DIY Workshop: ’57 Fender Vibrolux Project

Vintage bargains are still out there if you’re prepared to take the time to look around, and you aren’t too hung up on originality. Despite a re-tweed and a swapped speaker, Ed Oleszko couldn’t resist this ’57 Fender 5E11 Vibrolux when he found it languishing at the back of a guitar shop. What followed was an adventure in amplifier archaeology.

First look

Some sources suggest that the Vibrolux is essentially a 5E3 Deluxe with added tremolo. However, the Vibrolux circuit is much closer to a tremolo’d tweed Harvard. Although the Vibrolux cabinet has the same finger-jointed construction and dimensions as a Deluxe, the stock speaker is a 10-inch rather than a 12-inch.

At this point, I hadn’t even seen the cabinet, but by all reports it had been subjected to a dodgy re-tweed at some point, and attempts to ‘age’ it resulted in a look that was more milk chocolate than elegant amber. Fortunately, though, the original nameplate and handle were still present, so Ed sent the cabinet to Mark Phillips at AF Custom Cabinets in South Wales for a more sympathetic restoration.

While this was being done, Ed wanted the Vibrolux restored electronically, too. He has every intention of gigging this amp, so the priorities were safety and reliability. He recognised that much of the amp’s originality had already been lost, and so was open to circuit tweaks and modifications if they resulted in improved tone. The first task was to take a closer look in order to figure out what Ed had actually bought.

Digging deeper

The amp was working and the tone wasn’t bad, but it felt a bit lifeless and underpowered. To my ears, the overdrive was a bit ragged, and compared with most tweed-style amps with dual-6V6 output stages, the Vibrolux had a lot more clean headroom. The tremolo effect was weak and the speed range seemed to be restricted.

This Vibrolux had undergone running repairs over the years. Several capacitors and resistors had been changed, but almost all of the electrolytic capacitors were still in situ.

The replaced caps included a mixture of British and US components, some of which were vintage themselves. According to the schematic, a couple of these capacitors were not of the correct value, and the replaced resistors were a mixture of carbon and metal film types, rather than 50s-style carbon comps.

The serial number and date code on the original mains transformer and potentiometers indicated a manufacturing date of 1957. A ’57 Vibrolux would have been fitted with a Triad #108 output transformer, but this amp’s transformer product code read 125A1A – making it identical to the Schumacher-manufactured unit Fender installed in 60s Princetons, Deluxes and Deluxe Reverbs.

Its date code corresponded with 1963 and 1973, but the cloth-covered wires suggested the latter. Ed and I discussed sourcing a Triad #108 replacement, but further research revealed Fender began installing Schumacher 125A1A output transformers in the Vibrolux from mid-59 onwards, so we decided to keep it.

Safety first

Before going any further, I had to ensure the amp was as safe as possible. I was pleased that a grounded mains cable had already been installed, but at the other end of the cable I found a cheap two-prong US mains plug with the earth wire attached to a crocodile clip.

A replacement three-prong US mains plug was sourced from RS Components, and we were good to go .

Making connections

The original speaker and tremolo footswitch connectors were RCA/phono.

Although they do a job, the connections can be intermittent if you have to use quarter-inch jack plugs with jack-to-RCA adaptors stuck on the end. I decided to install open-frame metal jack sockets instead, and discovered the existing RCA socket holes were large enough to accommodate them.

Electrolytic capacitors

Amp restorers often have to balance originality with safety and reliability. If a vintage amp is pristine and bone stock, it’s understandable if a collector wants to keep it that way. But when amps have already lost many of their original parts, sympathetic restoration allows owners to enjoy playing them without the worry of failure, or indeed death.

With ancient electronic components, the terms ‘working’ or ‘not working’ don’t always apply. Components such as resistors often drift significantly from their specified value, and capacitors may go leaky long before they fail. Consequently, the amp may be functioning, but not as it was designed to.

After disconnecting the amp from the mains supply and ensuring all the capacitors were drained of charge, I de-soldered one end of each electrolytic capacitor and tested them.

