by halving the shunt resistor (R53) from 100Ω to 47Ω
Both ways give identical outcomes since the feedback circuit is all about the ratio between those two resistors. Doubling one resistor is the same as halving the other, and vice-versa.
Click to expand...
Given this, will the three-way switch mod linked to above need to be modified for the Vibrolux? Different resistor values? Or does it make things more complicated?
 
If I were to leave R64 alone and add a three-way switch to R53 with 100 ohm and 47 ohm resistors it will essentially be the same mod? Did I get that right?
 
smurfco said:
If I were to leave R64 alone and add a three-way switch to R53 with 100 ohm and 47 ohm resistors it will essentially be the same mod? Did I get that right?
Click to expand...

You can do a two-way switch for those two values, but I don't think you can leave that Ground open like you can the series resistor.
 
Thanks so much for the help, first of all.

So if I use a 3 way switch around R64 and use 820 and 430 on the Vibrolux I can still have the "none" option in the middle position? I think that's what I will do.
 
One other question - do I need these resistors to be rated at a particular wattage or voltage or anything?
 
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