Custom Vibrolux Reverb - Mark Moyer Mods

[fezzy]

I'm about to get started on the Mark Moyer "CVR to Vibroverb" conversion and would like to get an experienced person to double check my parts list before I start. This is one of my first mod projects so I'm hoping someone can help me out if I get stuck. :) Thanks!

Heres the conversion steps:
==================================

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.

==================================

So thats about it... just some desoldering & soldering. I have all of my 1/4 watt resistors ready to go. However, in the CVR schematic some of the resistors I'd be replacing are not 1/4 watt. Should I match those with the exact same wattage? Or will 1/4 be ok? The only part I'm missing is the .0033 Silver Mica cap so I'll just skip that one for now.

Thanks in advance for anyone who wants to help!!!

[CzarSketch]

done 'em. sounds great! do yourself a favor and get resistors with more accurate resistance, and yes, i'd use the same wattage, just to be safe. Nothing like a good case of melted resistor to ruin your day.

[fezzy]

Excellent! Thanks CzarSketch! A few specific questions:

Could you tell me the type of resistors you used for each step?

#1)
R23: 82k (1 watt?)
R35 & R11: 220k ( I think 1/4 watt is OK for these)

#2)
The original 500pf cap is 1KV. I can only find a 500V 33pf Silver Mica cap. Will this be OK?
http://thetubestore.com/ca-si-33pf-500v.html

#3)
R41: 10k resistor (1/4 watt OK??)
R42: 470 ohm (1/4 watt OK??)
R37: 6.8k (1 watt, correct?)

#4)
I have a 100pf ceramic disc cap rated at 50vdc. This should be fine I think.

Also, would anyone happen to have a schematic for the '63 Vibroverb? It would be nice to compare it with the Lux.

Thanks!

[CzarSketch]

you can tell by the size of the resistor, really. if you want to be safe, 1 watt will work for all of them. I think you've got them correct, though. The 500V cap is fine, i very much doubt there's 1000V anywhere in that amp...

oh and as for the '63 vibroverb, this basically converts it to that. there are a few other differences but not many. enjoy your VV!

[ziess]

The original brown one;
http://www.ampwares.com/ffg/schem/vi...6g16_schem.gif

Blackface;
http://www.ampwares.com/ffg/schem/vi...a763_schem.gif

or the later AB763 circuit;

http://www.ampwares.com/ffg/schem/vi...b763_schem.gif

Tommy.

[JAM]

Dont use 1/4W resistors in tube amps. 1/2W and 1W is what I'd use, 1W for all plate load resistors.

[fezzy]

Thanks again CzarSketch! I did some more reading about resistor stuff and now its obvious that 1W is the way to go for this project.

Ziess, thanks very much for the schematic links.

Jam, you're totally right. Thanks!

I'll let you all know how it turned out when I'm finished. Probably wont be for a week because I have to order some 1W resistors now. ;)

Also, if you guys can recommend any books I should check out I'd appreciate it. Lately I've been reading 'Valve Amplifiers' by Morgan Jones. A great book - but a little intense for a beginner.

[CzarSketch]

"The guitar amplifier handbook" or something like that by Dave Hunter and

"The Tube Amp Book" forget who that's by.

i own the first one, borrowed the other one from a friend, both are great.

[fezzy]

Excellent. I'll check 'em out.

[NTBluesGuitar]

Dont use 1/4W resistors in tube amps. 1/2W and 1W is what I'd use, 1W for all plate load resistors.

Good tip.

I use 1W carbon film on my builds except for in the fixed bias circuit, where I use 1/2W.

I just happen to have a 6G16 build in the planning stages now!

[JAM]

3. There needs to be a 470-ohm resistor between this point and ground! It’s R42 on the Vibroverb PCB.

There's a discrepancy here. The Fender layout for a 6G16 has a 470 ohm, but the schematic has a 4700. The Weber kit has 4700.

[fezzy]

There's a discrepancy here. The Fender layout for a 6G16 has a 470 ohm, but the schematic has a 4700. The Weber kit has 4700.

Interesting. Do you have a link to the Weber kit you are talking about?

