- 1963 -1967 “blackface” Bandmaster circuits AB763
- 1968 -1975 “silverface” Bandmaster circuits AA763, AB763, AC568, TFC-5005
- 1968 -1978 “silverface” Bandmaster Reverb circuits AA768, AA568, AA1069, or TFL5005
- 1988 -1980 “silverface” Bandmaster Reverb Master volum 45w or 70w
AA763/AB763 Tube layout (Seen from behind, V1 is to the right side)
- V1 12ax7 = Preamp normal channel
- V2 12ax7 = Preamp vibrato channel
- V3 12at7 = Reverb send
- V4 12ax7 = 1/2 Reverb recovery and 1/2 gain stage for vibrato channel
- V5 12ax7 = Vibrato
- V6 12at7 = Phase inverter
- V7 6L6 = Power tube #1
- V8 6L6 = Power tube #2
- ( V9 GZ34/5U4GB = Rectifier tube on silverface reverb amps)
The Bandmaster came with many different circuits during the blackface and silverface eras. The blackface head had a diode recifier, one normal channel and a vibrato channel with no reverb. In the silverface era the recitifier was changed from diode to tube and the vibrato channel got reverb. Hence, the name “Bandmaster Reverb”. As usual the later silverface models were modified to become cleaner and more powerful. The 70W model from 1978 with master volume and push/pull boost had a huge power transformer and big filter caps. It reminds us more of a classic 80w Twin Reverb than a Bandmaster Reverb, and it shared its circuit design with the Pro Reverb and Super Reverb at that time.
A noticable difference in the vibrato channel between the early silverface Bandmaster Reverb and blackface Bandmaster is the extra gain stage in the silverface amps. This was also the difference between the first 1964 blackface AA864 Bassman and the following AB165. This gain stage was introduced as a part of the reverb recovery circuit, the V4 12AX7 to be exact, common for all the reverb amps of the blackface/silverface era. Amps with no reverb recovery stage cleaner sounding and will remain pretty clean when you turn up the volume knob above 4. The extra gain stage in the silverface Bandmaster Reverb amps contributes with tube distortion and compression in the preamp section when the amp is pushed. Not only is there more action in the preamp section, the silverface models also has more sag and looser response due to the tube rectifier. The Bandmaster Reverb head will remind you pretty much of the amps in the Pro Reverb, Super Reverb and Vibrolux Reverb where the sweet spot is to be found around 4 on the volume knob. Not all silverface amps were developed in a bad direction. We think the
Let’s study the blackface Bandmaster AB763 and Bassman AA864. They are similar in the way that both are clean sounding with just one 12ax7 tube in the preamp stage ( vibrato ch in bandmaster and normal channel in bassman). The vibrato channel in the Bandmaster is even more clean than the Bassman because of the vibrato circuitry loading the signal chain and reducing the gain level in the premp section. Hence, the AA864 Bassman normal channel has more preamp “juice” and reaches the sweet spot at an earlier volume knob setting. The Bassman has a slightly bigger output transformer resulting in a firmer tone and more attack. A smaller output transformer will introduce sag and compression in the power amp section. The blackface Bandmaster is therefore ideal for those who look for a pure Fender clean sound without making ones ears bleed.
The blackface and silverface Bandmaster is a big sounding amp with a flexible speaker impedance of 4 ohm, allowing anything between one and four speakers (8 ohm each) to be connected via the main and/or external speaker jack. This makes it possible to adapt to small and big stages and gigs just by configuring the speakers.
Most of the maintenance and mod tips below will make the amp break up earlier, reaching the sweet spot at a lower volume. Tube mods (pulling V1 out and a 12ax7 PI) will take you a long way, and if you also invest in implementing the tremolo disconnect mod, you may get a fatter, warmer and fuller tone at practice volumes. Unfortunately does the diode rectifier not offer any options for rectifier tube swaps to select among different plate voltages, clean headroom levels and sag characteristics.
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.
Five fundamental tricks to create the holy grail of Fender tone
Here is a video demonstrating the effects of some of the mods and tricks described here. To see embedded text comments, go to video on Youtube.
