Fender Vibrolux Reverb | Tube WorkShop

Amp Service / Tuning

1979 Fender Vibrolux Reverb Amp

Some stories are simply written by life!

We bought a 1979 Fender Vibrolux Reverb at the end of 2021. Not a collector's item... modified... but it sounded good. Why? Simply because we think the Vibrolux Reverb with its 2x10 configuration is cool and we've wanted one for a long time. Since then, unfortunately, it's been leading a sad existence in our workshop, rarely played, and we've never opened and reworked it.

Somehow there was never the right "occasion"...

In June 2024, we met the exceptional 14-year-old guitarist Rudi Beinroth from Hamburg by chance at the Crossroads Festival at No.1 Guitar Center. And Rudi blew us all away with his tone and perfect Stevie Ray Vaughan licks. During the later chat, Rudi said: "My dream amp would be a Fender Vibroverb!"

Wait a minute: We have this Vibrolux Reverb... in many ways it's the 2x10 version of a Vibroverb. And the amp would definitely be delighted to finally be played properly again. For example by Rudi. So our Mario is now getting to work on making the Silverface Vibrolux Reverb fit for Rudi...

The Amp

What do we have here?

Here we have a late 70s Fender Silverface Vibrolux Reverb.

Great amp! Many people consider the Fender Super Reverb with its 4x10 configuration to be the best amp Fender has ever built. In many areas, the Vibrolux Reverb with its 2x10 configuration is the happy "middle" between a Fender Super Reverb (4x10 speaker) and a Fender Vibroverb (1x15 speaker).

OK, the end of the 70s wasn't really the "Golden Era" of Fender amps. Our amp actually comes from the last era of Vibrolux reverbs from 1978-1982, when Fender frantically tried to compete with Marshall with some kind of "boost" function. It didn't really work well. It sounded terrible too. But the good news is that Fender always stayed pretty close to their Blackface roots from the 60s with the Vibrolux reverb. Especially with the transformers, which are identical to the earlier models. Very good!

And we don't have an "unmodified" amp either.
Apparently, someone has used their modding skills in our model and integrated a mid-range control in the vibrato channel, as well as a master volume. Both are actually not a bad idea. But hey... isn't there a more elegant solution, like simply drilling two holes for potentiometers through the amp logo? On the back or something, and leaving the front panel original?

Anyway, this fish has already been cleaned. Let's look at it positively: We don't have to worry too much about "vintage correct" and "collector value" with our amp. That topic is "over". We can concentrate on making the amp a great "player" without constantly asking ourselves whether this will reduce the collector value.

Dating

Without Tinder...

OK, we've already written that we're looking at a '79 Vibrolux Reverb. How did we come to that conclusion?

Well, first of all, the presence of the "Pull-Boost" function in the Vibrato Channel limits our amp to the 1978-1982 version... [Link]

Then we can of course check the chassis number. This is A988307. A9 + 5 more digits already strongly indicate 1979. [Link]

And hey, we can also check the transformer codes. The latest EIA code can be found on the power transformer with 606-923. This was obviously manufactured in week 23 in 1979 by the Schumacher Electric Corp. All other transformers also come from Schumacher (EIA 606), and are identical to the earlier Blackface models.

So we can be very sure that our amp was made in 1979 .

Expectation Management

What are we facing?

Okay, so what do we know?
We have a '79 Fender Vibrolux Reverb. Someone has definitely played with it before, otherwise there wouldn't be any retrofitted mid and master controls. What else has the technician done? Has he already replaced all the electrolytic capacitors? Did he carry out his modifications "properly", or are we in for another "shit show"?

In general, the amp plays pretty well, without excessive humming or strange noises in either channel. So it can't be completely "off". Nevertheless, with a late 70s Silverface amp, we expect the typical "wax inferno" on the soldering boards (more on that later)...

The tremolo (vibrato according to Leo Fender's logic) is not working at the moment. We need to look into that.

The reverb works and is cool, but as is often the case with Fender, it is much too powerful at control settings >3.

The good news is that the 2 speakers are the original CTS speakers and they still work well. Incidentally, they are from week 27 of 1979. The CTS speakers deliver a great and very crisp clean sound. Typical Fender, where the speaker was very much tuned to deliver a great clean sound and "distortion" was a foreign word.

Therefore, one could assume:
Cool, there are probably only a few small things to fix on the amp and then it's off to Rudi. We could do that, but we won't.

Why? Fender Silverface amps have "problem areas".

You can always tinker with these here and there. But this usually results in an amp that keeps showing new problems. Or you can do it "right" once. Then you have an amp that is better in many areas than the factory condition and that can also handle tough rehearsals and live use without complaint.

Pants down!

Let's get to the bottom of things...

At the doctor's office, you would hear: "Would you please undress?"

