AB763 Blackface Amp Model Differences
What's the difference between a blackface Tremolux and Vibrolux?
By Rob Robinette, last edited 3/13/2017
Have comments or corrections? Email rob at: robinette at comcast dot net
This webpage is available in PDF form: AB763 Model Differences.pdf
'AB763' was Fender's internal model designation for the 1963 blackface circuit. "Blackface" refers to the panel color and 763 means July 1963. The circuit was used in the Deluxe, Deluxe Reverb, Twin Reverb, Super Reverb, Band-Master, Showman, Pro, Vibrolux, Vibroverb and Tremolux. Note there was no AB763 Champ, Princeton or Harvard. The AB763 circuit is considered by most to be the pinnacle of Fender amplifier design. This is why so many post-AB763 amps were "blackfaced" back to AB763 specs. The Deluxe Reverb schematic and layout below have been annotated with component functions so use them for AB763 circuit study. For more info on the AB763 amps see How the AB763 Works and AB763 Modifications.
The blackface tone is known for its "mid scoop." The AB763 TMB (Treble, Mid, Bass) tone stack sets this mid frequency scoop.
AB763 Mid Scoop With Treble and Bass Set to Maximum
With the Treble and Bass tone controls set to maximum the tone curve clearly shows the mid scoop. The Mid control is set to 6.8k to simulate the standard 6.8k Middle Resistor. Graph is from the excellent and free Duncan Tone Stack Calculator. See How the TMB Tone Stack Works for more info.
The basic AB763 circuit has two channels, both with a Hi and Low input, two or three preamp gain stages, a high output long tail pair (LTP) phase inverter driving two or four power tubes in a push-pull Class AB configuration. All of the AB763 amps have tremolo and some also have spring reverb. Most of the model differentiation comes from the power supply, the number of preamp stages, FX, power tube type and number, output transformer and speaker configuration. The lowly AB763 blackface Deluxe has anemic power and output transformers, two preamp stages, two small power tubes and a single 12 inch speaker driven by 22 watts. The mighty Twin Reverb has high output power and output transformers, three preamp gain stages, a no-sag solid state rectifier, four big power tubes and dual 12 inch speakers assaulted by 85 watts. The other AB763 amps fall between these two extremes.
All the AB763's with reverb have "verb" or "reverb" in their names. The blackface amps without reverb, the Deluxe, Vibrolux, Tremolux, Pro, Concert, Showman and Band-Master, have only two preamp gain stages compared to three stages in the Deluxe Reverb, Super Reverb, Vibroverb and Twin Reverb amps. The primary purpose of the extra gain stage is to make up for signal loss from the reverb circuit but the extra stage more than makes up for the loss and gives earlier dirt and more total distortion. The amps with two preamp gain stages can jumper their channels together for a fatter tone. Amps with three gain stages cannot jumper their channels because the Normal channel only uses the first two preamp stages and the Vibrato channel uses all three so the two channels are out of phase (each gain stage flips the signal voltage). Jumpering these amps won't damage them but the tone will be thin and funky. A simple AB763 mod to amps with three preamp gain stages can send both channels through the third preamp stage to allow jumpering.
The head cab AB763's, the Tremolux, Band-Master and Showman, use 50% smaller 500pF coupling caps leading to the phase inverter compared to the other amps' 1000pF (.001uF). This was done to reduce bass frequencies into the Fender closed back extension cabs which exhibit extended bass response compared to open back combo cabs. The smaller cap also tends to tighten up the overdrive tone.
The multi-speaker amps, the Tremolux, Band-Master, Concert, Super Reverb, Dual Showman and Twin Reverb use a "heavy" negative feedback (NFB) circuit compared to the other AB763 amps' "light" circuit. The feedback circuit is made up of two NFB resistors that form a voltage divider. The "heavy" NFB circuit uses an 820 / 100 ohm divider while the others use a "light" 820 / 47 ohm circuit. The 820 / 100 "heavy" NFB circuit attenuates the output transformer secondary signal voltage by 89%. The 820 / 47 "light" NFB circuit cuts the signal by 95%.
