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I just aquired one made in 2004, I thought it was pretty and got if for cheap. Anyway it's well known that they hiss badly, mostly because of the absense of NFB. There is a popular "mod" flying around the internet called the "Moyer Mod" that blames the hiss on a number of things as well as the NFB. The mod basically reverts the amp to a '63 Vibroverb Reissue. I already installed a NFB now but it's still noisy. I didn't change the tail resistor yet though, I don't quite get what difference 6.8k versus 39k would do ? AB763 circuits have a 22k tail resistor and modern Fenders have 6.8k, should I really go to 6.8k ? (Note that I like the current circuit, just want to get rid of the hiss)
Is it really a problem that both channels mix before the reverb ?
A smaller R23 would only give more headroom, not less, and would not cause any hiss whatsoever in my logic ? (more plate volts on that half-12AX7)
How and why would changing the ground reference resistors from 100 to 47 ohms do anything for the hiss ?
Should I really remove the diodes on the power tubes, and the shunting cap in the PI ?
**See under the Moyer Mods**
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.
Is it really a problem that both channels mix before the reverb ?
A smaller R23 would only give more headroom, not less, and would not cause any hiss whatsoever in my logic ? (more plate volts on that half-12AX7)
How and why would changing the ground reference resistors from 100 to 47 ohms do anything for the hiss ?
Should I really remove the diodes on the power tubes, and the shunting cap in the PI ?
**See under the Moyer Mods**
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.
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