Modding A Fender Vibrolux Reverb

Discussion in 'Amps' started by rmfroyd, Jul 25, 2015.

  1. rmfroyd

    rmfroyd Well-Known Member

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    Off to a slow start. Wife had our mail held while we were on vacation, and USPS failed to deliver it today. Kinda sucks because I knew we didn't have shit going on this weekend, so I would have had two solid days to work on stuff(which is why I ordered it while on vacation). Oh well.

    Here is the amp:

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    Picked it up used off of a craigslist trade. Sounds pretty good, but very very hissy, to the point of being annoying. I also thought the reverb was a little lacking. I figured if I am going to own one Fender amp, I might as well make it something special. Scoured the internet for info, and google persuaded me to do some mods to it. There are many different mods that you can do, the most popular being the 'Mark Moyer' mods. I was going to go that route, but I am lazy, and I like to have things in a nice convenient package(even if it costs a little extra).

    I decided to go with the Frommel Mod Kit:
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    The Fender Vibrolux Reverb is one of Fender's most classic circuits, and the reissue is a mere shadow of the original...until you install a the Fromel Supreme kit!

    The mods change all of the crucial components in the signal path and restore the glory of the Vibrolux Reverb. The mods remove the buzz and hiss that these amps are notorious for, and also restores the shimmering clean headroom of the original design. The weak reverb is made fuller with longer decay and lush tone, while the tremolo gains a warm pulse with plenty of presence and no harshness. At high volumes, the tube saturation and bluesy distortion is unbelievably articulate and warm, while the club volume clean tones are dynamic and crisp.


    Yes I know, I could have done a little research and bought this stuff on my own and saved a little bit of cash. What I am paying for is the manual that Frommel provides. This is my first Amp mod, and I like to have my hand held a little bit when trying new stuff. Before purchasing he sent me a page of the instructions, and they were full color, and very easy to understand. Right up my alley.

    I also bought a tube kit from Eurotubes(I know they aren't popular among some, but I have always been happy with them).

    Earlier today I decided to take the chassis out and prepare myself for much soldering and excitement that never actually materialized.

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    **Disclaimer** Amps are dangerous and can kill you fucking dead.

    The main thing to do is discharge the capacitors that store voltage. In this amp most of them are under the silver shield on the left.

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    I had left this amp sit unplugged for about 3 weeks while on vacation. I wasn't expecting a lot of voltage to still be stored, but it never hurts to check. I like getting shocked slightly less than I like getting kicked in the balls. Measured with the DMM set to DC at .325 volts. From what I read a person wants to discharge them at anything over 10 volts. So they were fine, but.....I didn't solder up a test lead for nothing this afternoon, so these fuckers are getting discharged. I didn't bother with a resistor in the loop since they were already so low. Just grounded to the chassis, and touched the positive end. Once everything was done I got them discharged to .008 volts. Since I have to wait another 2 days to get the rest of my shit, I will measure again to see if they creep back up before I remove them(I have heard of this happening).

    There was one more on the inside smack dab in the middle that I repeated the process on:

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    Tomorrow I have to remove the knobs and loosen the pots so that I can get the circuit board out and work on it from both sides.

    Don't let me forget to spray some deoxit into the pots as they are a little scratchy.
     
  2. rmfroyd

    rmfroyd Well-Known Member

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  3. backinit

    backinit Well-Known Member

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    Good stuff. Good prep is important, especially when you can cook your nuts.
     
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  4. bugspray

    bugspray Active Member

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    Good luck with your mods. I overhauled my Twin a few years ago and first go round she was hissing like a snake. Thought Plate load resistors was problem and replaced with dale resistors(Glad I did anyway). Turns out the reverb send tube was shot(duh). She is super dead quite to this day. I learned most of my stuff off the web, and yes i still consider myself a newbie by far....FWIW , I'm using old school Russian 6p3s-e for power section and do sound good to me biased around 40ma( changed bias from fixed to adjustable) Later...
     
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  5. rmfroyd

    rmfroyd Well-Known Member

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    Progress made, but really upset that we didn't get our held mail. The whole family is taking a nap right now, and I am really wasting this uninterrupted work time by not having my stuff.

    Pulled the pots and jacks out:

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    Then I started removing the screws that hold the board down, which was a mistake. It would have been far easier to remove all of the crimp connectors while the board was solidly screwed down, rather than it was flopping about. I coud've just screwed it back down, but I like complaining.

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    Made sure I took plenty of cellphone pics and I labeled all the wires, so it SHOULD go back together the way it is supposed to be.

    It will be much easier to work on the board now.

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  6. Chocol8

    Chocol8 Well-Known Member

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    It is hard to tell from the pics, but I don't see a cap discharge resistor on the filter caps. If I work on an amp that does not have one, I will add it.