The NFB voltage from an 8 ohm speaker output gives 41% more voltage than a 4 ohm. There's also a 41% increase when comparing a 2 and 4 ohm secondary. Fender used the "heavy" NFB circuit in the multi-speaker amps to compensate for their lower voltage delivered by their 2 and 4 ohm output transformer secondaries. The Concert and Super Reverb get 41% less negative feedback than all the other 6L6 amps due to their 2 ohm output.
Fender used the same NFB circuit in the 6V6, 2x6L6 and 4x6L6 amps even though their power amps put out different voltages. The result is the 2x6L6 amps get about 25% more NFB than the 2x6V6 amps and the 4x6L6 get around 30% more NFB than the 2x6L6 amps. Keep in mind the AB763 uses a crap ton of NFB and that's why Fender cut the NFB in half with their new 68 Custom Deluxe Reverb reissue by doubling the size of its NFB resistor to 1.5k. Personally I tend to prefer lower negative feedback with its thicker clean tone, earlier dirt and lazier transition to overdrive. Installing a switch to reduce NFB is an excellent AB763 mod.
The preamp tube plate voltage starts at a low 170v for the Deluxe and progresses up to 280v for the Vibroverb and Showman amps. Higher preamp voltage offers up more gain throughout the preamp and delivers more voltage swing to drive the power amp harder for more power tube distortion. The Deluxe and Deluxe Reverb's low preamp voltage doesn't hinder them because their small 6V6 power tubes don't need as much voltage input swing to overdrive them compared to the big 6L6.
The power tube plate voltage also varies between the AB763 amps. A higher plate voltage offers more power amp clean headroom with tighter lows and more overall volume.
The Concert, Band-Master, Showman and Twin Reverb use a solid state rectifier with less dynamic voltage sag than the tube rectified amps which leads to a tighter bottom end with more clean headroom and overall volume but playing dynamics are slightly reduced--the solid state rectifier amps feel a little less "tubey."
Table of Contents and AB763 Overview
Notes: Amps with reverb have 3 preamp gain stages in the Vibrato channel with slightly more gain and preamp distortion. *The 5AR4 rectifier is a GZ34 equivalent.
Notice how similar the Tremolux and Band-Master head amps appear. The only difference between the two are the Band-Master's solid state rectifier and higher preamp and power tube plate voltage. These differences would lead to a "stiffer," less "tubey" tone with more clean headroom for the Band-Master. It's very easy to convert an inexpensive Band-Master head amp into a Tremolux.
Small Bottle AB763s
The 6V6 powered Deluxe and Deluxe Reverb are currently very popular due to their light weight and less than ear splitting dirty volume. The same long tail pair phase inverter drives both the 6V6 and 6L6 amps but the lower power 6V6 is overdriven at a lower input voltage than the 6L6 so the Deluxe and Deluxe Reverb offer up more and earlier power tube distortion than the big bottle AB763's.
The Deluxe and Deluxe Reverb both "suffer" from undersized power and output transformers but this leads to enhanced playing dynamics with more sag and bloom. The undersized power supply causes voltage sag which occurs when bass notes and power chords demand extra current the power supply can't supply so the voltage drops. This voltage sag causes compression because loud passages are held down by loss of voltage. The Deluxe amps' voltage sag and compression lead to a very lively, dynamic, tubey tone that many players love but their undersized transformers also lead to their biggest weakness: lack of clean headroom and a sometimes farty low end. Replacing the speaker with a more efficient one and increasing the size of the first filter caps can add some clean headroom and make the amp more gigworthy.
The negative feedback circuit is identical in the 6V6 and 6L6 AB763 amps so the lower voltage supplied by the 6V6 power amp leads to about 25% less negative feedback than the 2x6L6 amps and 47% less than the 4x6L6 amps. This is one reason the Deluxe and Deluxe Reverb get dirtier at lower volume settings than the 6L6 AB763 amps.