We're now taking a look under the hood of the Vibrolux Reverb and are excited to see what dirty little secrets we'll find.

Tube set:

So, here we have a newer pair of 6L6WGC-STR power amps from TAD. Great, we can't say no to that, and since the amp doesn't hum much, the duo also seems to be well matched. In addition to the original Fender 5U4GB rectifier tube, there is a colorful sextet of preamp tubes from Fender, Groove Tubes, Siemens (oh dear!) and some where the labels have been cleaned off. None of them make a bad impression, but the truth will come out when we measure them on our eTracer measuring system.

Elko's:

More precisely: electrolytic capacitors.
They are in every amp. In the power supply and on the cathodes of the preamp tubes. Why are they always an issue? Because they have a limited lifespan. It is assumed to be around 20 years, depending on your "lifestyle". After that they can still work fine, but they don't have to. It's like a best before date. A yogurt that is six months "over" can still be edible, but do we really want to eat it? Especially with vintage amps that are regularly played at rehearsals and gigs, operational reliability is the top priority, and here the electrolytic capacitors simply have to be replaced. Period. Experimenting with "vintage electrolytic capacitors" is simply out of place here.

Ok, what does our Vibrolux look like?
Er... interesting! So, the electrolytic capacitors in the bias circuit have already been replaced. That was a good idea because they are vital for the amp's survival. Incidentally, I would have replaced the 45-year-old rectifier diode at the same time.

The cathode capacitors in the preamp were not replaced. Hmmm, you can do that, but you don't have to. And *shock* the power supply capacitors are still the original Mallory ones from week 29, 1979, 45 years old. You don't have to understand that. They are just as vital to the amp as the bias capacitors. The latter were replaced, but not the power supply capacitors? Really?

So: The Vibrolox has now won a complete cap service!

The wax problem :

Yes, 70s Fender Silverface amps have a wax problem! So does our Vibrolux Reverb...

Fender has always liked to use eyelet solder boards made of Pertinax. In the 70s, someone at Fender was concerned that these solder boards might absorb moisture and then become conductive. That is a legitimate concern. But Fender took completely the wrong countermeasure and thought that this could be prevented by simply dipping the assembled boards in wax to seal the surface.

Firstly, it makes a huge mess when you solder on a board like this, and secondly, Fender didn't take into account that over the years the wax layer tends to combine with all sorts of dirt particles and become conductive. This means that you can easily measure a few volts on the board surface where they shouldn't be.

This results in the funniest errors: crackling, toilet noise, etc.
And these are highly dependent on the use of the amp: played for 2 hours at a gig... suddenly the box was making a noise like a toilet flushing. Checked the next day in the rehearsal room: everything was OK.

Well, in the first case, maybe the wax melted due to the heat and caused a problem, and the next day the wax was solid again.

Perhaps this is where the reputation of Fender Silverface amps comes from, of being a kind of "constant construction site". That can be remedied. More later...

Grounding problems:

In German: "Masseführungsprobleme".
Hey, that really sounds very "German"! Let's start with the basics: Fender amps often have ground solder lugs inserted into the transformer's fastening screws. This is very economical, but unfortunately not durable (and nowadays it is no longer permitted in some areas). Unfortunately, the mains transformer in particular is usually the heaviest component that vibrates when the amp is played. This also causes the fastening screws and the corresponding solder joint to loosen. We recently had a Fender Bassman 135 where you could turn the ground solder lugs on the mains transformer in a circle with your little finger. We don't want that! It simply makes sense to separate mechanical connections from electrical connections.

The circuit and the soldering board:

OK, we know the amp was modded with a mid control and a master volume. This was done fairly well. In addition, Fender has already "raped" the solder boards from the Blackface era a little in order to implement the "great" Silverface mods. Let's see how far we can go "back to basics" here. And maybe we can also correct the famous "spaghetti wiring" of the late Silverface amps a little.

The bias circuit:

With the Silverface amps, Fender had the brilliant idea of deviating from the classic, adjustable bias circuit and instead installing an "Output Tube Matching" circuit, which also allows unmatched power tubes to be used. However, the amp techs of the world agree that this is nonsense and that they would rather have a bias circuit with which different "matched pairs" of power tubes can be meaningfully calibrated.

This was already changed by an earlier Amptech. It works reasonably well, but to be honest, there are more elegant solutions, such as with "fly wires".

Power amplifier tube socket:

Traditionally, Fender always inserts the grid stoppers and screen grid resistors directly into the power tube sockets. That's a great idea, and we do it that way at TWS. But now Fender has used carbon press resistors for this, which are relatively sensitive to heat. Then they were mounted so "tight" that they were directly exposed to the heat rising from the hanging power tubes. Not a good idea! Especially not long-term stable. It can be done better.

The Plan

To do list...

