JTM-45 and '62 Bluesbreaker
This page is designed to help you get the best out of your Marshall
JTM-45 and Bluesbreaker reissue guitar amplifiers. It is a collaborative
Internet effort---see credits below.
||The man who has been nominated "President of the JTM-45 RI society," Ted is our expert source for technical information. Without Ted, this page wouldn't have been possible.
||Skip is responsible for the original schematics. He also sent in some documentation on the JTM-45 mods which he had saved prior to the creation of this page.
||He's responsible for the "heads up" on grid swamp resistors below. He's also the tube supplier for the author of this page---very highly recommended.
||Mike had quite a few saved discussions on the JTM-45/BB RI mods. From those discussions, the author was able to fill in some of the details on the purpose and effect of the mods.
|Brian K. Wallace
||Brian made the "readable" version of the JTM-45 schematic. Thanks, Brian!
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Ted B's JTM-45 Reissue to Original ("Real") JTM-45 Outline
A quick overview describing how to convert the reissues to original
The Sovtek 5881s that come in these amps are cheap and
sound like it. Change ALL tubes. Get a Sovtek GZ34,
Valve Art or GT KT66s, and Ei Gold preamp tubes. Rebias
the KT66s to around 40ma at idle. Do not get Sovtek or
"Golden Dragon" KT66s, as these are not KT66s and don't
sound like KT66s.
Change the filter cap near the GZ34 to a 32X32uF. The
other one at the other end of the chassis should be
changed to a 16X16uF. These two steps will make a big
difference and change the character of the amp.
To go further, do the following.
Make small parts changes according to the schematic and
Ken Bran's handwritten notes which are found on the
first schematics page in Doyle's Marshall Book.
Change (C3) 330uF at V1 cathode to 250uF. A low
voltage (25v) electrolytic is fine.
Change mix resistors (R16, R18) from 470k to 270k.
Change Ch I mix resistor bypass cap (C6) from 220pF to 500--550 pF.
The 100pF bright cap on Ch I volume pot can be kept or it can be removed if a thicker, blusier sound is sought. Earlier Bluesbreaker amps had it, later ones did not.
1k cathode resistor R15 on V2 was originally 820ohms.
The .1uF cap at the Presence pot originally was connected from the pot wiper (center terminal) to ground.
Change the output tranny to a Mercury Magnetics or
Obsolete Electronics JTM45 type. Their repro JTM45
trannies sounds spot-on. Mercury Magnetics makes a
repro of the Drake tranny used in '66 models, and an RS
tranny used in the early amps. Obsolete Electronics
makes the RS repro only. Either makes the amp sounds
smoother and has audibly better tonal dimension.
If your amp is a repro Bluesbreaker, strongly consider a
speaker upgrade. Celestion alnico blues sound like a
million $, but the Weber P12B "Blue Dog" is almost as
nice, costs less, and handles more power. Unlike
Greenbacks (old or new) both of these speakers sound fab
in an open back cabinet, and give that mid-60s Brit rock
sound. Take your pick. After making these changes, when
you play it, you'll find it hard to believe it is the
Lord Valve's Heads-Up on Using KT-66 Tubes With the JTM-45
Lord Valve's field reports indicate that using the Valve Arts
(Chinese) KT-66 tubes in the JTM-45 (reissues and
originals) can be problematic. The amp design is prone to
parasitic oscillation that may kill your tube. The solution:
put 5.6K swamp resistors in series with pin 5 on each power tube
socket. First unsolder the wire connected to pin 5, then solder
the 5.6K resistor directly to the pin socket. Re-connect the
wire to the other end of the resistor. The goal is to make the
leads on the resistor as short as possible.
Lord Valve also suggests putting 1K/5W screen resistors across
pins 6 and 4. Here pin 6 is used as a tie point for the screen
supply. These may already be present on a factory JTM-45 or
When taking the reissues off standby, a loud "bang" is often
heard. This can be remedied by putting a 0.1uF/600V capacitor
across the standby switch. Decreasing the main filter
capacitors to the "original" value of 32+32uF is also suggested.
A random collection of mods, unsorted as of yet. Some elaborations on
previously mentioned mods.
- Power Tubes:
The reissues come stock with Sovtek 5881 power
tubes. Virtually everyone agrees that re-tubing
is the first modification you should make to the
reissue. Generally, the KT-66 tubes are
recommended---the originals used these and they
sound great. Others have reported success using
EL34 tubes. EL34 won't give the most
"authentic" (i.e. original) JTM-45 tone, but it
may be a preferable sound depending on taste.
Remember, you always have to rebias after
changing the power tubes. Also note that KT-66
are very large tubes (physically), and the
Bluesbreaker Reissue may require some creativity
to accommodate these large tubes.
After installing KT-66 power tubes, rebias to
around 40 ma at idle (i.e. 70% plate
GEC KT-66, very expensive (about
$200/pair). If you have the money, and
want the sound...
Valve Arts (Chinese) KT-66. These are
probably have the best price/quality
Groove Tubes KT66HP are also recommended.
At least one person has reported success
with JJ/Tesla 6L6GC power tubes biased
around 35 ma.
- Preamp Tubes (12ax7/ecc83):
Note you can play around with different preamp
tube configurations. The "first" preamp tube
socket (the one farthest from the power
tubes) is the most important tone-wise. For
example, if you happen to have an actual NOS
(new old stock) tube laying around, you might
want to put it in the first preamp tube socket,
and fill the rest with Ei Gold or possibly JJ
New Old Stock (or "NOS" as they're
usually called) are the preference here,
if you can find them and/or afford them.
Mullards don't dissapoint.
