AB763 Blackface Amp Modifications
Includes modifications for the 65 Deluxe Reverb Reissue and 68 Custom Deluxe Reverb
By Rob Robinette, edited 11/29/2016
Have comments or corrections? Email rob at: robinette at comcast dot net
This webpage is available in PDF form: AB763 Modifications.pdf
'AB763' was Fender's internal model designation for the 1963 blackface circuit. The "763" in the model name comes from the circuit change date of 7-1963. It was used in the Deluxe Reverb, Twin Reverb, Super Reverb, Concert, Bandmaster, Showman, Pro, Vibrolux, Vibroverb, Tremolux and no-reverb Deluxe. For information about the AB763 model differences see this. 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. I usually prefer switchable mods that keep the amp's original tone in place. The often unused Normal channel is also a great place to mod away with to make that channel interesting and usable.
Note the Preamp Plate Voltage difference between the Deluxe Reverb and the higher powered amps.
Table of Contents
Normal Channel Reverb Mod Send both channels through the reverb and tremolo effects.
"Fritz" Mod Send both channels through the reverb and tremolo effects.
Lead Channel Mod Do something constructive with your unused Normal channel.
Plate Load Resistor Mod Add preamp gain with a simple resistor swap
Boost Preamp Voltage Add gain to the low voltage Deluxe, Deluxe Reverb, Vibrolux and Tremolux amps.
3-Way Negative Feedback Switch Mod Make your amp more versatile.
Master Volume Mod Add a simple Trainwreck Type-3 PPIMV master volume.
Master Volume + Vox Tone Cut I love this thing.
Raw Switch Mod Eliminate the tone stack and boost volume.
Tone Stack Mods Why have two identical tone stacks? Mod one of them.
Firm Up the Deluxe and Deluxe Reverb's Loose Low End Make it sound more like a Twin Reverb.
Slow the Tremolo Slow is good
Reduce Reverb Noise & Oscillation Simple mod to clean up the reverb.
Tube Tweaks It's surprising how much you can do with simple tube swaps.
Click the image for the full size schematic. Every component function is listed.
This simple mod routes the AB763's Normal Channel through the reverb and tremolo effects and through the V4B third preamp gain stage. It does not affect the Vibrato Channel's tone at all. This mod also puts the two channels in phase so you can jumper the channels together for a slightly thicker tone. This mod is used by Fender in the reissue 68 Custom Deluxe Reverb. This mod has the Normal Channel use the Vibrato Channel's .022uF coupling cap and bypasses the Normal Channel's .047uF cap. Because of this the Normal channel will lose a little low end and share the same voice (except for the Vibrato Channel's bright cap around the volume pot). This is an excellent mod and I highly recommend it.
To send the Normal Channel signal through the reverb and tremolo you simply connect the V1B and V2B plates at the bottom (input, nearest tubes) of the two coupling caps. You must also clip or remove the V1B .047uF coupling cap on the right. I recommend just clipping a leg of the cap right at the eyelet so you can easily re-solder it if you ever want to reverse the mod. With this mod I would also recommend jumpering around the now unneeded Vibrato channel 220K channel mixing resistor (at far left) for a slightly boosted, slightly brightened tone. You can use a temporary alligator clip lead to try the mixing resistor bypass. Click the image to see the full size version. Original photo by John Chabalko.
To do this mod to the 65 Deluxe Reverb Reissue you would connect the input end (end nearest tubes) of capacitor C5 to the input end of capacitor C12. Clip or remove capacitor C5. Optionally jumper around resistor R35 at far right (now unneeded Vibrato Channel Mixing resistor).
The "Fritz Mod"
The Fritz Mod is very similar to the above "Normal Channel Reverb Mod," except that it keeps the Normal Channel coupling capacitor in the circuit so the Normal channel retains its slightly deeper voice. If you plan to change the voice of either of the channels the Fritz Mod is the way to add reverb because each channel retains its own coupling cap. This mod also puts the two channels in phase so you can jumper the channels for a fatter tone.
The channels are joined after the coupling caps.
The channel jumper is connected to the output (upper) end of the coupling caps. You must clip or remove the left (Normal Channel) 220k channel mixing resistor at far left. I also recommend an optional jumper around the now unneeded Vibrato Channel 220k channel mixing resistor at far left for a slightly boosted, slightly brightened tone. Click the image to see the full size version. Original photo by John Chabalko.
To do the Fritz mod to the 65 Deluxe Reverb Reissue you would connect the output end of capacitor C5 to the output end of capacitor C12. Clip or remove resistor R12 at far right. Optionally jumper around resistor R35 (Vibrato Channel Mixing resistor next to R12). The circuit board has these parts labeled.
Many AB763 players never use the Normal Channel since it has lower gain and no effects. If you don't use it you should consider voicing it as a "lead" channel by changing out only four components. Three of these component changes are from the 1987 Marshal "Plexi" lead channel preamp. They filter out excess bass frequencies that tend to boom or get muddy when severely overdriven. You will also gain some clean headroom and maximum volume because low frequencies use up a lot of the amps power so removing very low frequencies allows more amplification of the remaining audio frequencies.
With these four changes you can push the Lead Channel very hard with gain and boost pedals and get a nice, tight, modern overdrive tone. Reverb and delay effects and hot humbucker pickups will also sound better through this "lead" channel because the reduction in low frequencies will keep the amp from being overwhelmed. This mod will also make the channel more pedal friendly in general. I really love the Lead Channel Mod.
