Fender AB673 Normal Channel Modification

Introduction

I have received a surprising number of emails concerning my restoration of a Fender Deluxe Reverb guitar amplifier in 2007 on this web page. A few people were quite upset and claimed that I had I ruined and devalued this amplifier. Apparently altering a black face amplifier is considered taboo by some members of the guitar community. I am respectful of the opinions of others however, the many people who have heard and played through this amp think it sounds superb. To me, any amplifier is just a bunch of components organized in a manner which should provide useful sound or sounds for the operator. The real value of an amplifier is when it provides a tone that pleases and inspires you to practice and perform. At this point, I will cease to be an apologist and get on with writing this web page.
This web page describes some experiments I performed on the Normal channel of a Fender Model AB763. The Vibrato channel was left stock. This project allows a choice of "black face or "boosted-midrange and bass" guitar sounds by choosing the Vibrato and Normal channels respectively. The focus of these experiments was for clean guitar sounds only.


Background Information



The scooped midrange tone of the black face Deluxe Reverb amp is fantastic; to be sure! However, to me, it might not be the best tone for jazz guitar. I really like the sound of Gibson amplifiers from the 1950's such as the GA-50. These amps have more lows and mids and to my ears this is a desirable tonal quality for playing cleanly with an arch top guitar.
In my opinion, it is the midrange tones that give amps such as the early Gibson amps a little more punch and a gentle breakup with extra harmonics when certain notes or chords are played forcefully. The overall tone I wanted was warm and clean. To learn more about tone circuits, I surfed the world wide web and found scores of tube amplifier schematics to look at.
I tried about 10 different preamplifier + tone circuits and made notes about what was good and not so good about these various circuits by using my ears. The GA-50 preamp circuit was even replicated, except that I used a 12AX7 or 12AT7 tube instead of the 6SJ7. Overall, this circuit was a little harsh sounding and also the interaction between the tone controls was dramatic and affected both the volume and tone of the amp. Some of the other circuits I tried included the Stephen Delft "Moonlight", James/Baxandall, Marshall, Fender, Gibson, Vox and various 1 knob tone control circuits. Ultimately, I went back to the original Fender tone stack for awhile but modified it as shown in Schematic 1. The 0.15 uF cap and 4K7 resistor give a little more lows and mids than the conventional Fender black face tone stack. This is my favorite version of the classic 2 control, Fender tone stack.
Later, I remembered this circuit. In the early 1990's I converted an old Garnet 50 watt amplifier into a Dumble Clone and used this circuit. I remembered how nice the clean tone of the amplifier was when switched into the jazz setting. I have total respect for Alex Dumble, and his design philosophy. I decided to build the original 60's ODS preamp circuit permanently wired in the jazz mode as the Normal channel of my Fender Deluxe Reverb amp. Then I experimented with the stock circuit.


Further Experiments


Above photo. The DR out of the wood. As only the Normal channel is modified, it is still possible to get the classic DR tone by using the Vibrato channel. I do not use reverb and turning the reverb control to 1 quiets down the Vibrato channel a little. My modifications to the Normal channel did not increase the noise of this channel. For Normal channel one, use a metal-film or carbon composition grid resistor (22K - 33K) on V1-pin 2 and not the 68K value used in the original DR schematic. The Normal channel one 68K resistor was formerly in parallel with the Normal channel two 68K input grid resistor, but obviously not any more as the Normal channel 2 input has been removed.

Above in Schematic 2 is my final experimentally derived (jazz setting only) version of the 60's ODS preamp. I used appropriately rated Polysomething or Silver Mica capacitors and appropriately rated metal film resistors. I will explain some of the changes to the original circuit after disclaiming that this web page is meant to offer some information regarding amplifier modification and that your results will vary according your stock amp characteristics, guitar and speaker choices, personal bias and perhaps even by which day of the week it is. First and foremost, this web site is about experimenting with electronics. Please do not modify your amp if you want to keep it's vintage monetary value. Do not perform amplifier modifications if you are inexperienced and unqualified in high-voltage electronics. Warning: Tube amplifiers operate at high DC voltages. Repairing, modifying or building tube amps can be dangerous or in some cases fatal.

Front panel switch. I placed a switch into the former second instrument input on the Normal Channel. This switch places a 15K resistor in series with the 39K midrange control resistor or not (The parallel R Value is about 10k). With the switch closed, the amp exhibits a mellow, tone with a lot of low midrange and bass response. I call this the A position. When the switch is open, the high mids are boosted and the the bass response is slightly attenuated. The volume also increases slightly. I call this my "Barney" switch and thus call it the B position. Barney Kessell is my favorite guitar player and commonly played an ES-340 with a a single coil, blade pickup. Some players do not like his tone, but I love it. I wanted to be able to get a little more mids "aka the Barney Tone" for when using humbucker pickups.