The Deluxe and Deluxe Reverb do not have a bright switch like the other AB763 amps but they do have a bright cap on their Vibrato channel.
You can get a different tone from these amps by installing 6L6 power tubes but the power and output transformers can't supply the extra current needed to actually get much more volume at the speaker. Running 6L6 tubes will stress the transformers so I don't recommend it.
AB763 Deluxe (Non-Reverb)
2 preamp gain stages, Tremolo, 2x6V6, 1x12 Speaker
The non-reverb Deluxe is the simplest amp of the family because it has only two preamp gain stages and no reverb. It has two triode light dependant resistor (LDR) signal tremolo but no reverb, 7025 (low noise version of the 12AX7) preamp, dual identical tone stacks with treble and bass controls for each channel (no Mid control), 12AT7 long tail pair phase inverter, two 6V6GT power tubes using adjustable fixed bias putting out about 22 watts of clean power into a single 12 inch speaker rated at 8 ohms. The non-reverb Deluxe has only one 16uF filter cap compared to the Deluxe Reverb's two. This makes the Deluxe's bottom end even more flabby than the Deluxe Reverb's.
The 47 ohm Negative Feedback Tail Resistor (highlighted at center) is replaced by a 100 ohm resistor in the multi-speaker AB763 amps to compensate for their 2 & 4 ohm output transformer secondaries which put out lower voltage.
AB763 Deluxe Reverb
3 preamp gain stages, Tremolo and Reverb, 2x6V6, 1x12 Speaker
This is "THE" Deluxe Reverb. The addition of reverb and a third preamp stage (V4B) are the only significant difference from the non-reverb Deluxe above but that third preamp stage adds quite a bit of additional gain. It comes with two 6V6GT power tubes putting out about 22 clean watts into a single 12 inch speaker rated at 8 ohms. If you are a fan of the Deluxe and Deluxe Reverb amps check out The Evolution of the Fender Deluxe and Deluxe Reverb.
AB763 Deluxe Reverb Annotated Schematic
Every component's function is listed so use this schematic for AB763 circuit study. The Vibrato Bright Cap, third stage preamp, 47 ohm NFB Tail Resistor and speaker rating are highlighted in red. These components vary between the AB763 models. Click image for full size, readable schematic. PDF is here.
AB763 Deluxe Reverb Signal Path
Signal path shown in thick red line above and color below. The signal path of all AB763 amps will be very similar to the schematic above and layout diagram below. Click image for clean schematic.
Guitar Signal Path
Click image for full size layout. PDF is here.
AB763 Deluxe Reverb Annotated Layout
Use this layout for AB763 circuit study. Click image for full size layout. PDF is here.
2 x 6L6GC Power Tube AB763s
These big bottle blackface amps are more gigworthy due to their increased power output than their small bottle siblings above.
The negative feedback circuit is identical in the 2x6V6, 2x6L6 and 4x6L6 AB763 amps so the higher voltage from the 2x6L6 power amp leads to 35% more negative feedback than the 6V6 amps. The 4x6L6 power amp supplies even more NFB voltage and delivers 30% more negative feedback than the 2x6L6 amps. Extra negative feedback tends to "civilize" an amp, add clean headroom and tighten the transition from clean to distortion.
You can safely run 6V6 power tubes in these amps if you replace the GZ34 rectifier tube with a lower voltage output 5Y3 which will bring down the amp's voltage enough to safely run the smaller tubes. There will be a speaker impedance mismatch but it's small enough for safe play through the built in speakers. Of course this is not possible in the amps with solid state rectifiers like the Band-Master, Twin Reverb and Showman but you can use a variac to lower the amp's input voltage on the solid state rectifier amps. Note that you can not run EL34 tubes in AB763 amps because the power tube socket pins 1 and 8 are not tied together to support the EL34's suppressor grid. It is possible to relocate the 1.5k power tube grid stopper resistor tied to the power tube sockets' pin 1 and then jumper pins 1 & 8 together to allow EL34 use but its extra heater current demand will stress the power transformer.