When evaluating this mod be sure and try a boost pedal to get the gain and distortion up--this is when this mod really shines.
This mod will not affect the Vibrato Channel. Pairing this mod with the "Fritz Mod" above will send the Lead Channel through the reverb and tremolo effects and through the third preamp gain stage. You'll be able to run more reverb with the Lead Channel because the very low freqs aren't there to freak out the reverb circuit and springs.
Here's an excellent demo showing the difference between the 5E3 unmodified Bright channel and the "Lead Channel:" kdj 5E3 Lead Channel YouTube Demo The difference between the Vibrato and "Lead Channel" won't be quite so stark in the AB763.
The 2.7k cathode resistor is used in many high gain preamps and will bias the preamp cool and make creamy asymmetric distortion more likely. The smaller bypass cap will boost more mids and highs. The .0047uF coupling cap is standard in many modern high gain amps and will trim unneeded low frequencies to tighten up the overdrive tone. This smaller cap will sweeten the overdrive tone by reducing bias drift recovery time of an overdriven second preamp stage. The V1A 220k 1 watt plate load resistor will add gain to the channel's first preamp gain stage and make overdrive more likely in the amp's following gain stages.
For the mod you increase the V1A cathode resistor from 1.5k to 2.7k (1/2 watt), reduce its bypass cap to .68uF (micro Farad) 25v, increase the plate load resistor from 100k to 220k 1 watt and reduce the big V1B .047uF coupling cap to .0047uF 450V (or higher voltage and yes, that is with two leading zeros).
For the 65 Deluxe Reverb Reissue & 68 Custom Deluxe Reverb the V1A cathode resistor is R5. The V1A cathode bypass capacitor is C1. The V1A plate load resistor is R4. The V1B coupling capacitor is C5. On the 68 CDR cap C5 is deleted so don't worry about it. The circuit board has these parts labeled.
If you would like more gain in the preamp you can swap out one or more of the standard 100k plate load resistors with 220k resistors. A larger plate load resistor will give more signal swing and boost gain. I recommend going with 1 or 2 watt resistors to reduce resistor noise (hiss).
I like to do this to just one channel so consider changing the Normal Channel's V1A and/or V1B plate load resistor. I prefer modifying the Normal Channel first gain stage (V1A) plate load resistor with a 220k 1 watt first. I use this mod in the Lead Channel Mod above.
For the 65 Deluxe Reverb Reissue & 68 Custom Deluxe Reverb you would change out V1A Load resistor R4, V1B R11, V2A R16, V2B R29. The circuit board has these parts labeled.
The Deluxe, Deluxe Reverb, Vibrolux and Tremolux amps run with low voltage preamps compared to the other AB763 amps. The schematic for the Deluxe shows only 170 idle volts on the preamp tube plates while the Twin Reverb shows 270v. You can add whole-amp gain and distortion by changing the two power (voltage dropping) resistors to the Twin Reverb values which will bump up the phase inverter and preamp plate voltage. This mod will give you earlier and more preamp and power amp distortion (which might actually be too much for the 6V6 power tubes in the Deluxe and Deluxe Reverb).
The high preamp voltage AB763 amps like the Twin Reverb use a 1k 1 watt "C" power node power resistor and 4.7k 1 watt "D" power resistor.
C and D Power Resistors in the Twin Reverb
In the Twin Reverb the C Power Resistor is a 1k 1 watt resistor. The D resistor is a 4.7k 1 watt.
I recommend you start by modifying just the C power node power resistor first and sample the modified tone. Reducing the value of the C power resistor will raise the voltage on all the tube plates except the power tubes. Replace the C power resistor with a 1k 3 watt. I recommend a 3 watt rating for these resistors for longevity's sake. The C and D power resistors are located in the "doghouse" which is located outside the chassis next to the output transformer.
You don't have to use a 1k resistor here, you can chose any resistor value between the original value and 1k to tweak the tone change to your liking.
If you like the change and want more then replace the D power resistor with a 4.7k 3 watt which will boost only the preamp voltage (and reverb recovery in the reverb amps). Again, you don't have to use a 4.7k resistor here, you can chose any resistor value between the original value and 4.7k.
Deluxe Reverb Power Resistors
The C and D power resistors are located at upper left in the filter cap "doghouse."
Vibrolux & Tremolux Power Resistors
The C and D power resistors are located at upper left in the filter cap "doghouse."
65 DRRI & 68 CDR Power Resistors
The C and D power (voltage dropping) resistors are located at upper-center left in the filter cap "doghouse."
3-Way Negative Feedback Switch
The AB763 uses a crap ton of negative feedback which is the main reason the amp stays so clean until very high volume. It uses an 820 ohm negative feedback (NFB) resistor to control the level of feedback. The reissue 68 Custom Deluxe Reverb cuts NFB in half by using a double size 1.5k NFB resistor.
I like to use a 3-way ON/OFF/ON SPDT mini switch to give you: Normal/None/Light negative feedback (AB763 820 ohm/5E3 Deluxe disconnected/68 CDR 1.5k). The center disconnected setting gives you a 5E3 Deluxe kind of vibe with early dirt and a lazy transition from clean to distortion. Feel free to tweak the resistor values, you may like even less NFB than the 68 CDR's 1.5k resistor so try a 2.2k resistor instead.