[JAM]

fezzy,

Here's the schematic:
https://taweber.powweb.com/store/6g16_schem.jpg

Look at R63

[Kap'n]

You can't look at one specific resistor in a -fb loop.

For example, the original 6G16 schematic

http://www.ampwares.com/ffg/schem/vi...6g16_schem.gif

The 10k resistor in the -fb loop is a voltage divider with the 4.7K resistor to ground, knocking down the voltage to about 1/11 of the output voltage.

Even though the values are completely different, this is a similar ratio to the SF Vibrolux Reverb

http://www.ampwares.com/ffg/schem/vi...a270_schem.gif

With an 820 ohm resistor on the top of the divider, and a 100 ohm resistor on the bottom, giving 1/9 of the voltage back into the loop.

[JAM]

You can't look at one specific resistor in a -fb loop.

For example, the original 6G16 schematic

http://www.ampwares.com/ffg/schem/vi...6g16_schem.gif

True, but my point is the layout has a 470 ohm where the schematic has 4700. However, unlike similar LTPI, the other part of the loop is 10k, most have higher values. The modification fezzy is asking about has a 470 ohm, so I was wondering if whomever did these mods was looking at the layout and not the schematic.

http://www.ampwares.com/ffg/schem/vi...g16_layout.gif

[Kap'n]

That gives you about a 1/21 feedback (I know I should use dB, but I'm lazy).

That might reduce some of the hiss, but I'm not a big fan of reducing the -fb significantly below stock values. That's the major reason the CVR sounds like crud in the first place.

[fezzy]

OK, I've finally had time to start on this project. The original steps are listed below with an update below each. I have a few questions for anyone thats willing to help out. Thanks!


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.
UPDATE: I don't entirely understand what he is saying as far as the rewiring goes. Could someone elaborate for me?

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.
UPDATE: I was only able to find an Orange Drop .0033uf cap so I used that one. Reverb definitely sounds better but I'm wondering how much better/different it would sound with a Silver Mica. Does anyone know where I can order one? All I was able to find was a 33pf...

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.
UPDATE: This one is done. No complaints, not entirely sure of an improvement though. It may be very slight - when the reverb is switched off. There is still significant hiss when reverb is on...

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.
UPDATE: Done, not sure of an audible improvement though. Probably only beneficial at louder volumes?

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.
UPDATE: Skipped this one for now because it does not have anything to do with removing the hiss - which is my goal at this point.

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.
UPDATE: Skipped this one because R62 & R63 actually tested to 47 ohms so there is no need to replace them. However, on the schematic I believe it says 100ohm, which is strange and makes me think i have an outdated schematic. Any ideas?

Overall the hiss at this point has NOT been reduced by much. I'm wondering if step #1 (which i skipped) is the key? Also, when I first switched on the amp there was some strange stuff going on like static but it went away then came back after a bit of playing and went away again and hasnt come back. Any ideas what this means?

The amp does sound slightly better so that is pretty rewarding but the hiss is still there and especially when reverb is switched on. Any suggestions would be greatly appreciated! Otherwise I'll just keep playing around with it until I figure it out. Thats all for now. Thanks to everyone! :)

[CzarSketch]

#1 is a big part of it.

[fezzy]

OK, so here is my problem with Step #1:

"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."

Heres a couple images to make sense out of this:

First Image: There are two blue wires on pin 6 of v2. One already connects V1 to V2.



Second Image: This is how I understand how the change should be made. Is this correct or completely wrong?



Also, I tried to compare with the old 66 VV schematic but its too blurry to read the numbers of the pins so I cant really tell what is going on.

[fezzy]

Alright, I re-read Mark's instructions for the 20th time and I'm pretty sure that this new image shows the correct rewiring. The last one I posted is indeed wrong so ignore that one. Any comments?

[Kap'n]

Yes, that's what it seems to indicate.

Moving that wire separates the two channels.

[surfcaster]

hi. i received my custom vibrolux one month ago (2nd hand) and first thing i noticed is how noisy it was. i was thinking about doing all these modifications to see if it improves but before i decided to try with the tubes. i pulled out the GT white label that were in it and put a matched pair of 6L6 GC tungsol reissues in the power section and some NOS GE in the preamp. believe me: the hiss has disapeared. i still plan on doing the mods related with the reverb quality but the hiss with the reverb on, and the pops when connecting and disconnecting the tremolo are gone.

