Download Fender Deluxe Reverb Reissue Modifications

Fender Deluxe Reverb Reissue Modifications
April 8, 2001
Posted by swingin.little.guitarman on Ted Weber’s Amp Board
10/8/2003
Part 1 External Modifications
Tube Replacement, Bias, Speaker Replacement
I just finished modifying a Fender 65’ Deluxe Reverb reissue amp, and
had promised to share the details. This amp was a 1994 blonde tolex
reissue. Serial number was BLD-00021, one of the first of the reissue
amps. It belongs to my good friend “The Hof.”
The 65’ Deluxe Reverb is Fender’s attempt to bring back the famous
blackface amp of the 1960’s. The general complaint concerning the
reissues is that they don’t sound like the old 60’s amp. They tend to be
overly bright and brittle sounding, particularly at low volumes. The
overdriven harmonics tend to be harsh instead of smooth and
compressed. Player touch sensitivity and dynamics are virtually nonexistent. These are all valid complaints on the reissue amps I’ve
played.
The construction of the reissue amps versus the point to point wiring
of the original amps makes these amps a bit more difficult to modify.
Many people shun these amps simply because of the circuit board
construction. I’ve found the circuit board construction to be a very
small factor in the overall SOUND of the amp. The choice of
components and their values is the biggest factor in determining the
overall character of the amp. By modifying or replacing selected
components I was able greatly improve the sound of this Fender 65’
Deluxe Reverb reissue. In my opinion, this amp sounds BETTER than
many of the original blackface Deluxe Reverbs I’ve played.
The goal of this project was to evaluate the amp, and bring back as
much of the old 60’s sound without a total rebuild. The maximum parts
and materials budget was $100, not including labor. The cost of the
replacement speaker used can substantially increase the cost beyond
$100. New replacement speakers can be purchased in the $35~$100
range.
All of the speakers used for testing in this project were used.
The standard precautions apply when working on tube amplifiers apply
to this project.
1. The high voltages in these amplifiers can be dangerous. Make sure
the amp is unplugged and all power supply capacitors have been
discharged to prevent nasty and potentially deadly shocks.
2. The circuit board construction requires patience and a delicate touch
to prevent damaging the board traces. A good 20 to 30 watt soldering
iron and solder sucker are required. NO WELLER SOLDERING GUNS!
3. MOST IMPORTANT. If you are not familiar with and had experience
working on electrical circuits STOP. It is better to pay an experienced
tech to make these modifications for you than to risk injury or
damaging your amp.
Before disassembling the amp, I evaluated the amp and tried replacing
a few external components to improve the overall sound. I removed all
the preamp tubes from the amp. The stock Fender preamp tubes were
Chinese made 12AX7’s and 12AT7’s. None of the tubes were noisy or
micro phonic. After trying a variety of tubes I settled on a combination
of NOS, new manufacture, and stock tubes for best tone. The normal
channel preamp tube was not used. The socket was left empty
(farthest on the right when viewed from the back of the amp). The
owner seldom uses this channel so why burn the tube? Removing the
tube from the socket changes the voltage/current of the Vibrato
channel preamp tube. It provides a little more warmth and a slight
increase in gain. If you regularly use the normal channel replace the
tube. The change is tone is slight. The vibrato channel tube (second
from the right when viewed from the back of the amp) was replaced
with a NOS Phillips 5751. These are great sounding tubes with a little
less gain than the stock 12AX7, but the tone and harmonics are
superior to the stock Chinese tube. Also excellent was the GE 5-Star
5751 tube, however these are getting rather pricey. The Phillips 5751’s
are still available for about $15.
The reverb driver tube (third from the right when viewed from the
back of the amp) was replaced with a RCA 12AU7. This tube is a low
gain tube that is typically used as a driver in high-end amps. The gain
is about 20% of a 12AX7 and 28% of the stock 12AT7. This particular
amp had a bid of a mushy overdriven reverb sound. Beyond 4 on the
reverb knob was about useless. This tube helped to tame some of the
gain and cleaned up the reverb. There is a bit more detail to the
reverb that was lost with the original Chinese 12AT7.
The reverb return tube (fourth from the right when viewed from back
of the amp) was replaced with a new JJ 12AX7S. This is JJ’s latest
12AX7 version and is an excellent all around tube.