The NFB switch fundamentally changes the clean and overdrive tone of the amp. Playability and touch sensitivity are also changed in all three positions. This is a great modification and will really make the amp more versatile.
I suggest installing the SPDT ON/OFF/ON mini switch next to the tremolo intensity pot. Switch Down = Normal AB763 NFB, Center = No NFB, Up = 68 CDR NFB.
You must remove (or just clip a leg of) the original 820 ohm negative feedback resistor then connect the 3-way switch to the top and bottom resistor eyelets/turrets.
For the 65 Deluxe Reverb Reissue & 68 Custom Deluxe Reverb the negative feedback resistor is R64. It is 820 ohms for the '65 and 1.5k for the '68. The circuit board has this part labeled.
There are many master volume options but the Trainwreck Type-3 Post Phase Inverter Master Volume (PPIMV) is very easy to install and it works as well as any other master volume I've tried in any of my amps. This master volume controls the signal level feeding the power tubes so you can use it for lower volume distortion and to control the balance between preamp and power tube distortion. It works by mixing the two phase inverter output streams together and they cancel each other out. Less resistance = more signal mix and less output volume.
For the Type-3 master volume you simply add a 1 mega ohm audio (log) pot and two wires. It's easy to temporarily alligator clip the pot into the circuit to give it a try. When the Master Volume pot is set to max the master volume circuit virtually disappears and will not color the amp's tone.
You can place your master volume pot anywhere but you may have to use shielded wire to prevent noise or oscillation. I use RG-174 when I need shielded coax in an amp. Only ground one end of any coax cable in an amp, preferably the signal input end, to keep from forming a ground loop. If you keep the wire runs relatively short and away from the power transformer end of the amp you should be OK without using shielded coax.
Simple But Effective Trainwreck Type 3 Post Phase Inverter Master Volume (PPIMV)
Add a 1 megaohm audio (log) pot and two wires and you've got an effective post phase inverter Master Volume. As you turn the Master Volume pot down (counterclockwise, pot is shown shaft down) more of the opposite phase signals from the phase inverter are mixed together which cancels the signal out. It's a good idea to twist the two wires to the pot to minimize noise (not shown in diagram). I did this mod to my 5F6A Bassman and it works great.
5F6A Trainwreck Type 3 Post Phase Inverter Master Volume (PPIMV)
Add a 1 megaohm audio (log) pot and two wires and you've got an effective post phase inverter Master Volume. As you turn the Master Volume pot down (counterclockwise) more of the opposite phase signals from the phase inverter are mixed together which cancels the signal out. It's a good idea to twist the two wires together to the pot to minimize noise (twist not shown in diagram above). The pot is shown shaft down in the layout. The layout shows a 5F6A Bassman but the connection is the same at the 220k power tube grid leak resistors.
If you are anal you can completely eliminate the Type-3 from the amp circuit by adding a switch to disconnect the circuit. Use a 1MA pot with a push-pull DPDT switch and wire one leg of the master volume through the switch so when the master volume knob is down the circuit is completely disconnected. Pull the knob up to activate the master volume.
To do this you would run the wire from the #2 (center) pot terminal to the upper left DPDT switch terminal, then run a wire from the middle left DPDT switch terminal to the circuit board's right 220k resistor. The wire from the #1 (left) pot terminal would be wired as normal to the left 220k resistor (see layout below).
Type-3 With MV ON/OFF Push-Pull Pot
Master Volume knob Down = no master volume at all, Up = master volume on. This diagram shows a 5E3 Deluxe but the push-pull switch wiring is the same.
Another option is to use a switch and resistor for your master volume instead of a pot. You simply replace the master volume pot with a resistor on a switch. One guy used a 5k 1/2 watt resistor which gave him his preferred "bedroom practice" output level. Switch OFF = no master volume at all, ON = bedroom volume.
Master Volume + Vox Cut Control
The Vox Cut Control connects the two power tube grids with a 220k audio pot and 4.7nF (.0047uF) capacitor to allow variable high end cut. In a push-pull amp the guitar audio signals on the two power tube grids are 180 degrees out of phase with one another so mixing them together nullifies the signal, kind of like mixing matter and antimatter. The capacitor limits the effect to high frequencies but if you jumper around the cap the pot becomes a Trainwreck Type-3 Master Volume.
I'm a big fan of this very late tone tweak because it pairs well with an early tone control or stack. Use the early tone control to get the overdrive tone and substance you want then use the Cut Control to fine tune the tone. The Cut Control affects only the power tubes.
Wire the cut pot as a variable resistor so that as you turn the knob up (clockwise) resistance increases. Up = more resistance = brighter tone.
220KA or 250KA pot (audio pot wired as variable resistor) and .0047uF 200v cap connect the two phase inverter outputs.
Master Volume + Cut Control on 5F6A Bassman
The Cut Control is shown on a 5F6A but the mod is identical on an AB763. The .0047uF Cut Cap can be supported by a non-grounded terminal strip. You can turn a Cut Control into a Trainwreck Type 3 Master Volume by simply jumpering around the Cut Cap. An optional Master/Cut switch is mounted next to the Cut Pot. It is a SPST mini-switch. Close the switch for a Master Volume and open the switch for a Cut Control. If you don't want the Master/Cut Switch simply delete it and you'll have a normal Cut Control.
