- 1964 -1967 “blackface” circuits AA763, AB763
AA763/AB763 Tube layout (Seen from behind, V1 is to the right side)
- V1 12ax7 = Preamp normal channel
- V2 12ax7 = Preamp vibrato channel
- V3 12ax7 = Vibrato
- V4 12at7 = Phase inverter
- V5 6V6 = Power tube #1
- V6 6V6 = Power tube #2
- V7 GZ34 = Rectifier tube
The Blackface Deluxe Amp is often misinterpreted as a Deluxe Reverb without reverb. It might seem so if you take a look at the front panel and the knob functions. But a closer study of the circuit design will reveal that the Deluxe Reverb vibrato channel has the extra gain stage (one half of the 12AX7 V4 tube) common for the Fender reverb amps. The wet reverb signal is mixed together with the dry signal which has gone through the 3.3MOhm resistor, therefore needing a gain boost before entering the phase inverter. This means one less tube in the Deluxe Amp in the preamp section that will contribute to sustaining harmonics, compression and sag. The Deluxe-Amp is therefore cleaner when played high on the volume knob. Still, the clean sound of these amps sound fairly similar goven a common EQ circuitry, phase inverter and power amp section. When the volume is pushed beyond 4-5 where the amps will differ in tone. The Deluxe will remain relatively clean, while the Deluxe Reverb breaks up in a typical blackface manner. Being a middle size Fender blackface amp the Deluxe has not been modified and experimented with so widely as other Fender amps. It’s power, clean headroom, physical size and tonal characteristics have matched the expectations of more players than any other tube or transistor guitar amp of any brand on this planet. It is said that the Deluxe Reverb is the nr 1 most used amp in recording studios. We can understand why. It is powerful enough to cut through in a band with a drummer and reaches the sweet spot at a reasonable volume. It has the typical blackface 60′s sparkling, scooped clean sound as it’s bigger brothers, just with more smooth breakup, sag and compression. The smaller brother Princeton Reverb is rolling off more top frequencies and breaks up earlier with more warmth producing a brownface type of tone. It can not really cut through with Albert Collins’ icy licks like the Deluxe.
In general we don’t recommend the mods that increase the power and clean headroom unless you are playing your amp at gigs on medium/large stages and you need more of the sparkling clean notes or funky rythm chords to cut through. If you are using your amp at home or in recording situations we think you’ll be very hapy with a stock Deluxe. You will be able to shape your tone just by selecting the correct speaker(s) and some experimenting with tubes.
You’ll need schematics to implement some of these mods. http://www.ampwares.com/fender.asp. We usually start with explaining a mod from a functional perspective where we relate to components in the logical schematics diagram. Finally we point out location of components in the physical layout diagram.
12AY7 or 12AT7 as preamp tube – Less preamp gain.
If you want a cleaner and spankier tone you can replace the V1/V2 12AX7 preamp tube with 12AT7 or 12AY7. These tubes have lower voltage gain and different frequency responses, particurlarly when distorting. 12AX7 tube has a voltage gain factor = 100, while 12AT7 = 60 and 12AY7 = 45. One could describe these tubes to have less harsh and buzzy distortion. This mod does not alter the tone significantly when amp is played clean’ish. You’ll have to increase the volume setting to achieve a similar volume as before. If you hit sweet spot at volume=4 with 12AX7 you may go as high as 6-7 before reaching it with 12AT7 or 12AY7.
Pull out the V1 normal channel preamp tube for more preamp gain in vibrato channel
We’ll start with saying that this is a must have mod. It is so easy to enable and disable that it can hardly be called a mod. If you are like most players and only use the Vibrato channel (reverb, tremolo, the brigth cap and the extra gain stage), you should pull out the V1 tube. This is the preamp tube for the normal channel which you are not using. Vice versa; If you’re using the Normal channel, you can pull out the V2 tube. All AB763-similar circuits (Deluxe Reverb, Super Reverb, Pro Reverb, Twin Reverb, Vibroverb, Vibrolux) are designed so that signal is leaking between the two channels. The amp will play louder at the same volume knob setting when pulling the V1/V2 tube that you’re not using. The stronger signal will push the second gain stage (V4 tube) harder and give you increased sustain, compression and harmomics. This mod does not change the amp’s clean headroom.
This mod is one of Cesar Diaz’ tricks in the Fender Custom Shop Vibroverb 64 which he always did to Stevie’s amps.
Replace the 12AT7 PI tube with a 12AX7 or 12AU7 – Less clean headroom.
Very practical mod at practice and low volume environments. This mod reduces the amp’s clean headroom and you’ll achieve sweet spot at a lower volume. You’ll notice that the amp gets looser and with less attack. Tips: If you pulled the V1 12AX7 tube you may use it as V6 phase inverter.
12AX7 as phase inverer tube will give the most effect out of this mod. 12AU7 will be in between 12AT7 and 12AU7.
Diode rectifier – More clean headroom.
This mod is for those who wants more power, clean headroom and attack. It is very practical to bring with you those small Sovtek plastic adapters that easily can replace the rectifier tube. You’ll increase the power of the amp with a few watts. The diode rectifier will feed your amp’s filter caps with a higher and ”faster” DC voltage compared to a tube rectifier. The fact that it delivers a slightly higher DC voltage to your power tubes (via the filter caps) means an increase of the tube voltage gain factor. When you hit a hard chord the preamp and power tubes will require energy and current is flowing from the DC filter caps which are charged up by the rectifier. If the recitifer can deliver more energy more quickly, you’ll have more power and attack.
Please be aware of that using gz34 or diode rectifier will wear you 6v6 power tubes faster than with a 5u4gb due to the higher plate voltage. Some 6V6 tubes are better than others in handling higher voltages. The TAD and JJs are rough and can take a lot of beating.