The vibrato tube (fifth from the right when viewed from the back of the
amp) was left stock with the Chinese 12AX7 tube. This is the least
critical tube in the amp. Any good, 12AX7 free from micro phonics is
acceptable. Changing to a higher quality tube had virtually no audible
impact on the sound of the amp.
The power tube driver/phase inverter tube (sixth from the right when
viewed from the back of the amp and next to the large power tubes)
was replaced with a JJ 12AT7. Some people like to replace this tube
with a good 12AX7 for more gain and drive. The 12AX7 gave a little
more warmth and drive which was welcome when playing at lower
volume levels. It’s certainly an option. As a complement to the new
power tubes the JJ12AT7 was judge best for overall sound.
The power tubes (seventh and eight from the right when viewed from
the back of the amp) were replaced with new Electro Harmonix 6V6’s.
These are the best tubes for the money that I’ve found. They are
designed to handle higher plate voltages than the stock Sovtek tubes.
Most of the other new replacement tubes weren’t even close to the
Electro Harmonix 6V6’s in detail and harmonic content. I think they are
the best of the new 6V6’s available at this time. I installed a set of my
RCA black plate 6V6’s as a comparison. The RCA’s are still the best
sounding of the 6V6’s. Definitely, the best choice if you’re not on a
budget. However they are now $95 a pair and I wanted some money
left in the budget for a replacement speaker. I have tried the Brimar
6V6’s in other amps with excellent results. I would rate them on a par
with the RCA tubes. I did not have any Brimar 6V6’s available for
testing during this amp project.
The rectifier tube (farthest tube to the left when viewed from the back
of the amp) was left with the stock Sovtek 5AR4. These are good
tubes. Some people replace this tube with a NOS 5A4-G to reduce the
voltages in the amp. Deluxe Reverbs are designed with the plate
voltages on the power tubes exceeding most manufacturers’ design
limits for the tubes. The higher voltages can produce some problems
when used with many of today’s new 6V6 power tubes. I have not
experienced any problems with the higher plate voltages and the
Electro Harmonix 6V6 tubes. The plate voltage in this reissue amp was
only a modest 407 volts. This is really quite low compared to many of
the Deluxe Reverbs I’ve worked on. Many have plate voltages in the
450~460 volt range. This was probably an intentional design change
on the part of Fender to allow for the operating parameter of the new
6V6’s. Since Fender has a short warranty on their amp tubes, this may
have insured the stock tubes would last the new owner a reasonable
period of time.
The factory bias setting for the power amp tubes left a lot to be
desired. I attributed much of the amps brittleness and lack of warmth
to under-biased power tubes. I checked the plate voltage of the amp
by removing the power tubes, turning on the amplifier, turning off the
standby, and measuring the voltage from the number 3 pin on the
power tube socket to ground. As stated above, the voltage measured
407 volts. I turned off the amp and reinstalled the power tubes. I
turned the amp back on, turned off the standby, and allowed the tubes
to warm up for approximately 5 minutes. Using my Allesandro bias
meter I measured the tube current at 20 mA. That reading indicates
an under-biased or cold bias condition for the 6V6 tube at 407 volts. A
6V6 tube has a maximum designed plate dissipation of 14 watts. In a
fixed bias, class A/B amp the standard practice is to bias the tubes for
70%~80% of the maximum plate dissipation. The stock tubes were
biased at 58% of maximum plate dissipation. With the new Electro
Harmonix tubes installed I adjusted the bias for 27mA, yielding 11
watts of plate dissipation (0.027 amps * 407 volts = 11.0 watts) That
works out to 78% of the rated maximum. For amps that are used at
lower volume settings I’ve found biasing the tubes closer to 90% of
maximum gives a little more warmth. With the Electro Harmonix tubes
as well as the RCA black plates I have biased some amps as high as
100% with no detrimental effects. The tubes run warmer, but the
sound at “bedroom” levels is improved. . The adjustment is not an
exact science. I adjust the bias pot with more concern for the overall
tone of the amp rather than an exact bias level. If the power tube
plates begin to glow red; the bias setting is too high. Reduce the bias
current. Within the 70%~100% bias range use your judgement to
select the best sounding current setting.