So, now that we have revealed all of the Vibrolux's dirty secrets, what needs to be done is now on the horizon.

The goal is to make an excellent "player amp" that has eliminated all the weak points from the Silverface era, is in top condition, and is absolutely suitable for rehearsals and gigs.

We try to keep as many of the original parts as possible, but we don't worry too much about "vintage correct". If something can be improved, it will be improved.

During the conversion, we will largely rebuild the amp back to the AA964 "Blackface-Spec". We will keep the mid-potentiometer and master volume, which will be integrated later, in a modified form. Firstly because they are useful features, and secondly because we have these unsightly holes in the front panel that cannot really be concealed elegantly.

To do list:

- Replacing the power supply capacitors

- Revision of the Bias Board

- Revision of the ground points on the transformer mountings

- Revision of the wiring of the power tube sockets

- Removal of all mods

- Complete redesign of the soldering board according to AA964 Blackface Spec

- Implementation of a mid control and a PPIMV master volume

- Commissioning and fine-tuning

Let's go.

Cap job...

In another professions, B... Jobs are the order of the day. For us amp technicians, they are Cap Jobs. While in the other profession, it is usually the customer who asks for the B... Job, with amps it is usually the case that we technicians have to convince the customer that the Cap Job would be a good idea .

Why do capacitors need to be replaced?

Electrolytic capacitors are not made to last forever. This is due to their construction with foil and electrolyte. This will dry out over time and unwanted chemical reactions will occur. This is often noticeable when the capacitor begins to "swell". Then it's high time. As a rough rule of thumb, you can say that good quality electrolytic capacitors last around 20 years. What happens after that is a gamble.

Some last another 20 years. These are the "Nice Guys"!
Some are at least kind enough to simply slowly lose their capacity, which manifests itself in a slowly increasing mains hum.
Some are not so nice and slowly become a resistance, which leads to an ever-increasing current consumption and can fry the mains transformer.
And the really nasty ones will spontaneously short-circuit or explode (a huge mess!). These are the service cases with the description: "Suddenly there was a loud bang in the amp, and then it hummed loudly." Not so cool if that happens during a gig.

Welllllllll... :

Do you have a valuable vintage amp that you treat like a raw egg and only play at room volume every now and then? OK, we can talk about leaving the original capacitors in for as long as possible as long as they don't show any "symptoms".

If you want to play a vintage amp regularly at high volume during rehearsals and gigs, then there is really little room for discussion here: The electrolytic capacitors have to be replaced! Anything else is "gross negligence". And if a customer turns up who is devastated because defective electrolytic capacitors have killed the power transformer in his "all original" 1964 VOX AC30, hmmm yes... then unfortunately our sympathy is limited. Sorry to say! After all, you also take your car for inspection. But we digress. ...

This is how we do it!

There are various methods for replacing the power supply capacitors in a vintage Fender amp. Some of them are sloppy but cheap. Others are more complex but sustainable and according to "factory spec".

The "sloppy" methods usually involve simply cutting off the old capacitors on the cables, bending them into a loop, and hooking and soldering the cables of the new capacitors onto them. This is called J-hooking. It works perfectly electrically... it's quick... it doesn't look pretty... and it's not really dignified in a vintage amp. If you have an operation coming up, you'll tell the doctor: "Please patch it back together as cheaply as possible. It just has to work, not look good...?!?!"

And then you can do it "properly". In a Fender amp, the electrolytic capacitor board is completely stripped of its components, cleaned, and then reassembled. In such a way that the connections of the new electrolytic capacitors are already mechanically well secured and are additionally protected from vibrations with a dab of silicone.

Apart from the fact that we generally reject "DIY repairs", rebuilding a 70s Silverface amp makes sense. This is due to the wax layer mentioned above that is on all soldering boards and which makes soldering on them a nightmare. But more on that later when we look at the preamp board.

Rebuilding & Modding :

So, we completely removed the capacitor board, cleaned it and re-equipped it.

The original Mallory capacitors have a voltage rating of 450V and a temperature rating of 85°C or less.

We replaced these with TAD types that have a voltage rating of 475V and a temperature rating of 105°C. The increased voltage rating gives us additional "headroom" with our current mains voltages of >230V. The original 450V capacitors were quite undersized. The 105°C temperature rating simply gives us a longer service life.

The two carbon press dropping resistors were absolutely OK in terms of their values. However, it makes absolute sense to replace them with modern 2W metal film resistors with a higher load capacity that are insensitive to temperature fluctuations. These resistors have no sound function. They simply have to work as well as possible.

Modding:
And yes, we have already "modded" a little here. In the Silverface Vibrolux reverb, the first dropping resistor was chosen to be significantly smaller than in the Blackface amps. This gives the preamp and the phase inverter a slightly higher supply voltage. The aim was to keep the amp clean for longer. This was also achieved. However, the late Silverface amps are also said to sound a little too cold and quite "sharp". In a Nile Rodgers cover band, this would certainly be perfect.

