Converting a “Custom” Vibrolux Reverb into a ’63 Vibroverb By Mark Moyer 

The Differences: 

1. Reverb is present in the ‘Verb only on the bright channel. It is on both channels in the ‘Lux. This is accomplished by connecting pin 6 of V1b to pin 6 of V2B and eliminating R35, and R11, V1B’s plate load resistor (220k in the ‘Verb), and using the R22/R23 combination as a common plate load resistor for both V1B and V2B. On the ‘verb, R22 and R23 are 22k and 82k, respectively. On the ‘Lux, R23 is dropped to 47k. The Lux has a significant amount of hiss and more pre-amp gain with lower headroom due to this circuit change. 
2. The Lux has a 500pf-coupling cap from the 12AX7 of the Reverb return to the pot. The Verb uses a .0033uf. 
3. The ‘Lux has no negative feedback. The ‘Verb has a NFB loop. Negative feedback loops tend to negate hiss and other unwanted noise. 
4. R37, the lower cathode resistor in the PI, is 6.8k in the ‘verb and 39k in the ‘Lux. 
5. The ‘verb has a 100pF shunting cap, C20, across the PI plates. This is absent on the ‘Lux. 
6. The ‘Lux has 3kV spike protection diodes on the OT. 
7. The ‘Verb has very cheap sounding Oxford 10K5 reissue speakers. The ‘Lux has the decent-sounding Eminence alnico’s. 
8. The Original Vibroverb runs a 12AX7 with a 1k 1w-cathode resistor as its Reverb driver. The Lux and Verb run a 12AT7 with 2.2k 1w. 
9. The Verb’s ground reference resistors; R62 and R63 are 47ohms. The Lux uses 100ohm 


The Changes 

1. Reverb on both channels. This is a neat idea but IMO the negatives outweigh the positives. Adding Reverb to both channels of the Lux causes a significant amount of hiss. Going to a common plate load resistor and lowering R23 from 82k to 47k makes the pre-amp run hotter, which to my ear makes for less headroom. I also suspect some cross talk is going on. Further, the Reverb on the Bright channel improves by converting the Lux pre-amp back to the Verb. If you like Big Clean Reverb laden tone with no hiss you must convert the Lux pre-amp back to the Verb. To convert the Lux pre-amp back to the Verb unsolder the blue wire from pin 6 of V2 and solder it to the unused hole marked 6 on the circuit board where the rest of V1’s connections are made. The blue wire connects pin 6 of V1 to pin 6 of V2. Leave one end connected to pin 6 of V1. Replace R23 with an 82k. Place a 220k resistor in both of the empty spots pre-marked R35 and R11. 
2. Fuller Reverb: The Reverb on the Lux is much more filtered out then the Verb due to the coupling cap in the recovery stage. C16 on the Verb is a .0033. On the Lux it is 500pf. This is a significant difference. I highly recommend replacing C16 on the Lux with a .0033 Silver Mica cap. This makes the Reverb come alive on the otherwise Reverb challenged Lux. 
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. 
4. Shunting cap: The Verb has a 100pf-shunting cap across the plates of the 12AX7 Phase Inverter. It is C20. I recommend placing this 100pf cap in the Lux as it is inaudible and greatly reduces the parasitics that are inherent with a printed circuit board. 
5. Surge protection diodes: While theoretically these diodes are a good idea. In practice I have found them to be more trouble then they are worth. They tend to cause the amp to blow fuses when switching from stand-by to on. Further, they seem to drain off some of the high frequencies. I am talking about the diodes from pin 3 to ground on the 6L6GCs. None of the older Fenders had them and most of these amps are still going strong 30 or more years later. I recommend removing these Diodes. 
6. Ground Reference Resistors: Replace R62 and R63 with a pair of 47ohm resistors. I used 1-watt metal oxide. This may seem an insignificant change but it removes the last little bit of hum and hiss form the amp. 

