“De-CBSing” a ‘76 Fender Vibrolux Reverb, got some questions…

[Raoul Duke]

Hi All,
Decided to take a break from building and try to improve an old amp of mine: 1976 Fender Vibrolux Reverb (looks to be AA270 circuit with no master volume, no pull boost).

The amp is what I think of as a text book CBS amp; lots of extra wire, wax coated board and components, really random lead dress (if you could even call it that), odd measures to mitigate poor build practices such as stability caps on the power tube sockets (pins 5 to 8 to ground), creeping green goo coming out of much of the hook-up wire etc. The amp is well used, but cared for and not abused. It looks good and sounds pretty good - worth the effort I think.

My plan is to clean up as much of the build as possible while replacing anything that’s out of spec as well as making it safer and more reliable by simplifying it and tidying-up its construction. I have a schematic and layout and am following both as I progress. Here’s a few questions to start:

- Reinstalling a modern grounded power cable and noticing deterioration in the courtesy outlet. Thinking I might bypass the outlet and ground switch and bring the hot to the fuse tip and splice the neutral to the white primary wire. I know using the ground switch and outlet for tie-ins is the usual, is there any reason I shouldn’t go direct and bypass them?

- Been trying to find info regarding how to conclusively prevent the need for the stability caps and can’t seem to find anything consistent. My guess is they are there to prevent problems caused by “quick and dirty” assembly process; so a thorough going-over and implementation of best build practices will hopefully go a long way - then I can address specific issues afterward should any remain. Is this a reasonable approach or is there more to it that I’m not aware of?

- Being a Dumble fan and having some waxy and warped original boards - I’m actually considering building and populating a new G10 board vs. fiddling with cleaning, de-waxing, chasing leaking DC etc. I’m under the impression that any Sliverface Deluxe Reverb, Vibrolux Reverb, Super Reverb board plan will work - just component values are different. Am I incorrect and is this a crazy idea? Anyone here ever gone this route?

Any opinions, advice, or experience would be appreciated! As Brandt would say: “these are our concerns Dude…”

At least to start…

Thanks!

[Stevem]

Especially if you will will go thru the effort of making a new board and also aid in removing those stabilizing caps designe the new layout so that all the grid wire are as short as possible.

For example the PI output coupling caps should be right on pin 5 of each output tube.

The PI input cap also right on the tube socket.

So your making plate wires long and input grid wires as short as possible anywhere you can .

This especially goes for the tone section caps.

Terminal strips can be soldered right to the back of the pots and all of that grid wire to get those connections point from the board to the pots is massively reduced.

And no it will not look like a Highwatt on the inside, but it will sound great!

[Raoul Duke]

Great advice, thank you Stevem! Very much appreciated.

[martin manning]

I’m under the impression that any Sliverface Deluxe Reverb, Vibrolux Reverb, Super Reverb board plan will work - just component values are different.

I have reservations about modding any vintage amp, except perhaps changing to a proper grounded power cable. If you do more than that, I would make everything reversible. I would start by desoldering all of the flying leads at the sockets, pots, switches, and jacks, lift the whole mess out intact and bag it. Remove all socket mounted components and bag them too. Then, get or make a repro AB763 Vibrolux Reverb board, populate it with all new parts and wire it in. Keep the 5U4 rectifier to maintain B+ voltage, and keep all usable chassis mounted components (pots, sockets, etc., removing, cleaning, and replacing each one). The original AB763 schematic and layout are available to guide you in the rebuild.

[Raoul Duke]

Understood Martin, and certainly a concern of mine - which is why I haven’t begun hacking away just yet.

Even though it’s a Silverface, it’s still almost 50 years old and hasn’t been altered in any way. I had it serviced when I got it about 5-6 years ago and it still had original electrolytics on the board and 3 out of 5 were original under the dog house. All of those were out of spec and replaced along with a few resistors, but everything else was left alone. I’ve since had a few small issues with dry solder joints and lead dress which prompted me to open it up and take a peek. The inside has the “played in smokey bars” funky brown staining mixed with the wax - but that’s easy enough to clean, just takes time and effort.

This is why my initial thought was to just clean it up and optimize what’s already here; leaving it as it left the factory - just a little neater and cleaner. Looking at it this morning - a new board is a bit extreme. Thinking I’ll just stick with the original plan at this point.

Always appreciate your perspective Martin, thanks!

[martin manning]

The value of the silver face amps has been on the rise too, which suggests staying with the clean and restore route.

[ampdan]

Here's my 1965 for reference.

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[Raoul Duke]

Very nice, thanks Dan! I bet she sounds great. Having your pics as a reference for lead lengths is helpful. Are all those electrolytics original?

Today I got all the creeping green goo off the board leads and began cleaning up the power tube sockets. The goo is really bad around the pot jumpers, so those might be getting replaced vs cut back and reattached. Also thinking I may replace the heater wires as the casing is stiff and crumbling in many spots.