But since Rudi is more into classic blues, we adjusted the dropping resistors to "Blackface Spec", where the preamp runs at a slightly lower voltage and therefore goes into breakup earlier. It is said about Blackface amps: "If you turn the volume above 3-4, the amp is no longer clean, but starts to distort slightly." And yes, I think that's exactly what we want here!

Bias Board

Boooooooring , but important...

So, the power supply capacitors are happy again. Let's focus on the bias board.
What does that do? Hmm, well, it does a negative voltage which we use to set the power tubes to the correct operating point. You don't have to understand this in detail...

What happens if it gets faulty?

Well, that can push the power tubes to the edge of nuclear meltdown and maybe even kill the output transformer. In short, that's not so cool and we don't want that.

The bias board is an important, safety-relevant component in an amp, even if it is anything but "sexy".

What do we have in our amp?

Originally, our Silverface Vibrolux Reverb was supposed to have a circuit that Fender called "Output Tube Matching". This allows you to set up unmatched power tubes so that they run evenly. Great idea!

Unfortunately, in practice we always swap power tubes as a "matched pair" and we want to set the bias of this pair to the optimum level. Not too cold, not too hot... nice and cozy. You know what I mean. Yes, unfortunately that's no longer possible.

Long story short:

Nice try Fender! You have solved a problem that doesn't exist in practice. In return, you have robbed us of the opportunity to optimally calibrate different "matched pairs". A previous technician was kind enough to convert this back to the "normal" option for bias adjustment from the Blackface era. It works quite well, but it could also be a little "cleaner".

Clean-up work...

Usually happens after an accident. We don't really have an accident here, but the already modified circuit offers some potential for improvement:

Remove the nasty wax from the board
Replace the ole diode. It still works well, but as you know, if it fails, there will be a nuclear meltdown!
The old carbon comp resistors are "vintage correct", but at this point we prefer to use modern metal layer or metal oxide types with very tight tolerances and high temperature stability. Hey, we're not talking "sound" in this part of the circuit.
Integrating a second capacitor to better smooth the bias voltage was a very good idea. We can do that in a more elegant way...

Again, we took out the entire board, cleaned it, and then changed the board layout so that we could accommodate both smoothing capacitors and the dropping resistor in front of the bias adjustment potentiometer. Yeah, no more "air-wired" components! And at the end of the day we adjusted the value of the tail resistor in the bias circuit so that we got a more optimal adjustment range.

Grounding

We are so down-to earth!

To make it short: Grounding in an amp ensures 1st Safety and 2nd Less background noise in the form of hum.

There are sophisticated ground routing concepts to achieve this as optimally as possible. Fender didn't really follow all of these. But that's not so tragic, because Fender didn't build high-gain amps. That's why Fender amps can still be very "quiet" in terms of background noise.

But one thing is a "no-go":

In the past, it was common practice to use the mains transformer's fastening bolts for ground solder lugs at the same time. But now the mains transformer is the heaviest component in an amp, and the screw connections tend to loosen over the years, particularly due to the vibrations in a combo.

What do you have then? That's right: Loose ground connections!

We recently had a Bassman 135 serviced, and you could turn the solder lug of the center tap ground connection in a circle with your little finger. Not good!

As I told many times: Mechanical connections and electrical connections should be kept as separate as possible. Making me sound like an old record.

This is how we do it:

The nuts on the transformer screw connections only hold the transformer. Nothing else!
All ground connections are soldered directly to the chassis. For this you need a "big gun" soldering iron. Please don't try to do this with a 20 $ device from your electronics junk box. "Gluing" is different from "Soldering".

Tube Sockets

Well guys, are you still in good shape?

In every vintage amp you have to look at the tube sockets and their wiring.

Are the contacts oxidized and need to be cleaned?
Do the tube sockets still make solid contact with the tubes, or are they worn out and need to be tightened, or do we even need to replace the socket?
Is the wiring of the tube sockets well done?

Here we have:

Everything was fine! We still cleaned it with isopropanol.
Yep, everything is solid here.
Uhmm, nope, we don't entirely agree with what we have

Rewiring the power tube sockets:

With the preamp tubes everything is fine so far, but we are not really happy with the power tubes.

