One frequently asked question I receive is "What is the difference between the AA763 and AB763 circuit for a Super Reverb (or Twin Reverb or Deluxe Reverb)?" For those without access to schematics, here are the differences:
 
 
AA763
AB763
Tone Cap
.033 uf cap
.047 uf cap
Reverb
4.7M resistor
3.3M resistor
Tremolo
56K resistor
100K resistor
Phase Inverter
27K/100K resistors
22K/82K resistors
Grid stoppers
None
1.5K resistor

The most important difference is the addition of the grid stoppers (safety first!). The only major difference in tone between the two circuits would be attributable to the different tone caps (.033 vs. .047). The .033 cap would yield slightly more midrange, but I donít think it would be very noticeable.

For some models like the blackface Concert, there was no change in phase-inverter resistor values, and no changes in tone stack cap values (fascinating!). And for other models, the tone stack did not change from .033 to .047 uf, but rather from .033 to .022 uf. The Bandmasters also saw no change in tone stack cap values between the AA763 and AB763 circuits. The "universal" changes do seem to be the oscillator circuit, cathode resistor change from 56K to 100K, and the addition of the 1500-ohm grid stopper resistors.

Did you know that early blackface amps (1963) do not have white silk screening around the bright switches? Itís not certain when Fender added the white rectangle around the bright switches, but they are there by early 1964. Whether this was added before the end of 1963 is not known (yet). This feature would have been phased-in at slightly different times for the different amps as faceplates were ordered and used in somewhat different amounts and at somewhat different rates for each model.

Some general information about cabinet construction: solid pine, finger-joined cabinet construction from 1946 to circa 1972. An interesting feature on the tweed-style cabinets is the use of a dowel reinforcement in the top on either side of chassis. You canít see the dowels unless the tweed covering is removed, but this reinforcement prevented the wood from splitting due to the weight of the chassis.

Baffle boards were made of plywood from 1946 to 1962. Particle board (some call it MDF Ė medium density fiberboard) baffles debuted in 1963 and were used through the early 1980s. The baffle board was removable on amps made from 1948 to about 1972 and glued-in thereafter.

From circa 1972 to the early 1980s, the cabinets were no longer made from solid pine boards, but cheaper laminated, multi-piece pine boards. Each side of the cabinet was made from several pieces about three or four inches wide, glued side-to-side, to make up a plank the depth of the cabinet. These laminated cabs were not finger joined, but rabbet joined. The baffle board on these rabbet joined cabinets was mortised into the sides and bottom (i.e. Ė not removable) to hold the whole thing together. Thatís why these post-1972 cabinets have the grill cloth stretched across a frame that is attached by a velcro-like system to the baffle. CBS almost certainly went to this construction method to save money, though at the expense of overall quality.

And with Fender, there are always exceptions to the rule. I have received reports of some pre-CBS blackface amps with one or more sides made from a multi-piece board. As well, I have received reports of some particle board bottoms used in master volume-era silverface amps.

Iím often asked if the marking on the inside of the cabinets are date codes. Sometimes date codes are ink stamped on the inside of the cabinet (mainly blackface and silverface amps including the piggyback speaker cabs), but those handwritten numbers you see in wax pencil or lumber crayon are actually matching marks. As a worker would through a run of cabinets and fit baffles to each one, he would mark the cab and baffle so they could be "married up" again after the baffle was grilled. The cabs were probably numbered sequentially within the production run. The number did not have any relationship to a particular employee though Sam Hutton is known to have marked the cabinets he assembled (usually in yellow lumber crayon) with an "S" superimposed over an "H" which looks like a $ with two strikes instead of one.

Weíve received some interesting reports about some oddball amps. The first was a 1960 brown Super Amp. The latest date code on it indicated 30th week of 1960 and the circuit and layout were neither 6G4 nor 6G4-A. This must have been one of those "Leo messed with it" amps that Forrest White speaks of in his book. This circuit is unique and transitional - part 6G4 in places, part 6G4-A in places, part "unique experimentation" in places.

An October 1963 Deluxe Reverb was reported with transformers (all Schumacher) all dated to mid-1963, except the reverb drive transformer which dated to December 62! The tube chart indicated the AA763 circuit, but there were some very strange original resistor values inside. Specifically, the reverb drive tube's cathode bias resistor was a 1K, 1-watt, instead of the normal 2.2K, ½-watt. The tail resistor in the phase inverter was 6.8K, the plate load resistors in the phase inverter were 47K and 56K instead of the normal (for AA763) 100K. The bias feed resistors were 68K instead of the normal 220K, and there was a disc ceramic cap on the board connected between the phase inverter plates. The ceramic cap is more commonly found on brown and blonde amps to prevent parasitic oscillation.

Some amp techs have observed examples of blonde and blackface amps with power transformers without center-tapped filament windings. These amps are usually the ones that have "hum" problems if they donít have 100-filament resistors added. Somewhere along the line Fender went to a center-tapped filament winding and no 100-ohm filament resistors. These amps could be modified simply by lifting the center tap, and installing the 100-ohm resistors in the usual place on the power lamp socket. If a filament in a tube shorts (happens most often in power tubes) it is a lot cheaper to replace a 10-cent resistor or two, than an $100 power transformer.

Fenderís sudden transition from cloth wire to thick PVC wire (in pastel greens, white & yellows) is well documented by anyone who has ever pulled a chassis. Sometime in late 1968, the cloth covered wire went away. However, several amps from the late Ď60s (non-reverb Princeton, Vibrolux Reverb, Bandmaster Reverb, and possibly a Deluxe Reverb) with oddball wire have been reported. The Princeton Amp was an early-mid 1969 model entirely wired (factory original & stock) with thinwall, 22awg irradiated PVC (IPVC) wiring. IPVC wiring is usually found in electronics like computers, not lo-fi amps. Keep your eyes peeled for wire with very thin, cream to yellow insulation. Itís likely IPVC.