The History
Jerry Garcia Fender of choice was the Twin Reverb, in particular, Leo Fenders AB763 circuit. Early in the Grateful Dead's infancy Jerry only used the amp in it's stock form as a combo amp.
" My First Amplifier with the Warlocks was a Fender Twin Reverb. I bought it in the first year they came out, around 1963 or 1964, I guess. It may actually still be in use. I sold it to Mike Wilhelm of the original Charlatans." J.Garcia - Guitar Player 1978 Interview
The Warlocks 1965
Sausalito 1966 (Rehearsel)
In 1967 Jerry switches to 2 amps, a Fender Twin and Showman Head which fed a 4-12" speaker cabinet.
1967
By 1968, the amps consisted of a Showman Head and 2 Fender Twin Reverbs. By this time, modifications were done to the amps by Owsley Stanley and Ron Wickersham to boost the output and stabilize the power supply of Leo's original design as well as to keep them as noise and hum free as possible. Mods like the Potentiometer Upgrades(AB Type J/RV4), the zener diode bias regulation and Allen Bradley Cathode Bypass resistors were used(Wickersham). Also, at some point in this time period the beginning's of the Alembic Company began to Preamp Tap the Twins and Showmans , as well as having their Tube Power Sections tapped into and used as "Slave" units. This practice was only used for a very short time and the exact dates are unknown. The Alembic Preamp Taps were then fed, first to Dynaco MK III's for a short time, and then to Mcintosh MC75's which fed the external cabinets. The same power tube used in the Dynaco's produced considerably more power in the MC75's. The speakers in the actual Fender Twin cabinets were not used. The external cabinet speakers were wired in paralleled pairs for a total 4 pairs, each with it's own MC75 fed from the 2 Showman channels and each of the two Fender Twin Reverb Vibrato Channels used. A Box is used to split the Guitar input signal to 4 seperate signals, one for each channel used in his array of amps. This produced half the total output with reverb and half without.
Newport Pop Festival August 4, 1968
From 1969 thru Europe 1972 he moved to 3 Fender Twin cabinets with the 2 - 4-12" Extension speaker cabinets. In the Stack of Twins, the top one utilized both the Normal and Vibrato preamps. Wired the same as above only 1 channel was without reverb.
Woodstock 1969
Fillmore East 1970
Poynette, Wisconsin April 26, 1970
Gaelic Park Aug 26, 1971
Copenhagen Apr 17, 1972
For a very brief time at the end of 1972, Jerry began to power his speakers with Mcintosh MC3500 tube power amps. In this configuration only 1 Fender Twin cabinet was used and both the Normal and Vibrato channels Preamp tapped to individual MC3500's. By 1973 he was using the MC2300 solid state amplifier for power with a single Fender Twin Reverb cabinet with both Vibrato and Normal channels Preamp tapped. Again, this produced half the output with reverb and the other half without.
Watkins Glen July, 1973
RFK, 1973
Alembic Guitar Player Interview
From the early day's, to the end of the "Wall of Sound" in 1974, the work done to Jerry's Twin(s) was done by Alembic and the "Alembicization" mods were done to them. During the "Wall of Sound" he used a Blackface Twin, Alembic Preamp Tap. Both channels were jumpered together with a jumper cable from the Vibrato channel Input 2 to the Normal channel input 1. The Guitar used input 1 of the Vibrato channel and the jumper cable connection allowed for the one signal to pass to both preamps. Each channel was preamp tapped seperately to each side of an MC2300. Once again producing reverb on only half the total output. A Budweiser "Bud Man" Sticker is placed on Jerry's MC2300 and this particular amp becomes a favorite of his that he would use almost throughout the rest of his onstage performances.
1974 - The Wall of Sound
Oct 19, 1974
After the "Wall of Sound", and subsequent hiatus, Alembic was no longer in charge of the gear but was used for maintenance from time to time mostly for instrument work. During the hiatus in 1975, Jerry begins using a Mesa Boogie Mark 1A preamped into an MC2300. However, at the Orpheum shows on June 1, 1976 an MC3500 can be seen in his rig as well as an MC275 on top of his speaker cabinets. He eventually moves to 2 Mark 1's preamped to MC2300's up to the Fall of 1976.