This is a great mod for pretty much any amp with a TMB (treble mid bass) tone stack. Because this type of tone stack really loads down the guitar signal being able to eliminate the tone stack is a valuable option. Just add an SPST ON/OFF mini-switch to the 6.8k Middle tone resistor's ground. No ground = no tone stack which gives you a very significant signal boost and pure "raw" unaltered tone. The raw tone has a non-scooped mid similar to the no-tone-stack tweed amps such as the 5E3 Deluxe. I find myself using this switch to instantly find a known, standard tone. It also works great with EQ pedals because it lets the pedal do all the tone shaping.
Raw Switch On and Off
You can see how even with all three tone pots at max what a huge boost in signal you get when the Raw Switch is engaged (tone stack ground disconnected). The mid scoop is also flattened out. Chart is from the Duncan Tone Stack Calculator.
I placed my Raw mini-switch between the Treble and Bass pots.
Raw switch added to AB763 Normal Channel. The switch is grounded at the Volume pot ground. The switch is a SPST (Single Post, Single Throw) ON/OFF mini-switch.
The 6.8k Mid resistors are located on both Bass pots. Just remove the resistor from the pot and connect the pot's left terminal to a Raw switch terminal with a new 6.8k 1/2 watt resistor (the leads of the original resistor won't be long enough to reach the switch). Connect the other switch terminal to ground and you're done.
With the switch in the connected position the tone stack is completely normal. With the switch in the disconnected position the tone stack disappears from the circuit and you get a big, fat signal boost. I like to orient the switch so that down is normal (connected) and up is raw/boost (disconnected).
You can use a push-pull 1MA volume pot as the raw switch if you prefer not to drill a hole in your faceplate for a switch. Just replace the volume pot with a 1MA (1 meg audio) push-pull pot and connect the 6.8k Mid resistor to the center-left push-pull terminal and connect the ground to the bottom-left terminal. With the push-pull down you will have a normal tone stack. Pull the volume control up and you get raw/boost.
For the 65 Deluxe Reverb Reissue & 68 Custom Deluxe Reverb the Mid tone resistors are Normal Channel R9, Vibrato Channel R21 (at far right). The circuit board has these parts labeled. The resistors are mounted next to the Bass tone pots. Disconnect one end of resistor R9 and bend it to one side. Connect the Raw switch to the free end of R9 and the empty R9 mounting pad.
If your AB763 amp has a Mid tone pot then you don't need the 6.8k resistor at all. Just run the Mid pot's ground wire (on the left Mid pot terminal) to a Raw switch terminal and the other switch terminal to ground. The switch will now interrupt the tone stack's ground. Another option is to replace the 10KA (audio or log) Mid pot with a 25KA or even 100KA pot (I prefer the 100KA). When you turn the Mid knob higher it will act as a "Raw" control, boosting the signal and removing the mid scoop.
Since both channels' tone stacks are identical you might want to consider changing one to offer different tone shaping options.
The Tone Stack is a series of three RC (resistance capacitance) audio filters that bleed three bands of audio frequencies to ground. The Treble and Bass pots change the resistance of the RC audio filters to change the amount of signal filtered out. The blackface tone stack is a passive filter so it cannot boost any frequency band, it can only remove parts of the guitar audio signal. Note the Treble pot is wired as a variable voltage divider (potentiometer) while the Bass pot is wired as a variable resistor (the input and wiper terminals are tied together). In AB763 amps with a Mid pot it is a 10KA (audio or log) wired as a variable resistor.
The AB763 Tone Stacks
Both channels' tone stacks are identical. The stacks' output flows directly into the Volume pot.
A common mod is to replace the 100k Tone Slope (yes, that's where he got his name) resistor with a 56k or even 33k which changes the way the tone controls operate. Others prefer a 150k Tone Slope.
Standard 100k Tone Slope resistor on the left, 56k on the right. The effect is subtle but some prefer the 56k slope. Notice how the max bass and treble are balanced on the right with the 56k slope resistor. The 56k also boosts the midrange by 2.5dB. I'm a fan of the 56k tone slope resistor.
Another common mod is replacing the Tone Stack's 250pF treble tone cap with a 500pF which will extend the treble controls reach into the mid frequencies.
The reissue 68 Custom Deluxe Reverb uses a .02uF Mid cap instead of the AB763 standard .047uF. You can see the difference between the .047 and .02uF Mid cap in this chart:
Tone Stack Mid Cap Change
The Custom channel's reduced .02uF middle cap shifts the "mid" frequency band higher and reduces the mid scoop by over 2dB.
The 68 Custom Deluxe Reverb also has an 18k resistor between the Bass pot and 6.8k Mid resistor. The 18k resistor works as a minimum bass setting and keeps the bass from falling off a cliff as you approach the Bass pot's minimum setting. It makes setting low bass settings easier and more precise but it does limit you to how low you can set the bass.
If you are considering modifying the tone stack I recommend you download the free Duncan Amps Tone Stack Calculator so you can see graphically what the modifications do and how the Treble and Bass pots will react to the new component values.