The Bandmaster’s 2×12″ speaker cabinet was from the Fender factory loaded with either Jensen C12n or Oxford 12T6. Personally we favor the Jensen C12n for their natural and transparent “Fendery” tone and, most importantly, for not being to bright and harsh. The C12n sounds very punchy, wide and deep in a 2×12″ cabinet.
See page How to select speakers for speaker recommendations.
12AY7 or 12AT7 as preamp tubes – Less preamp gain.
If you want cleaner and spankier preamp distortion charcteristics, you may replace the V1 or V2 12AX7 preamp tube with 12AT7 and 12AY7. These tubes have different frequency responses than 12AX7, particurlarly when distorting. People describe these tubes to have less harsh and buzzy distortion. This mod does not alter the tone significantly when amp is played clean or when only the power amp section distorts. You’ll have to increase the volume setting to achieve a similar volume as before. The reason is that 12AX7 tube has a voltage gain factor = 100, while 12AT7 = 60 and 12AY7 = 45.
Pull out the V1 normal channel preamp tube – 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.
Replace the 12AT7 reverb driver with 12AU7 - Better reverb control.
Reverb is an important character with vintage amps, yet so individual and mysterious. We all know that speakers change their tonal character during age. So does the reverb. The reverb function sounds and behaves differently between “identical” vintage amps. Some amps have long, lush and soft reverb while others are mushy and overwhelming. We often find the reverb sweet spot around 2.5 on the reverb pot, varying from 2 to 4. Some amps are sensitive and difficult to control the reverb on. The whole dynamic area can be within a narrow interval, i.e. 2 and 2.5. These amps require a careful touch when dialling in the reverb, which irritates us.
The reverb circuitry consists of two tube sections (reverb driver V3 and reverb recovery V4) and the physical reverb tank. All these components will drift during age and minor differences in component values are noticeable to man’s ear.
If you replace the V3 12AT7 reverb driver with a 12AU7, you will reduce the effect of the reverb and it will be much easier to control with the reverb knob. So simple as that.
Use normal channel for reverb control – Adjust EQ and depth of reverb.
This mod is relevant only for two-channel amps with normal and vibrato channel. This trick is great for the reverb enthusiasts among us, and who is not? Plug your guitar into the vibrato channel, then unplug the reverb return cable on the back of the amp (the one that comes from the reverb tank output) and plug it into the normal channel input. You will need a converter to go from male phono/RCA jack to a 1/4″ male jack. You may now use the normal channel as a reverb control where you can adjust the depth and tone using the volume, bright switch, treble and bass knobs (and mid if you have a Twin Reverb). The reverb knob on the vibrato channel will have no effect any longer.
This mod is not applicable together with the Pull V1 mod, as you need the normal channel preamp tube.
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 AB763-similar circuits (Super, Twin, Virboverb, Pro Reverb, Deluxe, Vibrolux).
By original design the tremolo circuit will absorbe current/signal even when one turns the tremolo off with the footswitch. This mod suggests to entirly disconnect the tremolo circuit from the signal path by replacing the tremolo intensity pot with a switchabe pot (spst). One side effect with this mod is a noticable click and a volume difference between tremolo on and off using the new spst pot. If one uses the tremolo regularly one should still use the tremolo pedal to enable/disable the tremolo and leave the intensity spst pot at your desired level. The spst pot is not good for enabling and disabling the tremolo very often (because of the click and the volume difference) but is a nice when you play without tremolo. A good thing with this mod is that you have both the tremolo pedal and spst pot to use.
With a new switchable/spst pot set at intensity=0 the mod will kick in and raise the signal level in the preamp section, right before the phase inverter. Once you’re turning up the tremolo the circuit is connected again and the tone will remain original. 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 amp section harder (phase inverter + power tubes). We like this mod a lot for stratocasters wi the Fender AB763-similar circuits since they boost a relatively scooped and 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 intensity 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. Note that the figure below shows the Super Reverb. The Bandmaster would be the same.
(One may also use the tremolo pedal, in stead of the spst switch/pot, to entirly disconnect the tremolo. You will not need to replace the intensity pot with a spst, but let the yellow wire to the tremolo pot go through the tremolo pedal. In our opinion this is not a good idea since one loses the possibility to use both the spst switch and tremolo pedal. Using the pedal will now involve a significant click sound and volume increase.)
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 vibrato channel in AB763-similar circuits 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.
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