The 39K and 10K (parallel resistance) values were determined by temporarily wiring in a 100K midrange potentiometer and performing listening tests. I found that there were only 2 settings of the midrange pot that I liked -- you might prefer other resitor values. I measured these values with an ohm meter and afterward wired the fixed resistor values as shown. With the switch in the B position, (i.e. the "Barney-tone" the amp really cuts through the mix. This "B" setting gives the amp a little more midrange focus and harmonics and also a little more breakup when picking forcefully at modest or greater playing volumes.

With the switch engaged or not, the amp has more bass response than the stock DR circuit, although you can dial the bass response down by reducing the bass control.

Sound Byte Illustrating the Switch Function. A is the 2 resistors in parallel, B is just the 39K resistor. This is a Joe Pass lick that I really like. The guitar is a borrowed ES-175. The difference is even more apparent when playing rhythm guitar.

Above is a close up photograph of the parts around V1. The dark colored 250 pF Silver mica cap is barely visible to the right hand side. The carbon film 39K and 15K resistors can be clearly seen on the left hand side. I will convert these to metal film type resistors after my next parts order. The big "reddish" capacitor is the 4.7 uF cap used to bypass the cathode of pin 8 to ground. Normally this cathode shares a grounding resistor and capacitor with pin 8 of V2, however, it must have its own 1K5 resistor and 4.7 uF cap to ground for this mod. You can't see some of the other resistors and capacitors as they are directly soldered to the tone and volume control potentiometers.

The switch on the Normal channel can easily be seen in the above photograph.

Treble Capacitor The original circuit calls for a 330 pF treble capacitor. For my mod, to increase the lows and mids, a 250 pF is used. In the original schematic, the tone stack has 2 switchable settings and in the rock mode, this stage may drive subsequent stages to develop preamp distortion- It is easy to see why Alex chose the 330 pF value.

Treble Control Capacitor and Bass Control Wiper to Volume Control Wiper Resistors There are 3 capacitors in parallel (total 0.0053 uF) between the Treble potentiometer and ground. I did this as I did not have a 0.0047 uF cap in stock.
The 2 resistors which bypass the main channel volume control in the original schematic are a pair of 270 ohm resistors with a deep switched connected between them. The low frequency response of this channel is already strong enough to shake pictures and vibrate walls, so the Deep control of the 60's ODS control was deleted. I used a 390K in series with a 100K resistor for my version. You could also use two 270K resistors in series or even a single 470K resistor. My choice of resistors was pragmatic; I had 390K and 100K metal film resistors in my parts collection.

Miscellaneous I omitted the 500 pF volume control bypass capacitor,  the 220K pin 7 grid resistor and the the negative feedback circuit from the original 60's ODS schematic by choice.

My DR back in the wood for awhile. I would like to build a complete 4 tube, 40 watt version of this amplifier channel for extra headroom and volume. It would be pretty basic; a single triode preamp tube, a phase inverter triode, 2 6L6 finals, a solid state rectifier and a big, heavy output transformer. Less can be more!


Sound Bytes



These are recorded with a borrowed ES-175. My recording technique/equipment and playing do not do this amp justice, but offer a reasonable representation of the sounds the Normal channel is now able to produce. These deep, midrange peaked tones are just not available on a stock AB763 circuit.

Sound Byte 2  Part A is just the 39K resistor (i.e. the non-Barney position). Part B is with the 15K and 39K Resistors in series.

Sound Byte 3  Switch in A position (i.e. the non-Barney position)

Sound Byte 4  Switch in B (or Barney) position. I over compressed this file unfortunately.

Sound Byte 5  Switch in B position. Near the end I hit the switch to A position and you can hear the chord tone change. The low frequency response is well illustrated in this file. The highs are also clear as a bell.


Conclusion


There seems to be countless information on the web regarding hot-rodding your tube amp, but very little on how to jazz it up. In my opinion, the strength of this modification is that it provides a non-harsh channel with lots of highs and lows and a switchable (or potentially variable using a potentiometer) midrange control. I do not want to hype this modification and have presented it as a circuit for further experimentation. I will likely experiment further with this circuit in the future as time and will permits.


Feedback    added June 19, 2010

Some people have built and extended this modification and many seem to like it. From now on, I will anonymously post any emails I receive about it. Hype and self-promotion don't belong on hobbyist sites.

Hi Just wanted to tell you that I have just tried your schematic 1 DR amp mod for jazz guitar on my silverface DR amp. Works very well with my arch tops- much better than my previous midrange mods which increased the value of the treble cap, and made things too honky. Great idea - thanks. I quite agree with your approach to modding amps for a desired tone, if done reversibly. To prove the point, I have gone further with mine and installed a midrange pot, with the obvious benefit... I have switched between the two 'middle cap'' values and also have the pot, which gives great flexibility. Not much use for the trem effect in jazz, and one can always put things back if necessary. What I have found to make a huge difference in getting a good jazz tone from the DR is installing an Altec 4178H speaker; it weighs 11 lb, but is very worth it. It gives a 'closed-back' feel, and of course increases apparent headroom, being a 100dB/W/m speaker.
Chris


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