2 preamp gain stages, Tremolo, 2x6L6, 1x12 Speaker
The Vibrolux is basically the non-reverb Deluxe with 6L6 power tubes. It has only two preamp stages like the non-reverb Deluxe above and shares its low voltage preamp. The low voltage preamp of the Vibrolux and Tremolux puts out lower gain and does not drive the two big 6L6GC tubes as hard as the other 6L6 AB763 amps resulting in less power tube distortion. It puts out about 35 watts of clean power into a 12 inch 8 ohm speaker which makes it loud enough to gig with but isn't nearly as heavy as the big 2x12 and 4x10 amps. The lack of reverb is the only chink in the Vibrolux's armor.
Note the Bright Cap Switches on both channels (highlighted in red on the left).
I could not find a Vibrolux layout diagram but it should be identical to the Tremolux (below in next section) except for the phase inverter coupling cap. It is .001uF (1000pF) in the Vibrolux and 500pF in the Tremolux.
If you would like an inexpensive Vibrolux it's easy to convert an inexpensive Band-Master into a Vibrolux head or combo.
2 preamp gain stages, Tremolo, 2x6L6, Head Version of Vibrolux
The Tremolux circuit is almost identical to the combo cab Vibrolux above with tremolo and two 6L6GC power tubes putting out about 35 watts of clean power. The low voltage preamp of the Vibrolux and Tremolux puts out lower gain and does not drive the two big 6L6GC tubes as hard as the other 6L6 AB763 amps resulting in less power tube distortion. The output transformer is different than the one in the Vibrolux and is rated for a 4 ohm load to drive two 1x12 eight ohm speakers wired in parallel mounted in a matching closed back cab.
Note the 100 ohm negative feedback resistor (highlighted at center). All multi-speaker AB763's use a 100 ohm resistor compared to a 47 ohm for single speaker amps. Also note the 500pF phase inverter input coupling cap which was most likely used to reduce bass frequencies into the Fender closed back extension cabs which exhibit extended bass response compared to open back combo cabs. The smaller cap would also tend to tighten up the overdrive tone. All the AB763 head cabs used this smaller cap.
The Vibrolux combo and Tremolux head are much more sought after amps these days than the Band-Master below but their differences are trivial and converting a Band-Master into a Vibrolux or Tremolux head or combo is an easy mod.
2 preamp gain stages, Tremolo, 2x6L6, 1x15 Speaker
The Pro is pretty much a no-reverb Vibroverb with their signature 15 inch speaker. Many blackface amp fans think the Pro and it's head version, the Band-Master, as the sweet spot of the AB763 amps. Great gigging amps but not crazy loud like the 4x6L6 amps. The Pro has only two preamp gain stages like the Deluxe, Vibrolux, Tremolux and Band-Master. It has two 6L6GC power tubes putting out about 40 watts into a single 15 inch 8 ohm speaker. The Pro has no Middle tone controls. It comes with a beefier power transformer with higher output voltage than the Vibrolux. The lack of reverb is this amp's only weakness.
Power tube plate voltage is higher than the Vibrolux at 440 volts and its output transformer is beefier for higher output.
2 preamp gain stages, Tremolo, 2x6L6, Head Version of Pro
The Band-Master is considered to be the head cab version of the Pro but the Band-Master has a lower voltage power transformer rated at 320-0-320V AC + solid state rectifier, whereas the above Pro's transformer is rated at 350-0-350V and uses a GZ34 tube rectifier but the resulting rectifier voltage is identical. The Band-Master's solid state rectifier offers up a little less voltage sag than the Pro's tube rectifier so its tone is a little more tight and punchy with a firmer low end than the Pro. The output transformer is rated for a 4 ohm speaker load while the Pro is rated for a single 8 ohm speaker. The matching Band-Master speaker cab had two side-by-side 1x12 eight ohm speakers wired in parallel for a 4 ohm load in a sealed back cabinet. The Band-Master puts out around 40 watts.