    It is simply a high value resistor that leaks a small amount of current from the high voltage rail to ground. This does not impact the amp operation at all, but when you shut off the amp, it will slowly drain the caps and should get them below lethal voltages before you or someone lacking your level of caution can get the chasis open. It doesn't replace the need to check and discharge (it could fail) but it does add a big layer of safety for a couple of pennies and the time it takes to solder two joints.
     
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  7. tonebender

    tonebender Well-Known Member

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    Good stuff !
     
  8. doc-knapp

    doc-knapp Well-Known Member

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    I love the sound of the old school Fender Vibrolux. Smooth as glass.
     
  9. cybermgk

    cybermgk Well-Known Member Supporting Member+

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    Has to be said. When you hold mail, you CAN go down to the PO and pick it up.
     
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  10. rmfroyd

    rmfroyd Well-Known Member

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    Yeah that's what I thought. When my wife filled out the paperwork, she checked the box that said 'deliver held mail on date specified'. Which was Saturday. I had other stuff going on and I made the mistake of assuming it would get delivered. Regular mail came at 2 (with no held mail), which is exactly one hour past the time the PO closes on Saturdays.

    So I couldn't go pick it up anyways. Oh well. Should have known better since it is S Florida.
     
  11. rmfroyd

    rmfroyd Well-Known Member

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    Got the kit last night, but had no time to work on it, hopefully tonight. If anyone is interested, here are the mods I am going to be doing:

    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.


    I wasn't originally going to put the Bias pot in, but from what I have read it is highly recommended, and if @bugspray did it on his amp, I might as well do it on mine.
     
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  12. backinit

    backinit Well-Known Member

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    I picked up a bias pot from Webber..not 10k though.
    A google search shows multiple sources. You're all set.
     
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  13. bugspray

    bugspray Active Member

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    With an adjustable bias, you have options. Neat trick for adjusting the pot, I carved the tip of a skewer to slot. Eliminates chance of electrocution! Please folks, be careful..
     
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  14. Chocol8

    Chocol8 Well-Known Member

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    Assuming he is correct in that some tubes will need a different resistor with the bias pot, I would go with a 5k resistor and a 20k pot, or two 10k pots in series to give yourself a wider adjustment range down the road. Actually, I would do a dual bias setup anyway so that each tube can be adjusted independently. Even if you buy a matched pair of tubes, the two sides of the output transformer won't be the same and your idle current wont balance.
     
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  15. rmfroyd

    rmfroyd Well-Known Member

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    Going to need you to hold my hand when it comes to that....
     
  16. Chocol8

    Chocol8 Well-Known Member

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    Busy now, but if you have pdf's of the schematic and layout, I can mark them up for you tonight or tomorrow night.
     
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  17. rmfroyd

    rmfroyd Well-Known Member

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    I will try to find something tomorrow.

    First off, check voltage at cp16 to make sure everything is drained properly, and then come to grips as to what the fuck you are actually attempting.....

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    FUUUUUUCCCCKK.
     
  18. Chocol8

    Chocol8 Well-Known Member

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    Just take it one component at a time. Each step is not hard, don't get lost in the overall complexity.
     
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  19. Manodano

    Manodano Well-Known Member Supporting Member

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    Yep, under ideal conditions, I would perform these mods one, maybe two at a time, fire up the amp and test the results, and then undertake a couple more of the mods. That's going to be a lot of work, but if you do all of these mods at once, (and it's a LOT) and then fire up the amp and encounter a problem, you've got a boatload of trouble-shooting to do. And that will be an overwhelming problem.

    Stare at the board and the parts a long time before you do anything and plan every move like a surgeon.

    You'll get better as you go along, so start with the easiest mod first, and work up from there. These mods would be moderately difficult on an old tag-board amp. The guts of your patient look relatively delicate to me, so you need to hone some skill here as you go along to avoid a catastrophic error. Specifically, you don't want to damage the traces on the circuit board.

    I do think the results are going to be worthwhile assuming you execute all of this properly. Folks do it every day, you can, too. So take your time, post often, and have fun with it.
     
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  20. rmfroyd

    rmfroyd Well-Known Member

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    Well, I got pretty much nowhere last night. I was getting everything organized and laying out my parts and realized I was missing a 6.8k 1watt resistor. Spent an hour looking for it and looking through my stuff to see if I had one. By the time I accepted defeat, I was too pissed off to work on anything. Luckily I don't need it until I get to the Negative feedback loop mod, so I can do a few things while I am waiting for one to arrive. So aggravating when you buy something(and spend a fair bit of money on it) and it doesn't come with everything you need. Bah.

    Will do, staring at everything all at once made me a bit woozy.

    Good advice. I would love to be able to do one mod at a time and plug it in, but the sheer number of things that need to be reattached as well as moving boards around makes me a bit leery of wires breaking off the circuit board. After having that happen on a few pedals I have built, I don't want to deal with that headache on an amp.

    I will probably do one mod a night, then double check the next night and so on. There isn't a rush, other than I don't really like project that drags on.....
     

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