The 1.5k grid stopper resistors are installed extremely close to the hot 6V6. And they are heat-sensitive carbon comp resistors. Although they all still show good values, it is worth installing new resistors with some distance between them. We chose 1W carbon film types for this and installed them in such a way that there is some "airflow" between the tube and the resistors.
And the 470 Ohm carbon comp screen grids are also located directly in the rising heat of the power tubes. We swap these for super-robust 5W wire resistors that are installed to the side.
Then we have the 2000pF capacitors on the control grid of the power tubes, that Fender introduced to suppress oscillations. These "steal" a little openness in the sound. Well, the oscillations come from the sloppy wiring of the Silverface amps. We will correct this and make sure that there are no oscillations that need to be suppressed.
And while we're at it: The cathodes of the power tubes are soldered directly to the chassis (= ground). It is essential to check these connections. And while we're at it, we'll also add two very tightly tolerated 1 Ohm resistors so that the bias can be easily measured in the future.

Now to the big fish

The eyelet board and its wax problem!

History:
Where do we start with this topic? It's best to start with a little history...

Since the early days of Fender amps, eyelet boards made of Pertinax have been used. Simply put: hard cardboard impregnated with phenolic resin. It may sound strange, but it's true.

Even if they tend to warp over time, it is still a robust base. Easy to install and easy to service. They have also stood the test of time. I don't know any amp technician who says: "Oh, those old eyelet boards from the 60s... they only cause problems!"

So actually everything is fine. For some reason, in the 70s Fender came up with the idea that it would be super cool to dip the assembled boards in wax. Some clever person probably said: "Hey, we dip all our pickups in these wax baths... and they're sooooo much better afterwards. Let's do the same with the boards from our amps, then they'll definitely be sooooo much better." And everyone said: "Great idea! "

Since we cannot understand what the motives were, we talked to an expert: ChatGPT!

Here is a brief summary of ChatGPT’s statements and our comments:

Stabilizing the components... ahem NOPE, for that the components would have to be completely sealed in wax.
Protection from moisture... hmmm, maybe. However, we are not aware of any moisture problems with the earlier amps.
Preventing microphonics... for this, the components would have had to be completely encapsulated, and somehow we are not aware of any microphonics problem with older amps.
Easier assembly and maintenance... this is a bad joke, isn't it? ChatGPT sells us the waxed boards as being "easier to service"!

So either ChatGPT is a load of crap, or Fender's marketing department was desperately looking for ways to sell the Silverface amps as an "improvement" in the 70s. Maybe both is true, and the super-smart ChatGPT is just repeating Fender's marketing bullshit from the 70s. Who knows?

Whatever, fact is:

Waxing the Eyelet Boards was a shitty idea!

The amp gets hot when in use. This makes the wax sticky and magically attracts all kinds of dirt that settles on the wax. Over time, this creates a conductive layer on the board and leakage currents occur.
When you carry out service work on the board, the wax melts first and you end up with solder joints that are a mixture of melted wax and solder. This is not ideal electrically and it is an ugly mess!

This is one of the main reasons why Silverface amps have the reputation of being "Problem Childs". They keep making noises. Toilet flushing... crackling... etc. During a service, a component is often replaced and the problem disappears. Great! After 4 weeks, the problem returns. Why? The component was not the problem. But when the new component was soldered in, the wax layer melted, and that temporarily resolved the symptoms.

We've had Silverface amps where we measured 5-20V on the board surface. And that should actually be 0V! We've also had Silverface amps where we only partially cleaned away the wax (to keep costs down). This kept causing new problems in other places. And we've also had amps where we cleaned as much of the wax off the surface of the board as possible and it still produced loud background noises. When we then removed the Eyelet Board and lifted it up, the background noises disappeared, but loud cracking noises came out of the speaker. Yes, exactly... the wax soup is also on the underside of the soldering board, and here we can't even see what's going on.

What to do?

From our experience, there are exactly 2 things you can do with a Silverface Amp:

Keep your hands off the Eyelet Board! If the amp isn't making any strange noises, then it's best to leave everything as it is. Don't even replace the cathode capacitors. Just don't touch the board as long as it's not causing any problems.
Completely remove and disassemble the eyelet board. Remove wax. Clean it. Rebuild the eyelet board.

That may sound pretty radical, but it has simply been shown that everything in between is rubbish and always causes new problems. At TWS, we now refuse to tinker around with these boards at the lowest possible cost. These are exactly the amps that come back every few months and the owner complains that "something's wrong again". Either we leave the eyelet board alone, or we do it properly for once!

Our Vibrolux reverb hasn't shown any "symptoms" yet, but we want to make it super stable and super quiet for Rudi. And not just for a short time, but for the long term. So: We're going for the Grand Harbor Tour!

"German De-Waxing": The Process

Ok, De-Waxing a 70s silverface amp is not a specifically German invention. Good amp techs all around the world do it like this, or similar. But the play on words with "Brazilian waxing" was just too tempting...

So what does it look like when we completely remove the wax layer from a Silverface amp? And what do we need for that? Hot air gun, a roll of kitchen paper, flat-head screwdriver, isopropanol and patience...