September 28, 1975
The Orpheum June 1, 1976
October 31, 1976
The Fender Twins, a Blackface and Silverface, come back in the fall of 1976 both housed in a modded Twin cabinet with speakers removed. The primary Twin used at this time was the Blackface and the MC2300 was used for power. It's believed by this time the JBL K120 was the speaker of choice. This setup was used for the rest of the seventies thru the eighties, with the Silverface Twin preamp becoming the primary sometime around 1982. With the trade deal lost with Rhodesia in 1979 the worlds supply of Alnico forced speaker companies to change there materials used and the move to JBL E120 ceramic magnet speakers began sometime in 1980.
Oakland Stadium October 1976
"My second amp was also a Twin Reverb. I'm still using it, but only as a preamp. From the preamp it goes to a Mcintosh 2300 that drives 4 JBL 12" speaker. It's simple and gives me another point of stability. The more elements you have stable, the more you can concentrate on your playing and not be continually adapting your technique to your equipment." J.Garcia - Guitar Player 1978 Interview
Oct 30, 1980
Mar 28, 1981
During 1982 Jerry starts to use the Silverface Twin as his primary preamp. Also, towards the end of 1982 and during 1983 Jerry uses an MC2500 instead of Bud Man, but by the end of 1983 he is back to the MC2300.
1983
June 21, 1989 _Summer Solstice
In late 1989 and through Spring 1990 Jerry is seen using a Crest 8001 Power Amp that sat on top of his Mc2300(Bud Man). By the Summer Tour 1990 it was not used again.
1989
Mar 24, 1990
The Fender Twins were retired in 1993 when Jerry began to use a Groove Tubes Trio preamp and Real Tube Reverb units direct to the soundboard. The Grateful Dead began to use an IN-Ear monitoring system which also retired the use of the MC2300 and JBL E120 loaded speaker cabinet, but JGB shows still utilized them and that is why Jerry's "90's" rack to this day still houses an MC2300. There was various techniques used in handling the TRIO's output to the board such as Speaker Cabinet Simulators, as well as offstage Mic'd Speaker cabinets.
Mar 14, 1993
;
June 26, 1994
July 9, 1995
The "90's" Rig
WARNING!
Any & All attempts to "Upgrade" or "Mod" your own amplifiers to these or any of the"Upgrades" shown is strictly at your own risk. Lethal Voltages that WILL cause death can be stored in the filter capacitors and any attempt should be made by a qualified technician.
The Details
While the insides of these amps have never been seen by the public, and obviously were "work in progress", these "Upgrades" to the Fender Twin reverbs are mostly Per "Insider's" Instructions and personal comments on the subject. There are still unknowns, and further research is ongoing. This is a well researched attempt to show how and where certain "Modifications" were done to Jerry's Twin Reverbs.
Note: Early Alembicization Mods known are in GRAY. NOT all of these mods lasted the duration as the gear was always subject for improvements. Also some of the mods became outdated or simply not used as the Amplifiers use changed from combo to preamp tapped.
The Preamp
1.) Preamp Taps - Alembic(A) & Vibrato Isolated(B) versions.
A.) The Alembicization Preamp Taps shown and in the notes below may have been used early on. All functions are accessable tapped or not. The B.) Vibrato Channel Preamp Tap - This drops the Vibrato circuit and it's Intensity potentiometers 50k load to ground when used as a preamp. All functions return to stock when used as a combo. This form of preamp tap was also done to the Normal Channel. (Note: Both of these taps have been confirmed to have been used but it is unclear when the change was made. However it is known that the Alembic version was used first.)
2.) Input Mod - Removal of the 68k Input Resistors to remove "Hiss" associated with them.
3.) Plate Resistors - Early "Alembicization" plate resistors were handpicked Allen Bradley Carbon Comp's. Sometime in the early to middle 80's Non Inductive Metal Film Military Spec'd Type RN65 or RN70 -
4.) Cathode Bypass Capacitors - Early Alembicization used Stock or Swapped Low ESR Aluminum Electrolytics were used. Sometime in the early to middle 80's these were swapped out for Military Spec'd Axial Solid Tantalum 150D series/CSR13's.
5.) Tone Stack ~ Early Alembicization used Stock values. Later Values were; Bass cap .022uf Polyester Film(Sprague 225p) / Mid Cap .022uF Polyester Film(Sprague 225p) / Treble Cap 250pF Silver Mica/ Bright Cap 120pF Silver Mica.