Step one was a visual inspection to check there was no swelling or discharge from either end of the cap. Step two was using a professional capacitor tester.

A 20uF had taken the place of one of the 16uF power supply filter caps, and it was leaky. I decided to change it, along with both 25uF cathode bypass caps and the 25uFbias cap. The two other 16uF power supply caps tested fine, so I left them alone.

Orange cardboard tubes enclosed the original Astron 25uF caps, so I opened up one end, slid out the capacitor and then put the Sprague Atom replacements back inside. Although the caps were being changed, it was good fun to preserve the original look.

Signal capacitors

When you’re swapping capacitors, it’s important to ensure that the voltage rating of the replacement is equal to or greater than the value of the original.

You can choose any number of products, including some amazing-looking Astron replicas from Luxe, but I decided to go with Tube Amp Doctor paper/oils, because they were reasonably priced, similar in construction to the ones I was removing and had garnered some favourable reviews from people who’ve used them.

The primary purpose of a signal capacitor is to block DC while allowing AC to pass. DC comes from the power supply, and the only AC flowing through the amp should be the audio signal. When signal caps go leaky they can adversely affect tone, increase noise levels and potentially damage the amp.

We decided to replace all the signal caps in this amp for the sake of reliability, but the originals have been kept and could be reinstalled at a later date.

Resistors

The decision to reinstall carbon comp resistors where they had been removed was driven as much by cosmetics as sonics, but Ed likes the sound of amps with carbon comps. Since this was a complete rebuild, I also tested all the original resistors that had survived from the 1950s – and replaced those that had drifted in value significantly.

Bias check

Unlike the 5E3 Deluxe, the tweed Vibrolux is a fixed-bias amp with both 6V6 cathodes directly connected to the chassis. There is a simple way for amateur amp tweakers to measure the bias and check how well the valves are matched; it involves connecting 1-Ohm resistors between the cathodes and ground.

A tiny 1-Ohm resistor makes no difference to the way the amp functions, but it means you can measure the voltage across the 1-Ohm resistor and apply Ohm’s law to figure out the bias current. The 6V6s in this Vibrolux measured within 0.5mA of each other, so they were very closely matched, but the amp was biased fairly cold.

Apparently, this is a common issue, and some owners choose to install a bias adjustment potentiometer to replace the 56K bias resistor.

Rather than make a hole in the chassis, I experimented with resistor values until I achieved what I considered to be the best tone; then I checked that the bias was within recommended margins for a pair of 6V6 valves running in Class AB at the measured plate voltage. I ended up with a 33K bias resistor.

Power supply

Although the virtues of various resistors, capacitors and valve brands can be debated ad infinitum, this project demonstrated how setting the internal voltages is equally import as a determining factor for tone and dynamics – if not more so.

During my initial tests, this Vibrolux didn’t sound especially tweedy – and a quick voltage check revealed that the voltages were way too high. The Fender schematics allow for 20 per cent tolerance either way, but the first half of V1 was getting a plate voltage just below 250V, when the schematic specified 160V. The voltage on the 6V6 plates was more than 400V – when the specified voltage was just 330V.

The 470R resistor between the first two filter caps had been changed to 330R, and there was a 1K5 resistor connected to the selenium diode in the bias circuit – rather than the 6K8 shown on the 5E11 schematic.

I had checked out several pictures of Vibrolux circuit boards while researching this project, and many had 1K5 resistors in that position. So be aware that Fender’s published schematics don’t always correspond with actual production specs.

What followed was not exactly scientific. Instead, I began experimenting with various resistors in the power supply until I achieved the voltages I was looking for throughout. It should be stressed that this was achieved by using my ears just as much as
a voltmeter.

At higher plate voltages, the Vibrolux ran cleaner and clearer with a tight and focused bass response. Lower voltages corresponded with quick breakup, easy overdrive and a squishy or compressed response when the amp was being pushed hard.