Note the solid state rectifier at lower right. The 440 volts on the power tube plates is the same as the Pro. Also note the 500pF phase inverter input coupling cap which was most likely used to reduce bass frequencies into the Fender closed back cabs which exhibit extended bass response compared to open back combo cabs. The smaller cap would also tend to tighten up the overdrive tone. All the AB763 head cabs used this smaller cap.
These Band-Master amps are inexpensive and easy to convert into a Vibrolux, Tremolux, Pro, Concert or no-reverb Vibroverb or Super Reverb head or combo.
2 preamp gain stages, Tremolo, 2x6L6, 4x10 Speakers
The Concert is another rare AB763 amp which is basically a Super Reverb without the reverb. Its four 10 inch speakers make it a big, heavy amp with great bottom end punch--even more than the Super Reverb. The major difference between their circuits is the Concert's solid state rectifier that reduces voltage sag and gives the Concert an even tighter bottom end but it does slightly reduce playing dynamics. Another difference is it does not have a Middle tone control like the Super Reverb. The Concert's 2 ohm output transformer supplies the negative feedback circuit with 41% less voltage so it has less negative feedback than the 4 and 8 ohm 6L6 AB763 amps (it would need an 870 / 200 ohm NFB circuit to get the same NFB voltage as the other amps). Lower negative feedback leads to a thicker clean tone, earlier dirt and a lazier transition to overdrive. The Super Reverb, Concert and 68 Custom Deluxe Reverb use .022uF mid caps where all other AB763's use .047uF.
Tone Stack Mid Caps
The Concert's reduced .022uF middle tone cap shifts the Mid tone control frequency band higher and reduces the mid scoop. Chart is from the Duncan Tone Stack Calculator.
3 preamp gain stages, Tremolo and Reverb, 2x6L6, 1x15 Speaker
This is my personal favorite blackface amp. The Vibroverb is pretty much a Super Reverb with a 15 inch speaker--the circuits' only differences are the output transformer, the negative feedback circuit and the tone stack's mid caps. The Vibroverb has three preamp gain stages compared to the Pro's two for extra gain and dirt (the Pro is the other 15 inch AB763). The Vibroverb will be forever associated with Stevie Ray Vaughan's famous tone. This is the rarest and most valuable AB763 amp due to its short production run caused by the lack of popularity of 15 inch amps in the 60's.
The Vibroverb is big and bold due to its high power tube plate voltage, 15" speaker and 40 watts of output power but its tube rectifier still causes some voltage sag for a more dynamic, tubey vibe than the Twin Reverb and Dual Showman's solid state rectifier. The Vibroverb has no Middle tone controls.
Why is the Vibroverb my favorite AB763 amp? Because it has three preamp gain stages, reverb, high preamp voltage to drive the power tubes harder, a soft tube rectifier for extra playing dynamics, a match-more-external-speakers 8 ohm output and the bold tone of a 15 inch speaker. If you are considering building a blackface amp this is the one I recommend due to it's famous tone, gigability and its rareness and desirability will provide resale value for a well built amp.
AB763 Super Reverb
3 preamp gain stages, Tremolo and Reverb, 2x6L6, 4x10 Speakers
The Super Reverb can be thought of as the Deluxe Reverb's big brother. It features four 10 inch, eight ohm speakers that make it a big, heavy amp with great bottom end punch and lots of output volume--it's almost as loud as the Twin Reverb. With a whopping 460 volts on the power tube plates the Super Reverb puts out about 40 watts into a 2 ohm speaker load. The Vibrato channel has a Middle tone control. The Super Reverb's 2 ohm output transformer supplies the negative feedback circuit with 41% less voltage so it has less negative feedback than the 4 and 8 ohm 6L6 AB763 amps (it would need an 870 / 200 ohm NFB circuit to get the same NFB voltage as the other amps). Lower negative feedback leads to a thicker clean tone, earlier dirt and a lazier transition to overdrive. The Super Reverb, Concert and 68 Custom Deluxe Reverb use .022uF mid caps where all other AB763's use .047uF.