Check also out Ted Weber’s “Copper Cap Rectifiers” that emulates the sag of a tube rectifier.
Upgrade to 6L6 – More power and clean headroom.
If you want more power you can easily replace the 6V6′s with 6L6. You need to rebias the amp immediately to prevent wearing out the tubes. We recommend 32mA for 6L6 and 23mA for 6V6. We have only tried this mod on blackfae circuits (AB763-similar) with GZ34 tube rectifier. This mod is very effective since it is quick and requires very little knowledge or tools. If you’re at a gig and need more clean headroom, just upgrade to 6L6 and rebias. Be careful with vintage speakers that easily blow when taking 35-40 watts in stead of 20. Both the power and output transformer in a Deluxe is smaller than the 40 watt amps, and you’ll have more sag and less attack.
Tremolo disconnect mod - More preamp gain in vibrato channel.
The effect of this mod is similar to pulling the V1 normal channel preamp tube when playing the vibrato channel. This is also a very popular mod in the AB763-similar circuits (Super, Twin, Virboverb, Pro Reverb, Deluxe, Vibrolux). When you’re not using any tremolo (intensity=0) this mod will kick in. Once you’re turning up the tremolo the tremolo circuit is connected and the tone is original. The tremolo circuit will draw current from your signal path whether it is in use or not. Hence, it reduces some of the gain that has been added by the preamp tube stages. This mod does not increase the amp’s clean headroom. We would describe the effect as making the tone richer, fuller and more powerful with stronger mids that pushes the power tubes harder. This mod provides a noticable volume increase (and a bump) when enabled that you can compensate for by lowering the volume knob settint. Most important is the richer and fuller tone. We like this mod a lot in all AB763 circuits since they really turn alive a scooped and relatively thin sounding guitar and amp. In addition to pulling the V1 tube, this mod is one of those must have mods which we never undo once having implemented it.
This mod is one of Cesar Diaz’ tricks in the Fender Custom Shop Vibroverb 64 which he always did to Stevie’s amps. This mod can be implemented in two ways, either by unsoldering the tremolo circuit or replacing the tremolo intensity pot with a swicthable pot, so-called SPST pot. The switch is enabled when you turn down to zero. There are also switchable push/pull pots where you pull the knob to enable the switch.
- Order a 50k SPST from one of our recommended amp part dealers.
- Remove the original 50k trem intensity potmeter by unsoldering the wires to the potmeter lugs and unount the pot from the chassis. The black plastic wheel is to be used for the new pot.
- Install the new 50k SPST pot to the chassis. This is a switchable pot offering a mid positioned switch in addition to the variable resistanse 0-50KOhm between the lugs. At level=0 you’ll completly disconnect the tremolo circuit.
- See the wiring diagram below.
- You’ll have to solder the yellow wire to one side of the mid positioned switch in stead of to the right pot lug as before.
- Then solder a new (red) wire between the other side of switch to the right lug.
If you’re permanemtly disconnecting the tremolo circuit you can just clip of the brown and yellow wires and insulate the ends with tape.
Unlike it’s bigger brothers, the Deluxe came with no bright switch. Instead there is a hard wired bright cap 47pF in the Vibrato channel enhancing the upper treble notes. The Normal channel did not get this bright cap. You may choose to clip one of the legs on this 47pF cap to turn the bright switch off, or you may want to install a identical bright cap to the normal channel. See red circles in figures below for this cap. The cap is installed directly between the volume pot lugs.
Schematics (Deluxe Reverb is showed, but the Deluxe will be the same in this context)
Click images for large versions.
Increase/remove/switchable NFB resistor
This mod is applicable in all Fender amps with a negative feedback (NBF) loop. The purpose of the NBF loop is to clean up and cancel out high frequencies and upper harmonics (distortion). Without the NBF loop the volume knob gets more sensitive and aggressive. You’ll find the amp’s sweet spot at a lower volume knob setting. If you remove the feedback loop entirely you’ll achieve maximum effect of this mod. You do not get more or less clean headroom in the amp. You get a rougher and rawer tone with more of the higher frequencies, aka hizz. If you think your guitar and speaker is bright enough you probably do not want any more treble. This mod is for those who want even more of the razor edge sharp tone. Many players find the Deluxe’s Vibrato channel bright enough. If you do this mod, we recommend the switchable mod.
You may choose to implement the mod in several ways:
- Simply disconnect the feedback loop (a wire). The 820Ohm resistor can easily be removed with a soldering iron. See the red circled resistor in figures below.
- Increase the feedback loop resistance value. This reduces the effect of the feedback loop, making the amp break up more.
- One of the two alternatives above in combination with making the NBF switchable. You can solder out and remove existing wires to the ground switch if you plan not to use it (if you’re wondering what it does, you don’t need it). Then wire the ground switch in series with the 820Ohm NBF reistor. This will make the NFB mod switchable, which is very practical.
Click images for full size versions.
The Deluxe came unfortunatley without a mid pot. Many players are happy with the default 6.8K mid resistor in the EQ circuitry, but some want what the others have; a 10K potmeter mounted in series with the 6.8 Kohm resistor. When the potmeter is set at 0 you’ll have an original tone, and when turned up the mids increases and your amp growls with a British Vox’ish tone (especially together with a Celestion speaker). For a wider tone range you can replace the 6.8 KOhm resistor with a 4 KOhm, and use the 10 K pot to go beyond the original 6.8 KOhm and up to 14 KOhm. If you do not use the reverb or tremolo pedal it is very practical to install the middle pot here. It requires a simple solder and removal job to replace the phono plugs with a mid pot. Or you could install the mid pot at the left most side of back of the amp where the additional power supply is located.
Questions and comments are welcome at the bottom of this page.
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