Here we go:

Unsolder all cable connections from the board. Unless you can rebuild a Fender amp blindly: Take lots of photos of which cable goes where. And another tip for soldering: work hot, carefully and quickly. The insulation of the white Spaghetti-Cabling melts faster than you recognise.
Removing the eyelet board and the isolation board underneath.
Desolder all components and cable connections on the board. It is best to put the components back in the correct order on a layout diagram of the amp.
Each desoldered component is cleaned from wax (hot air dryer/kitchen roll) and checked for compliance with its values. Bad components are immediately replaced with new ones that have the same spec.
Cleaning all eyelets of old solder. Desoldering station, desoldering pump, desoldering braid... these are the tools of your choice in descending order. Or you can do the pro move by "knocking the eyelets out over a trash can". Melt solder in the eyelet... knock board on the edge of your trash can... eyelet clean!
Dewaxing Pt.1- Rough cleaning: Hot air gun, lots of kitchen roll... Melt the wax and wipe it away. Top/bottom of the board. Or you can simply place the board vertically in a container and melt the heaviest layer of wax with the hot air gun. Make a candle out of the wax you removed. Scent: Vintage Fender!
Dewaxing Pt.2 - Deep cleaning: When most of the wax has been removed, go back to the eyelet board with the hot air gun. It will start to shine again. This is wax coming out of the eyelet board. Heat... wipe away ... heat again... wipe it away. Until nothing comes out anymore.
Remove old flux: There is usually a crusty layer around the eyelets. This is old flux from the solder. We scrape it off with a flat-head screwdriver and then clean the entire board generously with isopropanol.
Deep cleaning of all eyelets: The board now looks great again. No celebrating yet! We now heat each eyelet with the soldering iron and are amazed at how much wax and flux residue around the eyelets is expelled. Wipe away with kitchen roll, clean with isopropanol, and repeat until nothing comes out except isopropanol.
Repeat on the back of the eyelet board and on the front and back of the isolation board

OK, this is not an easy process and you can't do it in 5 minutes. It is time-consuming. The reward is eyelet boards that are practically in new condition. Ready to be re-equipped with the original components and then provide them with an ideal home. And as an "extra" you get lots of wax-soaked kitchen towels, which are perfect fire-starters. The perfect occasion for a barbecue party!

Reconstruction

And modding...

So, here we are with our Vibrolux Reverb puzzle and we need to put it back together. It's a bad idea to just start soldering. A better idea is to think carefully beforehand about what you want to end up with. Otherwise you'll end up soldering in circles.

What do we want?

We want to convert the circuit to the AA964 Blackface spec
We do NOT want the obscure "Pull-Boost" feature
We want a midrange control in the vibrato channel
We want a well-functioning master volume

Some more dismantling...
Yes, sorry... we actually wanted to rebuild. Coming soon, really. Just a little more "beautiful living"...

The first thing to go is the wiring for the "pull-boost" circuit. Yes, you can simply deactivate it, but we don't like unnecessary cable clutter. Then the retrofitted mid-range potentiometer is removed. Why? We want a center potentiometer. Yes, we do... But a different one. And then the retrofitted master volume flies out. This was relatively poorly implemented at the input of the phase inverter. We definitely want a Post Phase Inverter Master Volume (PPIMV). But more on that later.

In a perfect world...
If we had a Vibrolux Reverb with a front panel that wasn't disfigured by 2 crude holes. What would we actually do then in terms of modification?

The pull-boost circuit would still be removed
We would use the hole on the back for the pull-boost footswitch socket to install a master volume. There would also be enough space here for a TWS PPIMV WonderVol.
Instead of a mid-range control, we would use the pull-boost switching function in the volume pot of the vibrato channel to realize a switchable mid-boost. In other words, a pull-mid boost switch.

Rebuilding & Modding :

So we are starting with a completely virgin eyelet board in perfect condition. We have located and replaced components that no longer meet their specifications.

In an original restoration, we would now simply re-equip the eyelet board with all existing or renewed components.

In our case, things are a little different. Since we are "blackfacing" the Vibrolux reverb, we have to take the corresponding circuit changes into account when rebuilding it. These are mainly changes in the phase inverter and the hall driver. In addition, the bias voltage for the power amplifier is no longer fed via two 220k resistors on the eyelet board, but goes directly to the PPIMV.

Eyelet board, all original?

In addition to the whole wax mess, all "old components" of course still have the potential to cause trouble. Even if we have measured them all and they show good values... a 45-year-old carbon pressed resistor can make noise. The same goes for a 45-year-old ceramic capacitor on the treble control in the tone control. The plum-colored polypropylene capacitors from Sprague are usually beyond reproach.

So are we swapping things "on suspicion"? No, we're not doing that... we want to preserve as much of the original sound substance as possible. That's why we first equip the board with the "old" components and only change them in the places where we "blackface" them with identical types with different values.