Tone Stack Mid Caps
The Super Reverb's reduced .022uF middle tone cap shifts the Mid tone control frequency band higher and reduces the mid scoop. Chart is from the Duncan Tone Stack Calculator.
The Super Reverb is the only AB763 with .022uF middle tone caps and a 2 ohm output transformer. Note the Middle tone control on the Vibrato Channel (highlighted center-left).
Note the Middle tone control on the Vibrato Channel.
4 x 6L6GC Power Tube AB763s
These are the big guns of the AB763 lineup putting out about 85 watts to offer up large venue volume.
The negative feedback circuit is identical in the 6V6, 2x6L6 and 4x6L6 AB763 amps so these 4x6L6 power amps supply even more NFB voltage which delivers 30% more negative feedback than the 2x6L6 AB763s. Extra negative feedback tends to "civilize" an amp, add clean headroom and tighten the transition from clean to distorted. This is one reason these high power amplifiers offer up so much clean headroom.
2 preamp gain stages, Tremolo, 4x6L6, Head Cab Version of Twin Reverb
The Single and Dual Showman are considered head cab versions of the Twin Reverb but they do not have reverb or a third preamp gain stage like the Twin. Both the Dual and Single Showman amps have a "Showman" label on the amp face. The only differences between the Dual and Single Showman are the output transformer (4 or 8 ohm) and NFB circuit. The Single Showman is the only AB763 head amp designed to drive a single speaker.
The Showman amps use a solid state rectifier for a firmer low end and more clean headroom. They put out about 85 watts of power like the Twin Reverb. The Single Showman is designed for a single 8 ohm speaker load and was typically mated to a single 15 inch speaker in a matching closed back extension cab. The Dual Showman uses a 4 ohm speaker load and was usually paired with a matching 2x15 extension cab with dual 8 ohm speakers wired in parallel. The extra 15 inch speaker made the Dual Showman a much louder amp.
Note this schematic is for both the Single and Dual Showman--see boxes at right. The Single Showman uses a 47 ohm NFB Tail Resistor while the Dual uses a 100 ohm to compensate for it's lower voltage output. Also note the 500pF phase inverter input coupling cap which was most likely used to reduce bass frequencies into the Fender closed back extension cabs which exhibit extended bass response compared to open back combo cabs. The smaller cap would also tend to tighten up the overdrive tone. All the AB763 head cabs used this smaller cap.
AB763 Twin Reverb
3 preamp gain stages, Tremolo and Reverb, 4x6L6, 2x12 Speakers, The Master & Commander of AB763 Amps
The Twin Reverb sports four 6L6GC power tubes and 85 watts of power into a 4 ohm speaker load. It also has additional tone control pots, the "Middle" control for both channels, where most of the other AB763 amps have a fixed Middle tone resistor.
Twin Reverb = Standard AB763 circuit + Middle tone pot + four 6L6GC power tubes + solid state rectifier + 2x12 Speakers = tons of clean headroom and stadium filling volume. The big downside of this amp is the extreme weight. If you must put casters on this amp make sure the wheels are soft rubber to minimize vibration and moving damage.
The solid state rectifier offers very little dynamic voltage sag so the Twin and Showman have tons of headroom and stay clean to crazy high volume levels. Because of this they make excellent pedal platforms--they stay clean so the pedals can do their thing.
Some of the major differences between the Twin Reverb and other AB763 amps are highlighted.
AB763 based amps are still strong sellers for Fender over 40 years after their release. Fender sells reissues of the AB763 Deluxe Reverb, Vibrolux, Twin Reverb and Super Reverb.