So what was the verdict of the tube and bias changes to the amp? A
HUGE improvement, much more than I would have expected. The amp
was still very bright, but much of the brittleness was gone. The tone
was much warmer, particularly at lower volumes. The amp picked up a
little sensitivity to my playing. The reverb was still a little deep for my
taste but the detail and clarity was a great improvement over the stock
sound. The total cost about $55. The total time spent was about two
and a half hours; most of the time plugging and playing tubes. If you
had your tubes already pre-selected you could probably do the job in
about 30 minutes. Definitely the “best bang for your buck” in terms of
money and labor to improve the sound of the amp.
The next external amp mod I made was a change of speaker. The
stock Fender speaker (made by Eminence) is not the best sonic choice
for the amp. This amp came with an older blue frame speaker, but it
still left a lot to be desired. I tried a variety of speakers in the amp.
Most of the replacement speakers sounded very good and were an
improvement over the stock speaker. I found that speaker choice
really comes down to personal preference. I personally preferred the
sound of “American” sounding Jensen style speakers as opposed to
“British” sounding Celestion style speakers in this amp. In the stock
amp with the new tubes and bias setting I preferred the Weber C12NB.
This is Weber’s C12N speaker with a British style cone. Hands down,
the best sounding speaker I’ve heard in a Deluxe Reverb. The British
style cone gives a little crisper top end, but retains all the fat lower
end of the original C12N. It’s great for clean and overdriven sounds.
It’s not a super efficient speaker so you don’t have to turn your amp
way down to play at lower volumes. The plug and play experience was
a lot of fun. I probably spent 4 hours comparing speakers. I found that
despite all the speaker specifications and description there was no
substitute for installing the speaker and playing the amp. All the
speakers tested were installed in the cabinet while testing. All the
speakers tested were either used or sufficiently “broken in.” The test
guitars used were a parts strat with Bartolini 3X pickups (very clean
and dynamic), an American Deluxe Strat with Gtarman custom wound
pickups similar to Fralin Vintage Hots (thanks Jeremy), a Yamaha AEX
502 semi hollow body with P-90 pickups, and a 1968 Les Paul with
original PAF’s (on loan but one can always hope). Below I’ve listed the
speakers I tested in the amp with my comments.
Weber C12NB
This is Weber’s C12N speaker with a British style cone. Hands down,
the best sounding speaker I’ve heard in a Deluxe Reverb. The British
style cone gives a little crisper top end, but retains all the fat lower
end of the original C12N. It’s great for clean and overdriven sounds.
It’s not a super efficient speaker so you don’t have to turn your amp
way down to play at lower volumes.
ElectroVoice EVM 12L
A very good speaker with an excellent smooth sound. Too efficient a
speaker to allow you to crank the amp and maintain lower volumes.
The best choice if you need extra headroom and volume for gigging
while maintaining a smooth top end. Very heavy. Made me wish the
amp had wheels.
Original Fender Orange Basket JBL D-120
A bit shrill for my tastes but sounded good with the hum bucking
pickups. A very efficient speaker so it’s difficult to maintain lower
volumes. May be just the ticket for ultra clean players, but not for
overdriven or distorted tones.
Kendrick Black Frame
An excellent speaker for clean and overdriven sounds. It sounds
similar to a Jensen C12N with a smooth top end but a much tighter
bottom. The bass remained clear and tight even at highly overdriven
volumes. A little less sparkle and chime than the Weber C12NB. A
recommended choice for those who play with the amp cranked.
Naylor Special Design 50
Very similar to the Kendrick Black Frame, but with a looser bottom
end. The best sounding speaker at low volume levels. The bottom end
is a bit too loose for high volume overdriven sounds. The top end does
not have the sparkle and chime of the Weber C12NB. It’s a little dark
sounding with the hum bucking pickups.
Weber C12B “Blue Dog”
Nice chime and top end. A very good speaker for moderately
overdriven, bluesy tones. The bottom end is not as tight as the
speakers listed above. It has a bit too much cone breakup for ultra
clean country sounds. This speaker is a best choice for those looking
for a British Celestion type speaker. Better than the Celestion Vintage
30.
Weber C12S
This is a great sounding speaker. Not quite as much low end as the
C12N type but lots of bright clean top end. A great speaker for low to
medium volume, ultra clean country sounds. Has some cone breakup
at higher overdriven levels. A good choice for those who like the sound
of original 1960’s Oxfords.