[fezzy]

Cool, thanks Kap'n. I'll try it today and see what happens.

Thanks for the info surfcaster! I'll try to replace the tubes as soon as I finish up these mods and hopefully I'll get the same result as you did.

Did you replace all of the AX7 tubes with NOS GE? Could you give me the model numbers of the tubes you used for the pre and output tubes? Thanks!!

Also, i replace the 12at7 with a Mullard NOS and that really helped out the reverb as well.

[ziess]

I really like GE 12AX7s. The 7025/12AX7 type they made in the '70s sound really good.

Tommy.

[surfcaster]

what i exactly have now in the preamp is

v1- NOS Tungsol 12AX7
v2-NOS GE 5751
v3-NOS GE 6201
v4-electroharmonix 12AX7 (i have heard this is the 12AX7 with the higher output, and to my ears it helps!)
v5-NOS GE 12AX7
v6-NOS GE 6201

and the power section with the reissued Tungsol 6L6GC.

[fezzy]

Thanks for the list surfcaster. Thats a pretty interesting setup. If anything, what could you compare the sound of your amp to now? Also, what are your 6LCGC's biased to?

An update on the Mark Moyer mods:
I did Step #1 last night. The result at this point is pretty impressive. The hiss/static has been significantly reduced and the amp sounds a little bit richer and sparklier. I also went ahead with Step #5 (removing the surge protection diodes).

The only thing left now is to install a bias pot. Heres the process for anyone interested:

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.

[surfcaster]

i don't have any pot instaled for the biasing. my tech decided not to install it as he thinks the R59 stock resistor is giving fine range. i can solder and desolder things but i am not into electronics so i decided to let him decide for me. i am not sure if i have the skills to describe a sound/tone with words. all i can say is that i am nailing now the sounds i listen in records i like... bedhead, the new year, etc.

i have philips 6L6WGC in my twin reverb... maybe i pull two of them and try them in the CVR...

[fezzy]

Let me know how the 6L6WGC's turn out if you decide to try it.

I have an interesting problem today:

When switching my amp on (from standby) I get this weird pop/click/squeak/chirp sound. Obviously, its related to one of the mods I just did - most likely something in Step #1.

I went ahead and recorded it so anyone that might be able to help could get a better idea of what the sound is actually like. I know that some old standby switches can cause a pop... this isnt anything like that type of pop. Take a listen:

Recording of pop sound

[Kap'n]

Sounds like it could be a cool drum sample.

Try pulling V1 out, and see if it still does it.

If it does, pull V2, and see what happens.

If it goes away, put V1 back in.

Report back.

[fezzy]

Just to make sure... By "Try pulling V1 out" you mean pull out V1's tube and turn the amp on - and then repeat for V2 if needed?

[fezzy]

And yes, it could totally be used for a goofy drum sample. lol

[Kap'n]

Just to make sure... By "Try pulling V1 out" you mean pull out V1's tube and turn the amp on - and then repeat for V2 if needed?

Yes.

[twangmeisternyc]

I found this forum today searching this very topic - How might one modify the CVR a bit... very timely to see this post running. I've had mine for over a year now, using it in my apartment in NYC, low volumes, many different guitars, for practice, etc....waiting for the day I can crank it up all the way.

I have made a second baffle with a 4ohm C15N Jensen, and plan on taking that baffle to the next step...a full enclosure not that much unlike the Vibro-King stack. Right now, it takes about 20 seconds to switch the 2-10's for the 1-15, but I kind of like the idea of having both baffles enclosed to be used seperately or adding the 1-15 via the extension speaker jack.

My comments thus far would be the amp has a bit of hiss, but at the volumes I currently play my signal to noise is worse case...as well as are the distances from the amp I play - generally sitting right next to it. I hear no real noticeable difference in the hiss whether every thing is dialed down or full.

Nor do I sense a marked increase in hiss with added REVERB...some, but its just there. I think that hiss becomes insignificant when I step across the room, use a longer cord, etc. being more so a proximity thing. It probably would be a problem should I ever want to mic/record.

