Fender AB763 Restoration

Introduction

Long before I began experimenting with radio electronics, I dabbled in amateur construction, modification and repair of tube guitar amplifiers. This was in the days before the Internet when high voltage parts and amp schematics were difficult to find and parts were often very costly. Today, tube guitar amplifiers continue to be popular and there are countless manufacturers and more importantly, parts suppliers who market their products on the Internet.

My project was to restore a 1964 Fender Deluxe Reverb (Model AB763) to its former glory and convert it from a combo amp (amp plus speaker) into a head (amplifier only). While collectors of vintage Fender Black Face amps may whince when they read about this project, I am not into collecting and just wanted to make a good sounding, revitalized, classic Fender amp for real-world use.

David Allen, from Allen Amps offers special "never-before" cabinets for vintage Fender amplifiers and I ordered one. The workmanship of this cabinet was impressive. The Fender Deluxe Reverb was only offered as a combo amplifier and so it was very interesting to see the outcome when I mounted the chassis into the newly made cabinet.



The original Fender combo amp cabinet. This thing smelled like a combination of mold, cigarette smoke and a dead goat. The wood at the bottom of the cabinet was rotten. The speaker also had dry rot and mold all over it. Cleaning only reduced the offensive odor it emitted by a few dB! This cabinet actually caused people with allergies to cough and sneeze when they spent any time near it.



Above photo. The original AB763 chassis with the B+ filter capacitor metal cover removed. When, I operated this amp for the first time I was very disappointed. It was very noisy and sounded terrible. I put in some new tubes and it sounded reasonably nice using a separate extension speaker. The original speaker cone was too "infected with mold" to work properly. A lot of white noise, hum and crackle sounds where heard during my initial testing.



My 2 favorite tube guitar amp "do-it-yourself" web sites are:

Allen Amps (David Allen also offers completed guitar amplifiers).  http://www.allenamps.com/

Hoffman amps. http://www.hoffmanamps.com/

These guys are very knowledgeable, helpful and have a great selection of parts for the home builder/repairer.



Warning: Tube amplifiers operate at high DC voltages. Repairing, modifying or building tube amps can be dangerous or in some cases fatal.



Restoration

I called David Allen (Allen Amps) and asked his opinion on how I might restore the tone of my vintage Fender amp. Here are some of his suggestions:

  1. Re-tube the amplifier.
  2. Replace all of the electrolytic capacitors.
  3. Replace all of the plate resistors with new metal-film resistors.
  4. Replace the 470 ohm screen resistors on both of the 6V6 finals.
  5. Tighten all connections which serve as AC grounds.
  6. Correctly set the DC bias on the finals.
  7. Re-tension all tube sockets.
  8. Convert to a grounded AC plug.
  9. Clean all pots, jacks and switches.
  10. Inspect for cracked solder joints and repair as needed.
  11. Consider replacing tone control caps and some of the coupling capacitors if DC leakage was found.

I ordered some parts from David and then slowly began the work. My first 2 tasks were to change to a grounded AC plug and solder in 5 new filter caps for the high voltage B+. The new capacitors made considerable improvement to the noise levels and tone of the amplifier.

Using a lug, the grounded wire of the AC plug was AC grounded to one of the power transformer bolts. Converting to a 3 prong plug reduced noise and also removed a ground loop noted when using an AC powered effects devices or a speaker emulator. With the stock 2 prong plug, whenever my right hand did not touch the guitar strings, noise and hum were noted. Converting to a 3 prong plug seriously attenuated this noise. On the back of the chassis is a switch which shunts either the hot or neutral side of the AC to ground via a 0.047 uF capacitor. After converting to a grounded AC power cord, I just removed this capacitor and left the switch intact.


Mouse over the images on the left to view a full size version.



New B+ filter capacitors are soldered into place. The original caps were leaky which contributed to noise and hum.

This is the stock Fender chassis after the grounded power cord conversion. Apart from dust, spider web material and insect exoskeletons, there were no obvious abnormalities.

Shown is an original 470 ohm screen resistor on one of the two 6V6 final amplifier tubes. This shot was taken before converting the AC power cord to a grounded type.

The two new 3 watt, 470 ohm screen resistors may be clearly seen in this photo.

The final amplifier stage negative bias capacitor was replaced with a 100 uF/100 volt electrolytic. The positive side is connected to chassis ground which is correct but at first glance seems a bit odd.

You can see 2 new Mallory 0.1 uF coupling caps and above them 2 new 100K metal film screen resistors. I cut the resistor on the right a little too short (it was the first part that I replaced), however, I was able to get a good connection to the turret. My solder joints are covered with flux and do not look shiny. I scraped them off after and they appeared shiny and adequate. The original Fender turrets have a lot of solder in/on them. My work looked progressively better as I re-gained experience with soldering inside turrets and learned how to size the lead length for each of the components. The parts described above were connected to the phase inverter.