Still on the fence about whether to completely bypass the outlet and ground switch or use one as tie-in points for the PT. If I bypass, that’s completely reversible and in my mind safer and more direct.

Decisions, lol…

[pdf64]

The 022848 OT has a bad rep for causing unstable operation when reverting later manufactured amps to earlier versions, eg
https://www.thegearpage.net/board/index ... t-38846765

https://el34world.com/charts/Schematics ... _schem.pdf

[lonote]

My plan is to clean up as much of the build as possible while replacing anything that’s out of spec as well as making it safer and more reliable by simplifying it and tidying-up its construction.

I’m actually considering building and populating a new G10 board vs. fiddling with cleaning, de-waxing, chasing leaking DC etc. ..is this a crazy idea?

Anyone here ever gone this route?

I ended up completely replacing a board after a couple attempts at cleaning. YMMV, as they say.

I kind of had various levels of rehab worked out in my mind when I started; first, various levels of cleaning, then rewiring with new wire (mine had the terrible pale green wire, in excess), then stripping, soaking & repopulating the board, then replacing the board.

I made it through the cleaning & then rewiring. Still had issues, so jumped to replacing the board. Prior to board replacement, the amp was working properly, but there were audible artifacts & random static that I eventually attributed to the board leakage. I was getting 2-3V at certain points on the original board, even after scraping, heat guns, isopropyl alcohol, etc. All good now.

I understand there are varying degrees of wax contamination between production years & even individual amps (mine was caked with wax), so you might start with a really good cleaning & try to measure board leakage near all the HT points to see what you are dealing with before deciding how deep you need/want to go.

To my mind, functionality wins out over originality. That said, it is never a black & white thing. I have 2 early 60s Fenders that board replacement would NOT be an option. They have a few random noises & hum on occasion but are still completely useable.

[ampdan]

Are all those electrolytics original?

They were until Summer 2024. I re-stuffed all the electrolytic caps with new.

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[Raoul Duke]

Are all those electrolytics original?

They were until Summer 2024. I re-stuffed all the electrolytic caps with new.

Hey that’s pretty creative. I’ve heard of that but never seen it. Looks great.

So today I planned on doing deep cleaning on the board around the likely DC leak areas. Before starting, I decided to do a thorough review of the schematic and layout just to be as familiar as possible before tearing it up. I found a number of discrepancies regarding values, so I decided that I would try and map out what I have as an official record of my starting point. At that point - I downloaded DIYLC and started to record - making notes and consulting the Digi-Key resistor color figureouter. Once I have an accurate finished product, I’ll post it here for info/critique.

pdf64 - thanks for the info; very helpful. I didn’t plan on Blackfacing and this info reinforces that decision. Very much appreciated!

lonote - I agree that form has to follow function also. I’m hoping my cleaning gets me there, but this amp seems really caked as well. Part of what convinced me to finally get DIYLC is the fact that what I generate for comparison can also serve as a board plan in the event I need one. Thanks for the benefit of your experience; it’s really helpful information and appreciated as well.

Thanks again all! I learn something from everyone who takes the time to offer info and I appreciate that very much!

Onward…

[Raoul Duke]

Really seeing a lot of variation in values in this amp vs the schematic; and these variations look original (from the factory) to me. In such a situation, which wins?

On one hand - the amp has sounded good the whole time I’ve owned it. Admittedly, not GREAT - but certainly good; relatively quiet, no weird behavior, kind of plain but in a pleasant and predictable way.

On the other - the circuit must have been developed a particular way for a reason. Could these values be the difference?

[sluckey]

Really seeing a lot of variation in values in this amp vs the schematic; and these variations look original (from the factory) to me. In such a situation, which wins?

If it ain't broke, don't fix it.

Fender has released a lot of amps where the actual component values differ slightly from the schematic. Who knows why. Anything we say is pure speculation. It could be something as simple as the assembly line girl ran out of 27K resistors and grabbed some 33Ks just to keep the line going. Who's gonna know. Or maybe even the engineers made a simple change but the schematic did not get updated. The Pro Reverbs seemed to be full of component value changes.

If this were my amp I'd use a red pen (or font) to mark up the schematic. Where different, write in red what the actual value is. Maybe then submit the marked up schematic for some wild speculative comments. Right now is an excellent time to annotate the differences. I'd like to know exactly what changes you are talking about.

[Raoul Duke]

I agree with “if it ain’t broke…” and I hope to have the list of differences completed tomorrow or Sunday the latest. I figured the more knowledgeable folks around here might find it interesting and I’m certain I’ll learn something more based on the observations.

BTW, I checked out that Double Shot and I’m sure I’ll be building it at some point. Seems to me to be a more straightforward and practical version of a Matchless DC30 - minus the bulbs inside the chassis and ~120% power tube dissipation, lol.