This may lead to minor problems later on, but you have to proceed with these amps like peeling an onion. First you remove the outermost layer and then you have to work your way through the different layers piece by piece. And if a "vintage component" does come across later, it can now be replaced quickly and without making a mess.

And yes, of course you can now start to discuss whether it wouldn't be better to use technically better or higher quality components when rebuilding the board. Lower noise metal film resistors on the anodes of the tubes, silver mica instead of the old ceramic disc caps, etc. Again: we don't want to build a new amp, we want to restore an old amp. And believe it or not, the much-hyped silver mica does not do a classic Fender amp any good in terms of sound. This usually makes these amps too sterile and they lose the typical, slightly greasy but organic-sounding highs. Just to give one example.

We are not quite so squeamish about the cathode capacitors in the preamplifier; these simply have to be replaced after 45 years. We use very high-quality bipolar types from TAD. In the same way, we give the vibrato circuit new "disc caps" in the oscillator. They don't make any "sound", they just oscillate happily to make the vibrato faster or slower.

And oh yes, the tremolo, er, vibrato...

We found out during testing that this doesn't work. Our main focus here is on the so-called "Roach". This is the classic Fender assembly, where an LDR and a neon lamp are combined with a shrink tube to form a single component. Over the years, the LDR often gives up the ghost. So we're replacing the "Roach" with a new one. Hopefully this will not only bring the tremolo back to life, but also improve the depth of the tremolo, because the LDRs built into them often "wear out" a little over time.

Master & Mid Control
So, our eyelet board has now risen from the ashes with new splendor. Let's take a look at the mid control in the vibrato channel and the master volume.

Admittedly, the way the controls were originally arranged is not really intuitive....
Speed... Intensity... Master... Middle.

The bad news are: There's really no other way with the existing holes! Learn to like it...

Master:
We would like a PPIMV in LarMar type here. Why? A Vibrolux reverb is actually an amp trimmed for "clean". But it can also be cranked and overdriven. The distortion is first created in the phase inverter.If we now install the master volume (as we have done up to now) before this, we have the effect that the amp distorts more and more the more you turn up the master.
We don't want that because you're constantly busy turning the volume and master. We want to be able to set a certain sound and then adjust the volume. So the master volume has to be behind the phase inverter, and we would prefer to have our TWS WonderVol installed here, which enables a constant sound even at very low volumes. Unfortunately, in a Fender chassis with the angled front, a normal, two-stage potentiometer only just fits in (with difficulty). And only right next to the intensity control. Otherwise it would collide with the bias board. That's why our Silverface Vibrolux Reverb gets a standard LarMar PPIMV, which does a good job.

Mid Control:
Great, it also fits mechanically via the bias board. Previously, a 10k potentiometer was installed there. Yes, we already know where that comes from: that was the value that Fender used for the mid pot in a Twin Reverb or Super Reverb. Was that cool? Not really.
Fender amps thrive on having a very thin mid-range.In the Vibrolux Reverb, the midrange was set to a fixed value with a 6.8k resistor. If you now install a 10k pot instead, you have to turn it almost to 7 to get the original sound. Below that, there is really only "even less mids", and you don't have much leeway upwards either.

We prefer a 25k mid pot at this point, similar to a Marshall. This means that for the factory setting without a mid control, you have to turn it down to approx. 3.5. However, you have a lot of room to move upwards, and you wouldn't believe how good a few mids do a Fender Vibrolux Reverb when you get into blues or rock territory. This control definitely defines the character of the amp.

We can't wait to see how the amp performs now!

Resurrection

And a few final tweaks...

Generally, we now have a completely rebuilt 1979 Vibrolux Reverb in front of us, which is significantly better than in its original condition:

All the ageing electrolytic capacitors have been replaced
The BIAS circuit has been optimized
The wiring of the power amp socket has been overhauled
Ground connections were optimized
ALL soldering boards were completely freed from wax and cleaned
The amplifier circuit was completely rebuilt according to the AA964 specification, but virtually all original components were reused.
The pull boost circuit was removed and a mid-range control in the vibrato channel and a post phase inverter master volume were integrated.

First impressions:

So we slowly turn the Vibrolux on with the Variac. Power consumption is normal... but we can't hear anything from the speaker. Strange. So master is on 10, volume on 5. Hmmm, OK... we can hear a very subtle white noise. Guitar plugged in. Holy shit, that's loud!

I don't want to brag, but in terms of noise, this is probably one of the quietest vintage Fenders I've ever heard. There's no magic behind it, just a lot of care and attention to detail when restoring an amp like this. Ultimately, our Vibrolux is proof against the common belief that "Vintage amps always have noise, that's normal!" No, that's not normal. A very well-maintained vintage amp can be incredibly stylish.