Celestion Vintage 30
Many people like this speaker, but in lower powered combos I don’t
like it. It has a gritty edge that doesn’t lend itself well to cleaner
sounds. If you like bluesy overdriven or high level distortion tones this
speaker may be a good choice. It has good efficiency so you get a bit
more volume and headroom from the amp.
Part 2 Internal Modifications
Component Replacement and Circuitry Modifications
This is the second in a two-part project for modifying the Fender
Deluxe Reverb Reissue Amps. The objective of this project is to modify
the reissue amps and come as close as possible to sonically duplicating
the original 1960’s blackface models for under $100 in parts and
materials. My first post focused on tube replacement, bias adjustment,
and speaker replacement. This post focuses on internal circuitry
modification and component replacement.
The Fender Deluxe Reverb reissue amps maintain the integrity of the
original 60’s blackface circuitry, however construction is modern circuit
board based rather than the point-to-point construction used in the
originals. Modification of these circuit board amps requires more care
and patience than the PTP amps. The circuit board traces can be
destroyed by using the wrong solder (Use only ROSIN core solder, not
plumber’s acid core solder), high wattage soldering guns (A 15-30 watt
soldering iron is all that is required.), or excess use of solder (Don’t
pile it on! Try to duplicate the factory connections in size.). If you do
not have experience with soldering or electronics I would recommend
you take my posts to a good, TRUSTED, amp tech or repairman. They
should be able to complete these modifications at a nominal cost.
Damaged circuit board traces can be repaired, but should be avoided.
The Fender Deluxe Reverb Reissue amp circuit boards are fairly sturdy,
but you can break or crack them if you use excessive force to remove
or bend them for access. I have tried to describe the modification
process as accurately as possible to help you avoid making any
mistakes, but there is no substitute for care and common sense.
The first step in working on the internal circuitry is to UNPLUG the amp
and discharge all the stored voltages in the system. To discharge most
of the voltages, first, UNPLUG the amp. Turn the amp’s power switch
to the “ON” position and turn the standby “OFF.” To completely
discharge all the capacitors I use an eight inch wire with alligator clips
on each end. Attach one clip to pin #1 of any preamp tube. Attach the
other clip to any point on the metal chassis. All of the voltages in the
amp are bled to ground within 10 seconds.
The second step in accessing the internal components of the amp is to
remove the top rear panel of the amp. This allows full access to the
chassis, tubes, and rear panel connections.
Next unplug the speaker from the back panel of the amp. Unplug the
reverb tank send and return cables from the underside of the chassis.
Note the red cable is connected to the RCA jack with the red dot next
to it. If your amp does not have the red dot I would recommend using
a black permanent marker to write “R” and “W” beside the appropriate
jacks so you can connect the cables properly upon reassembly.
Remove the small Phillips head screw that secures the power cable to
the inside of the cabinet. I always use a small plastic container to hold
all the screws and brackets so I don’t loose them while I’m working.
Most of these screws and brackets are NOT available at your local
hardware store. If you loose them you are out of luck and will have to
order them from Fender or another parts supplier. Remove all the
tubes from the amp chassis. It’s not worth the chance of breaking a
tube while you’re handing the chassis on your table or workbench.
Loosen the four chassis mounting screws on the top of the amp. The
nuts for these screws are located on the bottom of the chassis. The
nuts are not attached to the chassis and will require a small wrench to
hold them while you loosen the screws. After removing all four nuts,
you should be able to pull the chassis straps on top of the amp and
remove each pair of screws with it. There are small Phillips head
screws that secure the chassis to the top of the wood cabinet. Before
removing these I recommend turning the cabinet upside down with the
top of the cabinet flat on the table or workbench. Remove the screws
and set them aside. The amp chassis will now slide out the BACK of
the cabinet. I usually hold on to the transformers and lift slightly to
avoid scraping the tolex.
Set the chassis on your table or workbench. Use a small flathead
screwdriver to loosen the small screw holding each control knob to its
shaft. Remove the knobs from each potentiometer shaft. Use a
PROPERLY sized nut driver or socket wrench to remove the nuts
attaching each input jack and potentiometer to the front chassis panel.
Use care not to scratch up the front faceplate. It is not necessary to
remove the power light or front faceplate.
Look inside the top of the amp. All the volume, tone, reverb, and
vibrato potentiometers are mounted on a single long circuit board. The
board must be removed to access some of the tone circuitry. Viewing
the inside of the amp from the rear, there is a small nylon straplock
that attaches this board to the main circuit board. It is located on the
left side of the amp near an upside down potentiometer (the bias
adjustment pot). Use a small pair of diagonal pliers to cut this
straplock. This frees the control circuit board from the main board.