Duncan Amps Tone Stack Calculator
Once you get the Tone Stack Calculator running click on the Tone Stack Calculator's "Fender" tab at upper left. By setting the Mid slider (bottom left) to 68% you get 6800 ohms to equal the Deluxe Reverb's fixed 6800 ohm Mid resistor. You can double-click any component in the tone stack schematic to change its value so it's easy to see what happens to the control movements when you change the 100k slope resistor to 56k or adjust the value of the Mid resistor. The frequency response graph on the right will change as you alter component values or move the Bass and Treble pot sliders (at bottom left). Just playing with the pot sliders and watching the graph will tell you a lot about the interactive nature of the TMB (treble mid bass) tone stack. The Tone Stack Calculator is a very cool tool.
If you do install the Tone Stack Calculator be sure and see how a 25KL and 100KL Mid pot can act as a "Raw" control and boost the signal compared to a 6.8k resistor or 10KA pot.
Change the tone stack caps and resistors to suit your taste.
65 Deluxe Reverb Reissue & 68 Custom Deluxe Reverb Tone Stack Part Numbers:
Normal or Custom Channel Tone Stack
Tone Slope R6, Treble cap C2, Bass cap C3, Mid cap C4, Mid resistor R9
Vibrato or Vintage Channel Tone Stack
Tone Slope R18, Treble cap C7, Bass cap C8, Mid cap C9, Mid resistor R21
These part numbers are marked on the circuit board.
Firm Up the Deluxe Reverb's Loose Low End
The AB763 Deluxe Reverb and non-reverb Deluxe are infamous for their flabby low end that can lead to "farting out" at high volume. You can firm them up and make their tone more like their larger AB763 siblings by upping the value of the first filter capacitor. The filter capacitors are also known as reservoir caps because they store power that can be used when called upon by low frequency guitar notes and power chords.
Another more extensive modification option is to upgrade the power transformer but either of these mods will decrease the amp's touch sensitivity, playability and charm so it's not for everyone. I recommend you try the larger filter cap before messing with the power transformer.
WARNING: The first two Deluxe Reverb filter caps can hold their full charge if the amp is shut down with the Standby switch in Standby (disconnected). You MUST check for voltage at all the filter caps before trying this mod. If voltage above 25 volts is present then turn the amp on and make the amp ready to play by closing the Standby switch then turn the amp off. This should drain the filter caps but again, you MUST verify there is no voltage present with a volt meter.
You can temporarily clip in a parallel 16 or 32uF 450v (or higher voltage) cap to see if you like this mod. The higher the uF value the more the amp's character will change but you don't want the total capacitance of the first two caps to exceed 60uF which is the limit for the Deluxe Reverb's rectifier. If you don't want to try multiple caps I recommend you go with a 32uF. Alligator clip the new cap's + terminal to the first large cap's + terminal and connect the - to -. The filter capacitors are located in a "dog house" next to the output transformer and you'll have to remove the chassis from the cab to access it. Be careful removing the dog house cover because the caps might be charged with 430 volts. With the temporary cap clipped in the first filter stage will be equal to all three caps added together (16uF + 16uF + 16uF = 48uF) If you like the mod then replace the first large cap with your desired larger value cap.
AB763 Deluxe Reverb Filter Cap Board
First two filter caps on the right are connected in parallel. All of these caps would be 16uF from the factory. You don't want the total capacitance of the first two caps to exceed the rectifier's 60uF limit.
The 65 DRRI & 68 CDR's first two filter caps are 220uF 100v and 47uF 500v connected in series which is equivalent to a 38.7uF 600v capacitor. The 220uF cap has a 100k 1/2 watt bleeder and the 47uF has a 47k bleeder. A good upgrade is to replace the first two caps with 100uF 350v (or higher voltage) paired with 220k 1/2 watt resistors for an equivalent of 50uF 700v.
First two series caps on the right with their bleeder resistors above them.
The other AB763 amps first two caps are 70uF 350v filter caps connected in series. These two caps are equivalent to a 35uF 700v capacitor. One of the first two caps is upside down and they are connected + to -. They are accompanied by a 220k 1 watt bleeder resistor connected across each cap. These caps can also be upgraded to 100uF 350v (or higher voltage) for a little more power reserve.
AB763 Filter Cap Board
The first two caps on right are wired + to - in series. From the factory they are from right to left: 70uF 350v, 70uF 350v, 20uF 350v, 20uF 350v, 20uF 350v.
This SPST ON/OFF mini switch completely disconnects the Tremolo circuit which will slightly boost the guitar signal since the tremolo circuit places a load on the guitar audio signal. I like to orient the Tremolo Cut switch so that down is normal, Tremolo on; and up is Tremolo off, disconnected.
Splice in a SPST ON/OFF mini switch in the Tremolo Intensity pot's brown wire which is connected to the pot's middle terminal and the 220k Vibrato Channel Mixing resistor to completely disconnect the tremolo circuit.
You can also swap out the Tremolo Intensity pot with a 50K-RA (reverse audio) push-pull pot and wire it pull-on (my preference) or pull-off if you're a heavy tremolo user.
This mod is a little more difficult in the 65 Deluxe Reverb Reissue & 68 Custom Deluxe Reverb because there is no wire, only a trace between the Tremolo Intensity pot and the 220k Vibrato Channel Mixing resistor R35 so you must cut the trace and bridge the cut with the Tremolo Cut switch.