6.) ALL stock Fender Potentiometers were swapped out for ALLEN BRADLEY MIL SPEC TYPE J/ RV4 type.
The Power Supply
1.) Power Transformer - swapped stock 125p34a for export version 125p34ax
2.) Power Supply Filter Capacitors - Higher Voltage Filter Capacitors, "the largest you can fit in the can"(not shown).
The Power Amp Section
1.) Matched .1uF Output Coupling Capacitors
2.) Matched RN65 or RN70 220k Bias Splitter Resistors.
3.) Hand Selected Metal Film 100k & 82K Phase Inverter Resistors.
4.) Output Transformer - Upgraded for more Wattage when used only as a combo, ie.. no plug inserted into the preamp tap (Type Unknown) .
5.) Zener Diode - Bias Regulation - It's inception was to keep the power supplies stable with changing "house" voltages from venue to venue when used only as a combo, ie.. no plug inserted into the preamp tap (see note below), but with todays electrical standards, that voltages does not swing as much as the "old days". So, in essence this mod is a little outdated but is shown for completeness. It is a valid mod, but the Power Tube industry manufacturing processes of today leave more inconsistencies from tube to tube and manufacture to manufacture. If this mod is performed, it is recommended only NOS American Made 6l6's/5881 JAN tubes be used, or a very close spec'd equilvalent.
The Reverb Adjustments
1.) Hand select the reverb mix 3.3 Meg Resistor and 10pF Capacitor paralleled with it. A Silver Mica capacitor should be used for the 10pF to keep the tolerance tight.
2.) Hand Select the Reverb input 500pf off of the .02 uF coupling capacitor of the V4a grid network. A Silver Mica capacitor should be used to keep the tolerance tight.
3.) The stock Reverb 100k linear potentiometer swapped for a 100k Log/Audio.
4.) Jerry's Twins were stood on end so that the reverb pan was vertical to hinder any stage rumble from banging the springs against the pan.
The Alembic Input Mod:
Alembic Preamp Tap
This Tap does not isolate preamp channel's from each other and allows for the vibrato to still work when preamp tapped.
The Alembic Zener Diode Bias Regulation:
NOTE: THIS MOD IS OUTDATED AND SHOULD ONLY PERFORMED WITH "NOS" MATCHED AMERICAN MADE POWER TUBES AND ONLY NEEDED WHEN AMP IS USED AS A COMBO.
Alembicization Notes .PDF Courtesy of Janet Bowman
The AB763 Circuit Post Alembicization Modifications/Upgrades:
The Isolated Vibrato Channel Preamp Tap:
Jerry's Twin Knob Settings:
Volume - 4.5 (No More then 5 1/2)
Treb - 10 (All The Way Up)
Mid - 5.5
Bass - 1 (Completely Off)
Reverb 5 (Set to Room)
"Bud Man"
Jerry's #1 MC2300
Jerry's main Mcintosh MC2300, "Bud Man", was a remnant from the Wall of Sound. It was modded by Alembic with 1/4" input jacks versus the stock RCA jacks. Alembic also did the Banana Plug speaker jack modifications as well as configuring supports that held the amp chassis and the rack mounting rails to handle the rigors of touring. Later, Dan Healy stated it was modded to have it's Mcintosh Sentry Monitor(VI Current Limiter) disengaged/removed from the Driver Cards as well as "Hand Matched" Driver Transistors and Emitter resistors. . The Filter Capacitors Cans were custom made with a Higher Capacity and Voltages were increased for better filtering. The Output Transistors were Motorola 2n5303's, also "Hand Matched" with a curve tracer. Jerry used the 2 ohm tap of the MC2300, in Stereo Mode, using the Left Channel. The 12" JBL speakers were banana plugged.
Wall of Sound MC2300 with Alembic Mods
Note: Not Jerry's Actual MC2300, Just a Representation
of an actual Alembic Modded one.
Sentry Monitor "Lifted Leg" Disengage.
The Red X's are the legs that should be lifted to disengaged the Mcintosh Sentry Monitor.
| HT Speaker Clips | Preamp Mods | MC2300 | | JBL K120 | JBL E120 | Misc. |
| Irwin Interview | Custom Shop | Links |
| Contact |
Wald-Electronics © 2009 All Rights Reserved