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 2-15" 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. Ron Wickersham stated that 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.
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.
Woodstock 1969
Fillmore East 1970
Poynette, Wisconsin April 26, 1970
Gaelic Park Aug 26, 1971
Copenhagen Apr 17, 1972
For a very brief time during 1972, Jerry powered his speakers with Mcintosh MC3500 tube power amps. The input buffer stage of the MC3500 was bypassed and the Twin's Preamp signal was fed straight to the Output Stage. 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. Normal Channel was used for monitoring.
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 (below) 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. The Normal Channel was used for stage monitoring. 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 Rehearsels between May 27th and July 12, 1976 an MC3500 can be seen in his rig as well as an MC2100 on top of his speaker cabinets and 2 Mesa Boogie Mark 1's . He eventually moves to 2 Mark 1's preamped to MC2300's up to the Fall of 1976.
September 28, 1975
The Orpheum Rehearsel June 1, 1976(possibly May 28,29 or 30th)
Soundcheck July 12,1976
October 31, 1976
The Fender Twins, Blackface and Silverface, arrive in the fall of 1976 both housed in a modded Twin cabinet. The primary Twin used at this time was the Blackface(With Master Volume) and the MC2300 was used for power. The JBL K120 was the speaker of choice. This setup was used for the rest of the seventies thru the eighties, with the Silverface (No Master Volume) preamp becoming the primary sometime around 1982. With the trade deal lost with Rhodesia in 1979 the worlds supply of Cobalt forced speaker companies to change from Alnico Magnet speakers. For Jerry 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 onstage, 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/onstage 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 from "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.
The Modifications come in 2 parts, 1.) Alembicization 1967/68 - 1974 and 2.) Post-Alembicization 1975 - 1993
~Alembicization Mods~
1.) The Alembic Preamp Taps shown and in the notes below may have been used early on. All Stock functions are accessable tapped or not.
2.) Input Mod - Removal of the 68k Input Resistors to remove "Hiss" associated with them.
3.) Plate Resistors - Alembic plate resistors were handpicked Allen Bradley Carbon Comp's.
4.) Cathode Bypass Capacitors - Alembic used Stock or Low ESR Aluminum Electrolytics.
5.) 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.) Output Transformer - Upgraded for more Wattage when used only as a combo, ie.. no plug inserted into the preamp tap (Type Unknown) .
2.) 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, but with todays electrical standards, voltages do 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 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
~Post Alembicization Mods~
Post Alembic Preamp Tap
Jerry used the Vibrato channel, but NEVER used the vibrato. Unlike the Alembic tap used prior, this preamp tap drops the Vibrato circuit and it's Intensity potentiometers 50k load to ground when used. All functions return to stock when used as a combo. This form of preamp tap was also done to the Normal Channel. The use of the Switchcraft 14b switching Stereo Jack allows for;
a.) regular preamp tap on one or both channels
- a mono plug will short the ring to ground of the 14b jack. if either
preamp tap is used with a mono plug the power section is rendered dead.
b.) preamp tap with a stereo plug and the isolated ring floats the ground
used to shunt the power section. this allows the other channel to be used
as
i.) a combo
ii.) a power amp input with a stereo plug - the ring would be the input.
a network box would be used if a cable was not made specifically to have
the ring conduct on the tip of the other end. the network box has a stereo
jack and it's ring and tip each have there own mono jack, the ring would
be converted by the box to be a tip for the input device.
c.) an effects loop - stereo jack and network box is needed to accomplish
this. the tip is preamp out and the ring is power amp in. when used as
a combo.
d.) cascading - plug into either channels input and
use it's individual preamp out with a stereo plug plugged into the other
channels input. the power section can be used, or it can be preamp'd out
by the second channel.
e.) jumper input 2 of either channel into input 1 of the other channel
and combine that into the power section or preamp both channels out isolated
individually.
AB763 Mods
1.) Input Mod - Removal of the 68k Input Resistors to remove "Hiss" associated with them.
2.) Plate Resistors - Non Inductive Metal Film Military Spec'd Type RN65 or RN70 -
3.) Cathode Bypass Capacitors - Military Spec'd Axial Solid Tantalum 150D series/CSR13's. Cathode resistors changed to Carbon Film.
4.) Tone Stack - Bass cap .022uf Polyester Film(Sprague 225p) / Mid Cap .022uF Polyester Film(Sprague 225p) / Treble Cap 250pF Silver Mica/ Bright Cap 120pF Silver Mica.
5.) ALL stock Fender Potentiometers were swapped out for ALLEN BRADLEY MIL SPEC TYPE J/ RV4 type.
6.) Vibrato Output Coupling Capacitor changed from stock .1uF to .022uF Polyester Film(Sprague 225p).
7.) V1b and V2b shared cathode wire is removed. V2b's cathode resistor is changed from the stock 820 ohms to a 1.5K Carbon Film. V1's preamp tube is removed. This mod completely isolates the Vibrato channel when preamp tapped by seperating the normal channal at the front with the shared cathodes and at the rear with this era's preamp tap.
(Note: This mod renders the normal channel inoperative. To make it operational, swap V1a's cathode resistor from 1.5k to 820 ohms and run a jumper wire from V1a's cathode pin #3 to V1b's cathode pin #8. lastly reinsert the V1 tube. This allows the normal channel to be used if wanted/needed, but also allows for the proper Jerry modded operation when the V1 preamp tube is removed. This is represented in the Post Alembicization Mods Layout below.)
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.
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.) 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.
Post Alembicization Mods Layout:
Jerry's Twin Knob Settings:
AB763 Preamp/ Mcintosh Combo
Volume - 3.5 - 4 (Never Above 5.5!)
Treb - 10 (All The Way Up!)
Mid - 5.5
Bass - 1 (Completely Off!)
Reverb 5 (Set to Room)
Twin Pre and Amp Combo
(Healy noted Settings for a Twin Reverb(AB763)Combo - These a/b'd with an AB763/Mcintosh combo render about the same results.)
Treb - 4 -7 (Depending on the Room)
Mid - 10
Bass - 3 (no more than)
Reverb - 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. Healy and Crew configured supports that held the amp chassis and the rack mounting rails to handle the rigors of touring. 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 4 ohm tap(see Pic below) of the MC2300 with the 3 - 12" speaker cabinet, in Stereo Mode, using the Left Channel. The 12" JBL speakers were banana plugged in parallel.
Wall of Sound MC2300 with Alembic Mods
Sentry Monitor "Lifted Leg" Disengage.
The Red X's are the legs that should be lifted to disengaged the Mcintosh Sentry Monitor.
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