Sound check:

Oh yes, the "blackfacing" has done the amp good. Also the new midrange control and the PPIMV master volume. The clean sound was already very good before, but sometimes a bit "sharp" and too "hi-fi". The amp now sounds noticeably rounder overall, and the transition to overdrive is now much more forgiving.

The new midrange control is a real weapon, especially if you want the amp to distort a little. With the midrange turned up a little further, the amp distorts much more harmoniously, has more "meat" and no longer sounds as anemic as it did in its original state.

The master volume works great and actually allows you to turn the box down without having to accept too many changes or losses in the sound.

Optimization of the tube set:

So, the amp is working again, has virtually no background noise and sounds extremely promising right out of the box. Let's take a look at the tube set:

We replaced the 5U4 rectifier tube with a GZ34, like the one used in the Blackface era. With the bias set optimally, we now measure a voltage of 442V at the first filter node. Ah yes... so it was worth installing power supply capacitors with a voltage rating of 475V. And that was probably exactly the reason why Fender went with the 5U4 in the 70s. It delivers a little less voltage and allowed Fender to stick with the capacitors with a voltage rating of 450V despite a higher mains voltage than in the 60s. The guys at Fender have always been penny pinchers. But we can afford the GZ34 in our amp because we upgraded the power supply.
The two 6L6s in the power amp are a relatively new pair from TAD and work absolutely flawlessly. We have calibrated them to approx. 60% bPlate dissipation.
The preamp tubes are a mixed bag. Both the 12AT7 in the phase inverter and the 12AT7 in the reverb driver are a bit worn. We swapped the reverb driver tube for a plain JJ (doesn't do much for the sound, just has to supply power), and in the phase inverter we used a nice NOS JAN tube "Made in the USA". The Groove Tubes 12AX7 in the vibrato channel was a bit microphonic and was swapped for a NOS 12AX7 from Teonex (probably Telefunken or Valvo). The 12AX7 in the reverb recovery of the vibrato channel had a bit too much background noise for our taste at this sensitive point in the circuit. In the future we will rely on a very high-quality Sovtek 12AX7WB.

So only 2 of the original preamp tubes "survive", namely in the input of the normal channel and in the vibrato circuit. Has optimizing the tube set changed the world? No, but it has given the amp a little more headroom and it plays a little more confidently and dynamically overall.

Final Tweaks:

Reverbs:

As with almost all classic Fender amps, the reverb potentiometer is basically a switch between "no reverb" and "too much reverb". The reverb potentiometer is actually only useful in the control range 0-3. You can now adjust the reverb circuit by putting less level into the reverb coil. But that also changes the depth of the reverb. Rudi should now be able to fully enjoy the classic Fender Spring Reverb circuit, as you know it from a Deluxe Reverb or a Super Reverb.

Personally, I'm not a huge fan of it because I always find the reverb a bit too long and it doesn't really blend with the guitar for my taste. But hey, that's the "classic"!

What we have done, however, is that we have changed the reverb potentiometer to a logarithmic type. This at least gives you a reasonable control range of 0-6.

Tone control:

Fender amps have bass. Lots of bass! Professionals say: "Never turn the bass control above 3 on a super reverb!" And they're right. Especially when you push a classic Fender amp into overdrive, it's usually the bass range that becomes "fuzzy" and uncontoured first. Since we know that Rudi likes to indulge in SRV licks (and puts a lot of energy into the low strings), we decided to reduce the tone cap for the bass control from 100nF to 47nF. This doesn't change the basic sound of the amp, but we noticeably gain more tightness on the low strings without the amp sounding "thin". We like it! And then you can turn the bass control to 5-6 without feeling guilty... ;-)

Final Thoughts

Is an amp like this worth it?

If you made it this far without falling asleep, congratulations!

You have read what is necessary to turn an unloved Fender Silverface into a top amplifier that can compete with any Blackface model or is even significantly better.

For a normal order, you have to expect costs in the range of 500-600 EUR. That's pretty steep. Let's compare the used prices of a Fender Super Reverb:

A moderately preserved Silverface Super Reverb sells for 1,000-1,500 EUR
A moderately preserved Blackface Super Reverb usually sells for 2,000-2,500 EUR

Conclusion:

It is absolutely worth buying a Silverface amp cheaply and investing the money in a thorough overhaul by a specialist. In the end, you get an amp with almost identical ingredients to the Blackface era, but with a much better sound and less background noise.

Let us put it this way:
If you are a collector, buy a Blackface amp that is as original as possible and put it in the safe as an investment. If you want to play such an amp regularly: buy a Silverface model and have it serviced and made fit by a specialist!

What Rudi says...

... you can see on Instagram!

We are excited to see what he can do with this amp!

Just follow Rudi on Instagram at @rudi_bf to stay up to date.

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