This is a pretty common mod that allows a slower tremolo which I really like. The tremolo circuit has three disk capacitors. Two are .01uF and the other is .02uF. Replace the two .01uF caps with .02uF 400v (or higher voltage) to slow the tremolo. Many people like to use Orange Drop caps in the tremolo oscillator.
If after this mod you would like a faster maximum tremolo speed you can reduce the size of the 100k "Max Speed" resistor located on the Tremolo Speed pot.
This mod puts a little more strain on the V5 tremolo tube so if the tremolo stops working after changing the two caps just install a fresh 12AX7.
Disk cap numbers 1, 2 and 3 match the numbers in the schematic above. Replace #1 and #2 with .02uF 400v caps.
Stop Tremolo Ticking
The tremolo oscillator can induce ticking into the amp's signal stream. First try to separate the tremolo wires from any grid wires to stop the ticking. If that doesn't work adding a .02uF cap to the tremolo roach should stop it.
Add a .02uF 400v (or higher voltage) cap across the left side of the tremolo roach to cure tremolo tick.
If you regularly push your AB763 amp hard into distortion then reducing the power tube grid leak resistors from the Fender standard 220k, down to the Marshall standard 100k 1/2 watt can sweeten the power tube overdrive tone. Doing this reduces the likelihood of blocking distortion because it reduces the charge and discharge time of the coupling cap between the phase inverter and the power tubes. The lower value grid leaks also reduce the gain from the phase inverter which also reduces the possibility of blocking distortion. See Tube Guitar Amplifier Overdrive for more info on blocking distortion.
The 220k 1/2 watt power tube grid leak resistors are on the far left end of the circuit board.
Add an AB763 Reverb Dwell Control
Simply replace the 12AT7 1M grid leak resistor with a 1MA pot. The Dwell control is just a reverb circuit master volume.
12AT7 1M grid leak resistor clipped or removed. 1MA pot wired in its place.
Place your dwell pot or trimmer wherever you want. Remove the 1M resistor. Wire the pot's left terminal to any ground. Wire the right terminal to the now empty right resistor eyelet (or turret). Wire the pot's center terminal (wiper) to V3 (12AT7) pin 7.
Channel Mixing Resistor Bright Caps Mod
Putting 500pF bright caps around the 220k Channel Mixing resistors will slightly brighten the tone of the modified channel. You can do one or both channels. If you only want to do one then I recommend doing the Vibrato channel since it is the brighter of the two channels. You can alligator clip the cap in place temporarily to see if you like the mod.
The 220k Channel Mixing resistor on the left is the Normal Channel's, the Vibrato Channel's resistor is on the right. Original photo by John Chabalko.
For the 65 Deluxe Reverb Reissue & 68 Custom Deluxe Reverb the Normal Channel Mixing resistor is R12. The Vibrato Channel Mixing resistor is R35. The circuit board has these parts labeled.
If you want to change the maximum reverb that 10 on the Reverb knob gives you then this mod is for you. At the output of the Reverb circuit the wet reverb signal flows through the Reverb pot, then through a 470k Reverb Attenuation resistor that forms a voltage divider with the following V4B (3rd stage preamp) 220k Grid Leak resistor and dumps 68% of the wet reverb signal to ground. You can tweak the reverb output level by changing the 470k Reverb Attenuation resistor. To increase the max reverb level replace it with a 220k and the attenuation drops to 50% and 110k will give 33% attenuation. Replacing the resistor with a jumper (or bypassing the resistor) will give 0% attenuation.
You can also go the other way and decrease the wet signal strength by upping the Reverb Attenuation resistor value to 680k for 24% attenuation or 1M for 18%, but before you change the resistor value try a 12AU7 in the V3 Reverb Driver socket in place of the standard 12AT7. You might be happy with the cut in reverb signal with just a simple tube swap.
Decreasing the value of the 470k Reverb Attenuation resistor will boost the wet reverb signal level.
For the 65 Deluxe Reverb Reissue & 68 Custom Deluxe Reverb the 470k Reverb Attenuation resistor is R32. The circuit board has this part labeled.
Replace all the signal resistors (especially 1M input, plate load, grid leak and grid stopper resistors) with metal film resistors like the Vishay RN65 for less noise (hiss). Most AB763 kits use period correct carbon composition resistors but metal film resistors generate 1/10th the noise of carbon comp. In reality resistor hiss is usually drowned out by filament heater hum and power supply ripple so this won't be a dramatic improvement but every little bit helps with noise suppression. If you want to be thorough then replace every resistor except for the higher watt rated voltage dropping resistors located in the capacitor "dog house" between the filter caps. I also like to leave the phase inverter 22k tail resistor and the 100k & 82K phase inverter plate load resistors carbon comp (1 watt preferred) to ensure "vintage" carbon comp tone without generating excess noise.
This is one of those mods that's much easier to implement during a new kit build.
Reduce Reverb Noise & Oscillation
Fender added a .0022uF capacitor across the 220k V4A Reverb Recovery Grid Leak resistor in the A1172 silverface circuit. This cap can be seen in all Deluxe Reverbs through the Deluxe Reverb II. It's purpose is to filter out unneeded high frequencies which helps cut noise and prevent oscillation in the sensitive reverb circuit. If you have trouble with reverb in your amp this little cap can work wonders. The 68 Custom Deluxe Reverb already has a noise suppression cap in the reverb recovery circuit.