There are three ribbon connectors that wire the control circuit board to
the main board. Carefully pull these connectors loose from their
sockets in the main board. The control circuit board is now free and
can be carefully removed from the chassis.
Look at the control circuit board. It contains many small resistors and
capacitors as well as the control potentiometers. You can change the
tonal voicing of the amp by replacing the capacitors and resistors on
this board. It is extremely important to replace the resistors and
capacitors with voltage ratings that MEET or EXCEED the stock values.
That means capacitors should be good quality Mallory 150, Sprague
716P(orange drops), or other manufacturers with 400-600 volt ratings
(600 volt preferred). I used Sprague orange drops for this project, but
have had equally good results with some of Angela Instruments’ new
high quality replacement capacitors. I would not recommend going to
large, high voltage capacitors because of the size constraints. There
isn’t a lot of room, particular on the control circuit board, for the larger
capacitors. For resistors I used ¼ watt metal film resistors. I have
tried carbon composition resistors, but could not detect any sonic
benefit. Again, space is a premium on this circuit board. Check before
soldering any large components to be sure you have enough clearance
to reinstall the board in the chassis.
The final capacitor and resistor values I selected for this amp project
are not etched in stone (Stick with the recommended voltages). You
are free to experiment with values to suit your sonic tastes. The values
I recommend are the results of experimentation to match the original
1965 amp’s tone.
Cheap, surplus components are a waste of time for this project. The
goal is to improve the sound of the amp as much as possible within the
budget constraint. The total cost of all the high quality resistors and
capacitors used for this project was less than $10. The time invested in
this project is worth the couple of extra dollars for top quality parts.
The Normal channel of this amp does not get used. The modifications
performed to the control circuit board in this project will only effect the
Vibrato channel of the amp. You can modify the Normal channel as
well. It is very similar to the Vibrato tone circuit. For this project it was
left stock.
The Vibrato channel of Fender blackface Deluxe Reverbs has a small
capacitor attached to the volume pot. This 47pF (pico Farad) capacitor
is labeled C10 on the circuit board. This capacitor acts like a bright
switch that is always turned on. Removing or replacing this capacitor
makes a HUGE difference in the overall brightness of the amp. For
many people simply removing this capacitor will remove most of the
harsh brightness in the amp. I liked the amp with no bright cap
installed. When tested with single coil (stratocaster, telecaster, and
P90) pickups the tone was excellent. When tested with a Les Paul w/
original PAF’s and a 335 Dot reissue w/ Gibson ‘57 Classics the tone
was still very good although somewhat darker sounding. High gain
humbuckers tended to sound muddy. I replaced the capacitor with one
valued at 10pF, but found the high end harsh with traditional single
coils. For most applications I would recommend removing the bright
capacitor completely for use with single coils or replacing it with a 5pF
or small capacitor for humbucking pickups. This option is a matter of
personal preference. To match the tone of the original 1965 Deluxe
Reverb amp I selected a value of 0.5pF. This helped to retain some
presence that was absent from the reissue amp when the original
capacitor was removed. The tonal presence in the reissue amp was not
an exact match for the 1965 original. I tried several capacitor values,
but was never able to obtain a perfect match.
The treble tone pot has a 100Kohm resistor labeled R18 on the circuit
board. This “slope resistor” can be changed to a SMALLER value to
help remove some of the brightness from the amp. The smaller the
resistor value the more treble is removed from the circuit. I
experimented with values from 69Kohm to 110Kohm. A value of
90Kohm mostly closely matched the sound of the 1965 original.
The treble tone control uses a 250pF capacitor labeled C7 on the
circuit board. By changing this cap to a larger value 250pF-500pF it is
possible to roll off more of the high frequencies. Initially, I replaced
this cap with a 450pF silver mica cap. The result was an amp that was
capable of rolling off most of the treble frequencies and accompanying
harshness. However, future modifications resulted in an amp that was
TOO dark. I eventually replaced the 450pF with the stock 250pF value
(upgraded to a silver mica cap) to match the original 1965 amp. If you
later find the amp too bright for your tastes and would like to be able
to roll off more high frequencies with the treble control then replace
the C7 cap with one valued in the 400pF to 500pF range.