I do think the REVERB is a bit soft overall compared to other amps I've owned. But I do like it in both channels.

What my ears really do hear is TREMELO chugging, especially at slower speeds and higher intensity, and there is a pop should I try to TURN OFF TREM when REVERB is ON using the footswitch. I know every amp has nuances...and this one uses direct-coupled TREM circuitry.

My questions seeking comments:

1. Would any of these mod's preclude using the extension cabinet (i.e., the grounding of the POS speaker jack wire mod)? Does the extension jack need the same grounding?

2. Might these mod's reduce the TREMELO chugging wish-wash? (Right now, think "Riders on the Storm" with EXTRA WIND-DRIVEN RAIN!)

3. Any way to further isolate the footswitch to preclude the TREM OFF pop when REVERB is still ON?

[fezzy]

My questions seeking comments:

1. Would any of these mod's preclude using the extension cabinet (i.e., the grounding of the POS speaker jack wire mod)?
Definitely not. You should still be able to use your extension cab.

2. Do these mod's reduce the TREMELO chugging wish-wash? (Right now, think "Riders on the Storm" with EXTRA RAIN!)
Nope, I still have the same problem. Even after doing all of these mods. I'm hoping someone out there could give us a tip on how to fix this or get rid of some of the chugging sound.

3. Any way to further isolate the footswitch to preclude the TREM OFF pop when REVERB is still ON?
I still get a little pop sometimes when I switch the trem/vibrato off. None of these mods address that problem. Any one out there have a tip for us?

The mods listed above are really just for removing hiss from the amp.

[fezzy]

Yes.

Thanks Kap'n. I'll try it tonight and get back to you ASAP.

[twangmeisternyc]

Once I get the extension cabinet, I'll surely revisit these mods. Seems several are easily done...would be nice to see a pic of the speaker jack grounding you chose to do.

I've been through one round of tube swap-outs...I'll try surfcaster's set-up, too. Need to get a few of those other numbered GE's...6201 and 5751.

As for the TREMELO chugging, this direct-coupled design probably is 10x more reliable than bias modulation, but bias modulation is absolutely dreamy - hands down my fav.

[twangmeisternyc]

I stared at surfcaster's tube selections compared to the schematic a bit...since V1 and V2 are both pre-amp tubes (for NORMAL and BRIGHT) the 5751 is just de-gaining the BRIGHT channel a bit. V6 is the Phase Inverter, and it has been de-gained a bit using the 6201. Generally, one would keep V1 and V2 the same, maybe the mismatch is beneficial since the two channels are tied together for the reverb at that stage.

Hey, surfcaster...are you playing more so in the NORMAL channel?

Degaining the Phase Inverter when things come back together would in general reduce overall noise, too, at the expense of needing to dial-up VOLUME higher on either channel to reach the same amount of gain before hitting the power tubes.

I have some Sovtek 5751's and some JJ/Tesla ECC81's...they should be able to recreate whatever might be going on with those changes. Wouldn't be the first time happenstance solved a problem, eh?

[twangmeisternyc]

Fezzy,

I found elsewhere on this site the full Mark Moyer CVR mod write-up. At the end, it describes how to mod the footswitch to stop the popping when REVERB is engaged and TREMELO is being cycled OFF. Real simple, just move a wire...

[fezzy]

Fezzy,

I found elsewhere on this site the full Mark Moyer CVR mod write-up. At the end, it describes how to mod the footswitch to stop the popping when REVERB is engaged and TREMELO is being cycled OFF. Real simple, just move a wire...

I think I know what you are talking about. Can you point me to the post you found?

[fezzy]

Sounds like it could be a cool drum sample.

Try pulling V1 out, and see if it still does it.

If it does, pull V2, and see what happens.

If it goes away, put V1 back in.

Report back.

Kap'n, last night I did the following...

Removed V1 - pop was still there.
Put V1 back in and removed V2 - pop was still there.

I noticed that when I switch the amp out of standby that the reverb tube (V3) gets a surge of power, glows really bright, then fades back down to normal like the other tubes. What could this mean?