Solder a .0022uF bypass cap around the 220k V4A Grid Leak resistor.
I haven't tried any of the Dumble mods but I include them here for completeness.
Dumble Cool Mod + Boost Switch
Turns the Bright Switch into a Boost Switch. Remove the blue stuff and replace it with the red. Note the Normal Channel tone stack's mid .047uF and bass .1uF caps are not changed even though they are shown in red.
These mods were designed to be coupled with an additional gain stage so they by themselves will alter the Normal Channel's tone but not really in the way intended. You may or may not like what they do but they should work really well with gain or boost pedals.
The Cool Mod lowers the value of the Normal Channel first gain stage cathode bypass cap from 25uF to 5uF to reduce the emphasis on lower bass frequencies which helps keep the overdrive tone tight.
The mod adds a 220k grid stopper resistor to the Normal Channel's second gain stage (V1B) which helps prevent blocking distortion which comes in handy when you add a gain stage before between V1A and V1B like the original Dumble AB763 Mod did. The added grid stopper also helps when you hammer the amp with a big boost or gain pedal. Another option here is to parallel the new 220k grid stopper with a 390pF bright cap if you need the added highs.
The Cool Mod adds a local feedback loop to the Normal Channel's second gain stage. It runs from V1B's plate back to the grid through a 22M resistor and .047uF cap. If this part of the mod tightens up the tone too much try a 32M or 44M resistor. This is a pretty cool yet extremely simple circuit that should be tried in all high gain preamps.
The mod also cuts the size of the Normal Channel coupling cap in half, going from .047uF to .022uF to trim some unneeded low frequencies which will tighten up the overdrive tone.
The Cool Mod alters the Normal Channel tone stack with a change from a 250pF to a 330pF Treble cap which boosts the mids and allows the Treble tone control to reach deeper into the mids. A 1.8k "minimum bass" resistor is added to the tone stack that keeps the bass from falling off a cliff as you approach the minimum Bass pot setting. The downside is it slightly limits how low you can set the bass. The 68 Custom Deluxe Reverb has an 18k minimum bass resistor in the same position. You can add this resistor to any AB763 by replacing the wire that runs between the Bass and Middle pots (or Middle resistor) with a resistor.
The mod also replaces the 10KA (audio or log taper) Middle pot with a 100KB (linear) pot which acts as a "Raw Control." The first 1/10 of the Mid pot movement is equivalent to the entire old 10KA Mid pot. As you roll in more Mid control the mid scoop is removed and the signal is boosted across the frequency spectrum.
The Boost Switch works as a standard "Raw Switch" which lifts the tone stack's ground effectively removing it from the circuit which completely removes the famous blackface mid scoop and offers up a big boost in signal.
Dumble Cool Mod + Rock/Jazz Switch + Optional Separate Boost Switch
The Rock/Jazz Switch in the Jazz position filters out some bass and mid frequencies. Note the optional separate "Boost (opt.)" SPST (single post single throw) switch between the Volume pot and Input jack.
Dumble Cool Mod + Rock/Jazz Switch + Push-Pull Pot Boost Switch
The only difference from above is the Boost Switch is shown on a 250KA (audio or log) push-pull Treble pot.
Differences between the 65 Deluxe Reverb Reissue and 68 Custom Deluxe Reverb:
British voiced 12” Celestion G12V-70 speaker.
Reverb and tremolo are available on both Vintage & Custom channels. On the 68 CDR they call the Vibrato Channel the "Vintage" Channel and they call the Normal Channel the "Custom" Channel. 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 by doubling the size of the NFB resistor.
Vintage (Vibrato) channel bright cap has been removed from the volume pot.
The Custom (Normal) Channel's tone stack has a smaller .02 Mid cap and an extra 18k resistor between the Bass pot and Mid resistor. The smaller Mid caps shifts the mid band and mid scoop up frequency. The 18k resistor acts as a "Bass Floor" and keeps the bass from falling off a cliff as you approach minimum Bass pot settings but it limits how low you can set the bass. If you want to be able to set less bass then lower the value of the Bass Floor resistor. Some 68 CDR owners have jumpered around this resistor to allow lower bass settings so you may not want to add this resistor to your 65 DRRI.
Click on the image to see the full size, readable layout.
Step 1. Do the very simple Normal Channel Reverb Mod. This mod is exactly how the 68 CDR sends the Custom (Normal) Channel through the reverb and tremolo effects and third preamp stage. You will also be able to jumper the channels together after this mod for a slightly fatter tone. If you think you may alter the voicing of one of the channels then I recommend you do the Fritz Mod instead to add reverb and tremolo to the Normal Channel. The Fritz Mod retains the Normal Channel's coupling cap so you can change its value to alter the Normal Channel's voice.
Step 2. Change the Negative Feedback Resistor R64 to 1.5k or do the 3-Way Negative Feedback Switch Mod. This mod has an 820 ohm and a 1.5k NFB resistors so you can select normal 65 DRRI feedback or no feedback or 68 CDR feedback.
Step 3. Remove or clip a leg of the Vibrato Channel Bright Cap C10 located next to the Vibrato Volume pot. You could also put the cap on a switch. Use a SPST mini switch to break the connection between the Bright cap and Volume pot center terminal.