The reissue amp lacked the midrange warmth of the 1965 original.
R21 is a 6.8Kohm resistor that permanently sets the midrange of the
amp. It is located next to the bass control potentiometer on the circuit
board. I increased the value of this resistor to 8Kohm and finally
10Kohm. At 8Kohm the amp retained a more pronounced “scooped
mid range” tone for which blackface amps are famous. The 10Kohm
resistor more closely matched the sound of the original 1965
comparison amp. For thick, overdriven tones you may want to try
values as high as 20Kohm. 12Kohm to 15Kohm will add considerable
midrange punch to the amp. For those who prefer the sound of tweed
Fender amps this may be an option you prefer.
Approximately 15 years ago I read about a modification that was
commonly done to old Fender blackface amps. It involved upgrading
two of the tone/coupling caps labeled C8 and C9 on the circuit board
with high quality 0.022mF (micro Farad) capacitors. I’ve found this
modification to be a great improvement in tone for original blackface
amps was well as the reissues. The two capacitors are mounted side
by side on the circuit board. I used 600 volt Sprague “orange drop”
caps in this amp. The size of the replacement capacitors and the
limited space on the circuit board eliminates the possibility of mounting
the capacitors side by side. I mounted one capacitor in its original
position. I left the second capacitor with longer leads and mounted it in
further towards the edge of the circuit board. BEFORE soldering the
second capacitor in place I insulated the leads with small pieces of
heat shrink tubing. I have used quick dry epoxy when heat shrink was
not available. It is very important the soldered capacitor leads do not
touch one another or the sides of the potentiometer cases. Try to keep
the capacitor as close to the circuit board as possible. This modification
is the only area where space was a premium. I STRONGLY recommend
you test fit your modified circuit board back in its original position in
the chassis. Depending upon your location of the second capacitor it
may be difficult to reposition the board back into the chassis without
bending the capacitor leads. Again, make sure your capacitor leads are
not shorting against each other or other components.
That’s it for the control circuit board. DO NOT mount the board back
into the amp chassis yet. You will need to access the bottom of the
main circuit board. You will have much better access if the control
circuit board is not mounted in the chassis.
The main circuit board is mounted in the bottom of the chassis and
secured by a number of small Phillips head screws. In order to solder
components to the board you must gain access to the underside of the
board. The first step is to remove the small Phillips head screws. Some
may be hidden under wire harnesses so check closely that all screws
have been removed before attempting to remove the board. The bias
adjustment potentiometer is wired to the main circuit board and
attached to the chassis. Remove the nut on the underside of the
chassis. This allows the bias pot to be lifted with the main circuit board
without straining the wire connections. I did not remove the circuit
board from the amp to perform the remaining circuit modifications. I
used a small foam “nerf ball” to help support the board in a raised
position so I could access the soldering connections underneath. If you
decide to remove the board entirely (not recommended) use care to
avoid damaging any wiring harnesses or connections. Removing the
board will require wiring removal and replacement. During
replacement you will need to duplicate the original lead dress (wiring
layout) from the main circuit board to the rear panel connections.
The coupling cap from the Vibrato channel is label C12 on the main
circuit board. To duplicate the original 1965 amp tone replace this
0.022mF capacitor with a 0.03mF capacitor rated at 600 volts. If you
are looking for a little more warmth and breakup at lower volume
levels try using a 0.047mF capacitor. The larger cap value will help the
amp’s tone and overdrive at “bedroom and apartment” playing levels.
If you want to maintain the headroom of the original amp use the
stock 0.022mF or 0.03mF values.
Up to this point all the circuitry changes have affected the Vibrato
channel only. The modifications from this point on will affect both the
Vibrato and Normal channels of the amp.
Capacitor C25 is the coupling capacitor between the preamp and phase
inverter (power amp driver) circuitry. The stock component is a
0.001mF capacitor. The Fender silverfaced Deluxe Reverbs actually
replaced this capacitor with a 0.01mF value for IMPROVED tone and
gain! In order to duplicate the sound of the 1965 original amp I
replaced the original capacitor with one valued at 0.01mF. With single
coil pickups the amp will begin to overdrive with the volume set at 4-5.