I preferred preamp voltages that were bang-on tweed specs, but the power amp sounded better to me running at 375V. This figure was in between the 330V specified on the Vibrolux schematic and the 410V specified for a Princeton Reverb.

Remember that a Princeton output transformer had already been fitted in this Vibrolux. My findings may have been different with a Triad #108. In a sense, this Vibrolux has ended up with a tweed preamp running into a blackface-like power amp, so it overdrives and compresses nicely, but the power amp prevents excessive sag.

It’s also worth knowing that the 5E11 Vibrolux with the later transformer is a few component values away from being a 6G2 Princeton. You can even run the Vibrolux with a GZ34 rectifier and get pretty close to a blackface tone – minus the reverb, of course.

Tremolo

No audio signal flows through the tremolo circuit and, in a tweed Vibrolux, essentially it’s a simple oscillator that modulates the bias of the power valves. The valve tested fine, so it seemed that faulty capacitors were causing the weak tremolo in this amp.

Once the originals were removed and tested, it could be seen that they had drifted off spec, and some were leaky. I replaced the tremolo capacitors with Mallory 150s, making one small modification in the process – substituting a 0.1uF from the later 5F11 circuit for the stock 5E11 0.25uF cap that’s tied to the depth control.

This helped to reduce the pulsing low-frequency noise that was noticeable with the tremolo engaged. The Vibrolux tremolo really came into its own, producing deep wobbly throb with
a much wider speed range.

Negative feedback

Another big difference between the Vibrolux and a 5E3 Deluxe is the use of negative feedback. Fender would connect the positive from the output transformer to a point in the preamp circuit to stabilise the amplifier and increase the clean headroom.

Clean headroom is not something we generally look for in low-power tweed amps, so when I disconnected the loop for Ed, he loved what he heard. The Vibrolux had more gain, a looser feel and loads more bite. We decided both configurations had their merits, so I replaced the tone control with a 1M push/pull pot.

Verdict

Ed carried out the final assembly when his cabinet came back from the restorers. The 70s 10-inch speaker wasn’t up to much, so Ed had asked for a 12-inch speaker baffle to be fitted, and he installed a Celestion Blue. Despite attempts to tighten the valve sockets, the power and rectifier tubes still felt slightly loose, so attaching three valve retainers was deemed a wise precaution. It may seem incredible, but at the age of nearly 60 this amp has just gone back to work and it’s probably sounding as good as, if not better than, ever.

Parts List & Suppliers
• Mains plug RS Components
• Carbon composite resistors RS Components
• TAD oil capacitors Hotrox
• Mallory 150 capacitors Allparts UK
• Sprague electrolytic capacitors Watford Valves
• Valve retainers Watford Valves
• Push/pull switch Watford Valves

How does Ed rate the finished refurb?

‘Following the rebuild, the ’57 Vibrolux is now a simply staggering tone machine, excelling at all manner of tones – from whispering delicate cleans to full-on Delta blues, with gain and squish all controllable with your guitar’s volume and tone pots. Winding down a Strat’s volume control gives a touch-sensitive, delicate `Hendrix-y’ clean sound, whilst chords and notes still ring out with glassy natural compression and sustain.’

‘Wound up loud, there are hints of Bassman and Clapton’s Bluesbreaker, as harmonics and overtones swallow and envelop longer notes, blooming into a musical sustaining feedback with a PAF-equipped Les Paul. It really does make pedals redundant.’

‘With a GZ34 rectifier, the amp takes on more of a blackface character, with more solidity in the low end and less raw aggression. Yet it’s equally warm and enjoyable to play. The tremolo has a deep 3D quality that now feels an integral part of the amp’s core tone. I often leave a little hint on just to add a subtle depth and movement to chords.’

‘The total cost of the amp, including restoration, equates to around £1,100 – and that’s a bargain compared to `boutique’ replicas. ’57 Lily (named after the lady who assembled the amp all those years ago) will hopefully have another 60 years of gig tales to tell, and huge thanks must go to Huw and to Mark at AF Custom Cabinets for the superb re-covering job.’