The Options: 
1. Installing a Bias Pot: Use a 10k linear pot. Mount the pot through the bottom of the chassis below and to the pilot-light side of the tremolo pots. Remove R59. Solder two lengths of wire to the PCB where the resistor was. Connect one of the wires to the middle and one end tab of the pot. Solder a resistor to the remaining tab and connect the other end of it to the other wire. For values, I recommend starting with a 10k. What you’ve just done is replaced the 18k bias set resistor with a 10k resistor in series with a 10k pot. Where before you had a fixed 18k, you now have 10k-20k range. Button things back up and check your idle current. You will want to see anywhere from 30-40mA per tube. If you can’t get the idle current high enough (which is likely if you use NOS American tubes or the Telsa 6L6GC), replace the 10k resistor you just added with a 5k. If you use Svetlana tubes the 10k works fine. Phillips 6L6WGBs may require the 5k. The Telsa 6L6GCs will definitely require the 5k. I have tried the Svets, Phillips and Telsas in my amp. The Svets have nice mid range. A good sounding all around tube. The Phillips are clean and bold at low volume and break up nice. A good blues tube. The Telsas remind me of 7581As. They are big, full and clean. They seem to add wattage to this amp with lots of big clean headroom. They are also tough as nails and can be run hard. The Svetlana are my choice for this amp. I run them @ 36ma per tube. 

2. The Original Vibroverb, not the re-issue, ran a 12AX7 as its Reverb driver. Replace R26 with a 1k -1w resistor and install a 12AX7. The 12AX7 will increase the dwell of the Reverb and is a much more common tube if you want to experiment. Be sure to use a 1-watt resistor here! You may find that this gives you too much Reverb. 
3. Reverb Switching: These amps always have some Reverb, even when it’s turned down to ‘1’ and foot switched out. All Fender Reverb amps do this to some degree, but on these it can really be annoying, especially when cranked. 
The solution is to rearrange the foot switch setup to the original Fender style. On the ‘verb/Lux, the foot switch grounds the signal from the Reverb recovery circuit at R65. This should kill the signal, but for some reason it does not. On the original Reverb amps, the foot switch grounded the Reverb signal at the grid of the recovery tube. It’s simple to put this back in order. 
First, trace the gray wire attached to the foot switch jack back to the PCB. Either unsolder it at the PCB or just snip it off flush. Then solder it to pin two of V4. Viola! 
4. The stock Reverb cables are crap. Replace them with some nice heavy-duty video quality cables. I disconnected the originals from the board and soldered the new cables right to the board. This improves the signal quality and reduces interference making for better quieter Reverb. 
5. As in all Fender Reverb amps the 12AT7 Reverb driver takes a beating. A good fresh RCA or Mullard will last the longest and sound the best. I tried the Phillips 12AT7 and I was disappointed with its short life and shallow tone. 
6. As you may have noticed, I am a Reverb junkie. I like the smoothness of the three spring Reverb pan in this amp. The number is 9AB3C1B. 
7. There is a 47pf cap across the volume pot of the Bright channel. This is what makes the Bright channel Bright. This is the same set up as the Deluxe Reverb. I feel that the amp is too bright with this cap. If you share my opinion simply remove C11. It is on the Bright channel daughter board. For what it is worth, I remove the 47pf cap from the Deluxe Reverb as well. 
8. The stock blue Alnico speakers are decent but there are better. After much research I decided to buy and install a pair of the Weber C10Qs. This final change was the icing on the cake! They transformed my already fine sounding amp into a truly professional sounding workhorse. 


The Conclusions: 
The Fender Vibroverb, in my opinion, is the best sounding amp that has come out of the Fender factory in years. I am very big on REVERB and the Vibroverb, when properly set up, has the finest Reverb of all! 
Converting a Lux into a Verb is fairly simple and will take you about an hour if you know what you are doing. 
My goal in converting my 1995 Blonde Lux into a ’63 Vibroverb was to get rid of the hiss. I also had a parasitic on certain notes. Further, the Vibrato would make an audible pop when I switched it on and off. I liked the basic tone of the Lux but I could not live with the noises and shallow Reverb. Converting my amp made it virtually silent. The basic tone is about the same but the Reverb is much bigger and fuller. I do not miss the Reverb on both channels.