Standard humbucking pickups will start to “break up” at 3-4. Changing
this capacitor to a 0.02mF value will give more warmth and breakup at
lower volume levels. With single coil pickups the amp will begin to
overdrive with the volume set at 2-3 With standard humbucking
pickups the amp will start to “sing” with the volume set at 5-6.
The final modification I performed to the reissue amp was to decrease
the negative feedback. Looking into the chassis from the rear of the
amp you will see a small gray wire from the speaker output jack to the
main circuit board. There is a 820ohm resistor on the main circuit
board that limits the amount of negative feedback from the output jack
back into the phase inverter circuitry. Reducing the negative feedback
(by INCREASING the resistor value) gives the amp a little more
harmonic richness. Some people prefer no negative feedback. To
eliminate the negative feedback entirely simply cut the gray wire and
insulate the ends to prevent shorting. The elimination of negative
feedback can be a problem in some cases. Parasitic oscillation can
result causing uncontrolled feedback, squealing, and in some cases
apparent shutdown. I did not eliminate the negative feedback circuit
from amps. I prefer to install a small 2Kohm trimpot in series with the
original resistor. My past experience has shown that most Deluxe
Reverbs sound best with approximately 1500ohm of feedback circuit
resistance. This effectively cuts the stock negative feedback in half. By
using a trimpot I can adjust the resistance to get the best sound. Once
adjusted, I typically leave the setting alone. I cut the gray wire in the
middle and solder the ends to the lugs of the trimpot. I use a small
dab of clear silicone to attach the trimpot to the bottom of the chassis.
DO NOT push the trimpot down into the silicon so the connection
comes in contact with the chassis. The silicon will hold the trimpot in
position and keep it from moving about and shorting other electrical
components. To match the sound of the original 1965 amp the trimpot
was turned up about ¼ to ½. This should be giving a final resistance of
350ohm to 1000ohm + the original 820ohm = 1170ohm to 1820ohm.
The final adjustment to this pot is made with all the circuit boards,
connections, and tubes reinstalled. The adjustment is not an exact
science. I adjust the pot with more concern for the overall tone of the
amp rather than the exact resistance value of the feedback circuit.
Reinstall all the main circuit board in the amp and connect the three
ribbon cable connectors to the control circuit board. Bolt the bias pot
onto the chassis. Use a new straplock to connect the control and main
circuit boards together. Slide the chassis into the cabinet and connect
the speaker cable and reverb tank cables to the chassis. Reinstall the
tubes. DO NOT bolt the chassis to the cabinet at this time. You will
need access to the negative feedback trimpot while playing the amp.
Adjust the trimpot for the best sound. Once you’ve dialed in the
negative feedback setting, bolt the chassis back into the cabinet.
That’s it, the complete set of circuit modifications with a few tonal
options. The result was an amp that came extremely close to matching
the tone and player sensitivity of the original 1965 Deluxe Reverb. The
sound was SUPERIOR to three other original blackface Deluxe Reverbs
used for comparison. In my opinion the results of the project were a
success. The modified reissue amp is comparable with a GOOD
sounding original blackface Deluxe Reverb. The total cost for the
project was just over $100 including the speaker, tubes, and electrical
components. I invested about 40 hours experimenting, testing, and
documenting the project. A good electronic tech could easily duplicate
the speaker, tube, and circuitry modifications in fewer than two hours.
Best of all, nearly all these tweaks and modifications are applicable to
original blackface and silverface Deluxe Reverbs as well as the
reissues. Many of the mods can be applied to ALL Fender blackface and
silverface tube amps.
There are a few modifications I did not make to the amp because of
the time and budgetary constraints. Replacement of the output
transformer with a high quality replacement model would probably be
the next logical modification (Hoffman Amplifiers, Kendrick, Mojotone,
and others have high quality replacements that sound like the
originals). The cost, $50 to $100, would have exceeded the budget
limitations, but would have yielded worthwhile tonal improvement
based upon my past experience. The replacement tubes I selected for
this project were also limited to the funds at hand. Better substitutes
are available, and would improve the tone of the amp, but at a higher
cost. It is possible to improve the presence of the overall sound by
adding a capacitor across BOTH the added negative feedback trim pot
AND the existing negative feedback resistor. Values from 0 to 1.0mF
will give the effect of a presence control from off to full on. It’s an
inexpensive mod that allows you to control the “sparkle” of the overall
sound.
Posted by swingin.little.guitarman on Ted Weber’s Amp Board
10/8/2003