AB763 Based 65 Deluxe Reverb Reissue
3 preamp gain stages, Tremolo and Reverb, 2x6V6, 1x12 Speaker
The AB763 circuit on a PCB instead of an eyelet based circuit board. Amp repairs and modifications with a PCB can be more difficult. The 5AR4 rectifier is a GZ34 equivalent. Download the Reissue '65 Deluxe Reverb Owners Manual. See the 65 DRRI modifications at the AB763 Modifications page. It's pretty easy to convert a 65 DRRI into a 68 CDR.
'65 Deluxe Reverb Reissue Signal Path
Click image for full size clean schematic.
65 Deluxe Reverb Reissue PCB Layout
Click the image for the full size layout.
AB763 Based 68 Custom Deluxe Reverb (Reissue)
3 preamp gain stages, Tremolo and Reverb, 2x6V6, 1x12 Speaker
The 68 CDR is not a silverface amp, it is an almost exact copy of the 65 Deluxe Reverb Reissue with just a few valuable tweaks. I like to think of it as a silverface amp that's been blackfaced and then modified. Differences from 65 Deluxe Reverb Reissue include: Reverb and tremolo are available on both Vintage & Custom channels and both channels go through the third preamp gain stage V4B so both channels are now in phase so you can jumper channels for a thicker tone. Negative feedback is cut in half compared to the Deluxe Reverb which brings on earlier dirt and a thicker clean tone. Vibrato channel bright cap has been removed. Fender describes the Custom channel as having a "modified" Bassman tone stack. 10k grid stopper resistors are added to the 2nd and 3rd preamp stages to help control overdrive behavior and sweeten the overdrive tone.
The 5AR4 rectifier is a GZ34 equivalent. The 68 CDR's PCB circuit board is very similar to the 65 DRRI's layout above in the previous section. 68 CDR mods are here.
Click the image for the full size schematic.
68 Custom Deluxe Reverb PCB Layout
Click the image for the full size layout.
Tone Stack Comparison
68 CDR Vintage channel on left, Custom channel in center, 5F6-A Bassman on right. The center Custom channel tone stack is supposed to be a modified version of a Bassman tone stack but I don't see much similarity with THE Bassman stack. The Bass cap is five times larger, the slope resistor is 80% larger, the Bass pot is 75% smaller, two Mid resistors replace the 25k Mid pot to simulate a 25k pot set to 6.8k. The added 18k resistor also acts as a "bass floor" to limit how low you can set the bass. The treble control is a high-pass filter and the bass and mid controls are band-pass filters with the 100k slope resistor setting the upper frequency limit and the bass pot and 6.8k mid resistor setting the lower. See How the TMB Tone Stack Works for more info.
Tone Stack Mid Cap Change
The only difference between the two 68 CDR tone stacks is the 18k resistor and the middle cap. The Custom channel's reduced .02uF middle cap shifts the "mid" frequency band higher and reduces the mid scoop. Chart is from the Duncan Tone Stack Calculator. Download the 68 CDR Vintage channel, 68CDR Custom channel and 5F6-A Bassman Duncan Tone Stack Calculator files.
Customizing and adding the effects to the old "Normal" channel makes the 68 CDR more versatile because many Deluxe Reverb players never used the Normal channel. The 68 CDR's Vintage channel is the old "Vibrato" channel. The bright cap on the Vibrato volume pot has been removed to reduce ice pick highs. The added tone stack 18k resistor acts as a "bass floor" and sets the minimum bass. Some 68 CDR owners have jumpered around this resistor to allow them to set a lower bass level. British voiced 12” Celestion G12V-70 speaker is used. Blue jewel light (sweet!). Reviews of this amp are very favorable.
It's pretty easy to convert a 65 DRRI into a 68 CDR.
This webpage is available in PDF form: AB763 Model Differences.pdf
By Rob Robinette