Mark Moyer mods to "Custom" Vibrolux Reverb

[mrkgtr49]

I just started in on doing the Moyer mods to a 2000 Fender Vibrolux Revereb. In the segment on adding a negative feedback loop, he mentions adding a 470 ohm resistor to the circuit and states that it is R42 on the circuit board of his '95 Vibroverb reissue, and that resistors place on the circuit board is filled with a jumper wire. That also describes the board in my Vibrolux; same jumper wire in place of the same designated resistor.
So what I'm wondering is: can the 470 ohm resistor be installed in that location and do what it's supposed to do in the NFB loop mod? In his description of adding the resistor, he says it goes between the point where the wire from the tip of the speaker jack goes to the circuit board and ground. When I do a continuity check between the jumper in R42's location and ground, I get continuity. But when I check between the newly installed wire and the R42 jumper, I get nothing. Seems like if that's where the 470 ohm resistor goes, and it's supposed to be between the wire connection point on the circuit board and ground, I should read a dead short between the jumper and the wire through the board, right?
Has anyone out there done these mods, and is my thinking right on this point?

Thanks in advance, group!

[Enzo]

perhaps if you linked us to the mods you are attempting, we'd have something to show us your circuit.


Is there a '95 Vibroverb Reissue? Or is it the '63 Vibroverb Reissue bought in 1995? I find R42 in that one.

So they appear to use the same board for the two models. That happens a lot.

What wire? Wire from where to where?

Look at the schematic for the Vibroverb. The NFB comes from the tranasformer through R41 and joins R42. R41 is empty holes on your Vibrolux. Did you put a resistor there? Does your wire from the speaker jack connect along the edge of the board near the + end of C37? That seems to be were the NFB line hits the board. If you are doing that, there has to be a resistor or a wire where R41 is or you get no continuity.

And make sure what you are measuring. If you did install a 10k, and are checking continuity instead of resistance, that 10k resistance is probably high enough your continuity tester thinks it is open.

NFB is not "a resistor", it is a circuit. In the Vibroverb, the sample signal from the output feeds through R41 and R42 on the way to ground. Those are 10k and 470 ohm. They form a voltage divider of ROUGHLY 20:1. SO if there is 20v on the output, there will be about 1 volt fed back into R42 and thus the cathode of the phase inverter.

[oc disorder]

Gooday mrkgtr49

Its a bit hard to comment and help you with this "paint by numbers" approach of which there is a proliferation on the net. I suppose there is demand for this and that approach sells units to newbee modders without having to trawl through theory and learn technical terms.

eg move the pink wire next to the watsit that looks like a Liquorice allsort.

For those unaware I'm assuming these are the instructions being followed:-

"3. NFB: To further reduce the hiss found on the Lux and increase your headroom you must add a NFB loop to the ‘Lux. Solder a 10k resistor in the empty space marked R41. Then run a wire from the speaker jack (positive, or tip, terminal) to the unused connection point ‘2’ above the right-hand corner of C37 and next to a green wire. ONE MORE THING! There needs to be a 470-ohm resistor between this point and ground! It’s R42 on the Vibroverb PCB. My 1995 Blonde had a jumper wire acting as R42. One Other Phase Inverter Change: Replace R37 with a 6.8k resister. This goes hand in hand with the NFB loop change."


However a lot of players get satisfactory results that way judging by some of the forums about.

What you are basically doing in this instance is converting a Vibrolux circuit to a Vibroverb circuit or
adding negative feedback to a circuit that doesn't have it.

I might add there are heaps of posts doing the exact opposite ie removing neg f/b or making it switchable.

I'm posting schematics of that section, luckily as Fender used the same PCB the component numbers are the same.

The tip of the output jack has to connect to the unused hole/pcb pad on the board which leads to one end of the new 10K resistor.

Even if the shorting jack is open - because of the few turns on the speaker side of the output transformer it will measure as a short circuit unless you have a precision low ohms meter.

I hope this is enough info to help you or at least enable others to do so.

As an aside I wonder how long it will be (if not already) until we have DIY surgery based on sketches and YouTube videos without any medical training?

(P.S. I wrote this before I read Enzo's reply.)

[mrkgtr49]

Update: The Moyer mods are complete and the amp is transformed. No more hum and hissing, and the reverb sounds more like what a Fender reverb ought to sound like. Replacing the reverb driver with a 12ax7 increases the dwell too much for my tastes, so I put the 12at7 back in. All in all, it sounds great, has good clean head room, yet breaks up at a reasonable volume setting--around 4-5 with a Strat.

[Enzo]

SO you asked us about your lack of signal at R42. What was the problem? We made suggestions, but never heard the result.

[mrkgtr49]

Sorry Enzo; I had thought I'd posted another update, but it doesn't show here.
I made a stupid mistake and missed the reference to installing a resistor in the empty R41 position. That was the component that completed the circuit. I felt really dumb when I noticed the reference to it in your response earlier. Thanks for that! I should have compared the Vibroverb schematic to the Custom Vibrolux Reverb schematic, especially since they apparently use the same PCB. It's been a learning experience!
As an aside, I highly recommend these mods for the Custom Vibrlux Reverb. They weren't difficult to do, and the results are copletely satisfying.

Thanks for all the help!

[bryarblue72]

Hello,

New to the board here and I know this is a 3 yr old + topic and hoping it's ok with you guys to bring it back up.

I know someone selling a 1990 Fender Vibrolux Reverb Custom amp. It's interesting that the amp chassis is signed by Mark Moyer with a date of 1990, although it is not apparent that the modifications discussed here were made to it. Was Mark Moyer a fender build tech at some point in time thus perhaps the reason for the signature? Also, does anyone know the year models that the Mark Moyer mods will work on? I found a YouTube video page that includes a link to the build notes, i.e.,

https://www.youtube.com/watch?v=rmQcDqkxVIY

The video is based on the Moyer mods performed on a 2011 model amp.

Thanks folks in advance for your help!

Regards,
Roger

[oc disorder]

Hi, afraid I can't help re who Mark Moyer is or was except he seems to have become immortalized with this mod !

As I understand it it's only for the reissue custom redesigned by Bruce Zinky.

This is the model .. confusing to me as the schematic is dated 95.
Fender Custom Vibrolux Service Manual.pdf

This full service PDF has a picture on the front.

The main noise as I see it was from lack of neg f/b.

This recently sold one on Ebay was listed as 90's....


I assume it must have Custom on the front panel.

[Steve A.]

FWIW a very common mod for BF/SF amps is to replace the NFB resistor with something like a 25k linear pot in series with a 470R resistor so that you could dial in the desired amount of negative feedback.

Steve Ahola

[bryarblue72]

Thanks folks for your replies.