The chassis (minus tubes) on a piece of foam. The replacement parts are laying in front of the chassis. As appropriate, each cathode pair of each triode received a separate 22 uF electrolytic capacitor. On the original circuit, 3 of the triodes had it's cathode resistors sharing a common 22 uF capacitor to ground.

Shown is the chassis and reverb tank mounted inside the new wooden cabinet. The Tolex work was fabulous. The reverb tank mounting bolts were already fitted in the wooden cabinet. The rubber shock mounts and nuts were ordered along with the cabinet from David Allen. I will nicely secure the reverb input/output wires once all of my experiments have been completed. The back plate is removed in this photo.

Shown are 2 close up photos of the stock, Hammond, Canadian-made reverb tank. The reverb patch wires are new 3 foot offerings sold especially for amplifier heads from David Allen. I wrote the tube types on the wood so I can easily replace them in the future without having to look at the schematic.


Update

I ordered  a few more capacitors - this time via Doug Hoffman from Hoffman amps.

I pulled the chassis out of the wood once again.

Shown is an assortment of coupling capacitors I soldered into the chassis. Contrary to what I have read about them rarely failing, 3 of the original coupling caps were leaking DC. In addition at volume pot settings greater than 6, this amp had a "farty" tone. Furthermore, the highs were a little brittle for my ears. Testing with humbucker and single coil equipped guitars was performed. Sprague Orange Drop 715 series (polypropylene), silver mica and Mallory polyester caps were soldered in and out and the results were tested by ear and "hand tuned". Component values remain stock; coupling caps were replaced with either polyester, silver mica or polypropylene types to improve the tone as posssible.



I eventually replaced all but 2 of the original ceramic coupling caps in this amplifier (The amp sounded better with these 2 stock ceramic caps in place). Perhaps, I was overzealous with the replacing the coupling caps, however, I have no regrets. This amp now sounds superb! On the normal channel, I used 2 Orange Drop caps and a silver mica for the tone stack. This gave the channel a slightly more pronounced high midrange response than the vibrato channel.  I tested this amp with Jensen, Celestion and Eminence speakers. They all sounded great, but my preference was a borrowed Raezers Edge Stealth 12 cabinet for the enhanced headroom and awesome bottom end response.


Conclusion

This was a fun, non-radio project. Replacing the dried up, old capacitors was certainly worthwhile, however, I want to say that in no way was my experimentation with different types of capacitors quantitative. There is no doubt that certain types of capacitors can effect the tone, but the effects are often subjective and more passionate than scientific. This is not a hi-fi amp and one can get easily carried away in believing the "cult of tone" musings from guitarists and amp builders about amplifier components. I know the Fender purists will resent this project and I respect their view point.

After following the advice of David Allen and then afterward, tuning the stages by ear, I am very impressed with this amplifier. It is very quiet and the tone is fantastic. It is easy to understand why this little 22 watt amp has such a huge following. The AB763 schematic and it's variants; plus or minus a middle tone control, different output transformers and output tubes comprise some of the best guitar amps of all time.

Awesome AB763-related link from Wayne Reno


Sound Bytes

There have been several requests for sound bytes of this amp All are low volume recordings and I need to develop a more effective digital recording strategy.

Sound Byte 1

Here is my first try. This is a Warmoth Strat copy into the Fender DR with no effects. This is a humerous, absolutely squeaky clean middle/neck tone with no signal clipping, although nice tube compression emerges. The hum is a ground loop on an AC-powered recorder (big mistake); not from the amplifier. This amp sounds dead quiet, beautifully Fender-esque and numerous players have borrowed it for studio work, and/or offered me cash for it.

 
Play Sound Byte 1

Sound Byte 2

Some more amp samples this time using an thin-body arch top with a single coil pickup. I'm pretty rusty, but nice tone arises.

 
Play Sound Byte 2

Sound Byte 3
Some Texas blues rhythm guitar on mostly the bridge pick up of the Warmoth Strat. This was recorded with a small mono, recorder laying on the floor. Play Sound Byte 3

Sound Byte 4
More jangly, clean Strat-esque tones with the reverb control on 3. Play Sound Byte 4

Sound Byte 5

This is a recording of a single-ended, 8 watt "British Rock" amp I designed and built which used a single 12AX7 into a EL34 power tube many years ago. The sound byte was recorded with the amp volume control on 10 while driving a Marshall 4X12 cabinet. The microphone was a cheap electret condenser draped over the cabinet front. This mic emphasized the highs and the actual amp tone had much more lows and mids than is evident in this recording. No equalization was added. The guitar was a G and L Legacy with single coil pickups. While I prefer the 6V6 or 6L6 power tube, the guy who bought this amp loved it.
Play Sound Byte 5


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