Step 4. Modify the Normal Channel tone stack to match the 68 CDR Custom Channel. Change the Mid capacitor C4 to .022uF. The 68 CDR schematic calls the 18k 1/2 watt Bass Floor resistor R72. To insert the resistor you must cut the trace on the pot control board that runs from the Bass pot to the intersection of the 6.8k Mid resistor R9 and the C4 Mid cap. Then bridge the cut trace with the 18k resistor. If the 18k resistor limits your minimum bass setting too much you can try a smaller value resistor like a 12k.
Speaker Suggestions for the AB763
Weber 12F150 Vintage American Voice, 12", ferrite magnet, 1.5 inch voice coil, light dope $114. Weber's and my recommendation for the blackface amps that use 12" speakers. 10F150 is my recommendation for blackface amps that use 10" speakers, $94. Both require a long, 50 hour break in unless you have Weber do their free break in service.
Weber Ceramic Signature 10" and 12" for the budget minded. Some actually prefer the Sigs to the more expensive 10F150 and 12F150. Four Sig 10's in a Super Reverb sound absolutely glorious.
The 68 Custom Deluxe Reverb comes with a British voiced 12” Celestion G12V-70 and the amp gets great reviews.
Eminence GA-SC64 Vintage American Voice, "Tones of 1964 still ring true in this vintage voiced speaker by seasoned amp guru George Alessandro. Well balanced from top to bottom, the GA-SC64’s warm, dynamic character lends itself to vintage amps as well as modern gain and distortion. The traditional build materials offer warmth, dynamics, low note articulation and proper distorted harmonic content layering."
Eminence Cannabis Rex "The Eminence Cannabis Rex guitar speaker lets your laid-back style come through. Clean and full, with lots of body and sparkle. Smokey smooth with high-end definition. Country, Jazz or Classical guitarists take notice."
JBL D120F Loud, clean, efficient but heavy and adds some mids the amps needs. Best defense against ham fisted drummers. You can always find a blown one and have it reconed.
Celestion 12M Greenback Warm and Woody with relatively early breakup.
Eminence GB128 Vintage British Voice, "The Eminence Legend GB128 guitar speaker helps clean up your act. Cleaner British voiced tone with a full low end, warm, throaty mids and a very open top end. Turn up the volume and turn back the clock."
The WGS ET65 is an amazing replacement speaker for Fender amps even before you consider its low low price. "The ET65's creamy lows blend with tight midrange and sparkling highs for some of the best tones ever created. This speaker is for classic rock, country, and blues. Upgrade from your Celestion™ G12-65."
Pull the Normal Channel preamp tube V1 and use the Vibrato Channel. Of course this assumes you don't use the Normal Channel because tube V1 is the Normal Channel first and second preamp stages. Pulling V1 changes the amp circuit because the V1B and V2B triodes share an 820 ohm cathode resistor. Two triodes sharing an 820 ohm cathode resistor is the equivalent of each triode having its own 1.5k resistor. If you remove V1 then the V2B (Vibrato Channel second gain stage) preamp goes from normal center bias to a warm bias with an 820 ohm cathode resistor. In other words pulling V1 is exactly like changing the Vibrato Channel second preamp gain stage from a 1.5k cathode resistor to an 820 ohm. This warm bias increases gain and makes creamy sounding asymmetric clipping more likely. This works really well with hot boost pedals or even hot humbucker guitars.
Replace V1 or V2 with a 12AY7. You can go the opposite direction and reduce preamp gain by replacing the 12AX7 in V1 (Normal Channel) or V2 (Vibrato Channel) with a lower gain 12AY7. The 12AY7 was used as the first gain stage for many of the tweed amps and will offer up more clean headroom because its lower gain will reduce preamp distortion. This swap can really shift the distortion balance between preamp and power amp to favor power tube distortion when you combine it with the V6 phase inverter tube change as discussed in the next paragraph.
Try a 12AX7 in the V6 Phase Inverter for more power tube distortion. The 12AX7's extra gain will hit the power tubes with a hotter signal. This is a way to shift the AB763's preamp/poweramp distortion balance toward the power tubes. To give it a try just move the V1 12AX7 into the V6 socket and use the Vibrato Channel.
Replace the V3 12AT7 Reverb Driver tube with a lower gain 12AU7 to give you more usable room on the Reverb Control. The Reverb pot can be a little touchy with a usable range from 0 to 3 or 4. A 12AU7 won't push the reverb circuit as hard so the usable range can extend up to 5 or 6 on the dial and make dialing in just the right amount of reverb easier. The lower gain 12AU7 can also reduce reverb circuit noise.
Click the image for the full size layout.
AB763 Twin Reverb Schematic
Some of the major differences between the Twin Reverb and Deluxe Reverb are highlighted.
AB763 Twin Reverb Layout
The 65 DRRI has the AB763 circuit on a PCB. Click the image for the full size schematic.
65 Deluxe Reverb Reissue PCB Layout
Click the image for the full size layout.
The 68 CDR is not a silverface amp, it is an almost exact copy of the 64 Deluxe Reverb Reissue with just a few valuable tweaks. Click the image for the full size schematic.
68 Custom Deluxe Reverb PCB Layout
Click the image for the full size layout.
This webpage is available in PDF form: AB763 Modifications.pdf
By Rob Robinette