Ei Gold ECC83. These may be hard to
find. Check to see what kind of testing
your tube supplier does for preamp
tubes, as many Ei-brand preamp tubes
have a tendancy to be microphonic. They
have a sparkly, brilliant sound.
JJ ECC83. These are the next-best
preamp tubes, and are generally more
robust. That is, they are typically
less microphonic, and often of very high
quality. They tend to have a little
less gain and a more round, mellow
- Rectifier Tube
Sovtek GZ34 seems to be the best one
Regardless of what kind of speakers you decide on, you'll
probably want them to be Alnico. (Alnico is an alloy of
aluminum, nickel and cobolt.) The alternative is ceramic
speakers, but virtually everyone agrees that Alnico is best
for the JTM-45/Bluesbreaker.
- 12" Speakers.
Many folks believe that the stock Bluesbreaker
RI speakers, Celestion Greenback Reissues, do
not work well in open back cabinets.
Apparently, the speakers do not have enough
magnet mass, which makes them sound mushy.
Celestion Alnico Blues. If you can
afford these, they are probably the way
Weber Alnico "Blue Dogs" (P12B). A
great speaker at a more realistic
Replacing the big filter capacitors will make the amp
less "boomy" and give it better clarity. The filter
capacitors are the large blue cans that live on top of
the chassis, with the tubes and transformers. The stock
reissues have two 50x50uF capacitors. The standard
modification is to change the power amp filter capacitor
(the one closest to the GZ34 rectifier tube) to a
32x32uF/600V LCR capacitor. The other can (the preamp
filter capacitors) should be changed to a 16x16uF/450V
LCR capacitor. This also puts less stress on the
rectifier tube, which should reduce the risk of
A note on these big capacitors: in case it's not
immediately obvious, the capacitor values given above
are in YYxYY format. These can capacitors are actually
two capacitors with a common negative terminal. That in
mind, it may be easier to find two 32uF/600V capacitors,
connect the negative terminal, and continue as described
above. The same holds for the preamp filter capacitors:
it is permissible to use two 16uF/450V capacitors with a
common negative terminal instead of the single
16x16uF/450 "can" capacitor (made by LCR or Sovtek).
Either way you do it, the electrical result will be the
same. The tonal result should be the same. Note
however, some people want to retain the aesthetics of
the JTM-45 chassis, and keep those big capacitors
sitting on top.
- Output Transformer.
Install a Mercury Magnetics or Obsolete Electronics
JTM-45 output transformer. This is usually considered
one of the more sound-changing modifications. It really
helps give the RI a more accurate JTM-45 sound---it
gives the sound the dimension of the originals.
Comparing the replacement tranny to the old one is like
flipping the mono-stereo switch on an old stereo. These
fine repro output transformers are virtually identical
to the original JTM-45 output transformers.
- Small Parts Changes
Replacing all carbon film plate resistors in
with carbon composition will give a slightly
"richer" sound. Carbon composition resistors
will probably increase background noise. The
background noise probably won't be as noticeable
if the amp will mostly be used at higher volumes
(i.e. the music will drown out the noise). On
the other hand, the carbon composition resistors
will give the amp more of a "vintage"
Replace all capacitors with Sprague Orange Drop
polypropylene caps or Mallory 150 capacitors.
These small parts modifications should further
increase the clarity of the amp. Capacitors in
the pF range should be of the silver mica
variety, or ceramic disks can be used for a
little extra "edge".
Change (C3) 330uF at V1 cathode to 250uF---the
correct value cap seems to make the amp perhaps
slightly less muddy.
Change mix resistors (R16, R18) from 470k to
270k. This seems to give both channels perhaps
slightly more gain.
Change Ch I mix resistor bypass cap (C6) from
220pF to 500--550 pF. The 100pF bright cap on
Ch I volume pot can be kept or removed. Try
disconnecting one leg of the cap and playing the
amp. Then reattach it and play it again. Leave
it the way you prefer.
1k cathode resistor R15 on V2 was originally
820ohms. The .1uF cap at the Presence pot
originally was connected from the pot wiper
(center terminal) to ground. I have not
examined the audible difference of these two
mods. They are so easy to correct to match the
original circuit, that it only makes sense to
change them while you are in there.
- Output Impedance.
The standard JTM reissue output transformer has a
primary of about 6.5k, which is about 65% higher than
your typical two-tube Marshall iron in an EL34 amp (e.g.
50w plexi). If you want more of an aggressive sound,
set the selector one notch higher than the load, i.e.
set it at 16-ohms for an 8-ohm cabinet, which tends to
make the tone much more Marshally. It has been
said that using the impedance selector in this manner
will make the amp louder and it will sound less
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Vintage amp service, repair, and sales. Great amp bulletin board---with lots of relevant JTM-45 and Bluesbreaker discussion. Also offers parts and accessories.
Alt.guitar.amps Web Pages
FAQs, technical information, business listings, and lots of other useful information.
Tube amp parts, building supplies. Now with a tech forum. They also sell a drop-in replacement plexi circuit board for Marshall reissue amps.
Amps and amp parts. Very information technical information section, with some FAQs.
This site's goals include providing general musical equipment information, with an emphasis on tube guitar amps, resources for those servicing musical equipment, and sales of custom or hard-to-find items. Of particular interest to the JTM-45 and Bluesbreaker community is this link.
Various high-quality parts and accessories. Many folks who perform the JTM-45/Bluesbreaker transformer mod purchase their transformers from Mercury Magnetics. The relevant transformer page can be found here.
The website says, "A variety of products for vacuum tube enthusiasts, guitar players, and repair technicians." They manufacture tubes and effects. They also have a line of capacitors, speakers, technician tools, transformers, etc.
Antique Electronics Supply
Vacuum tubes, books, transformers, capacitors, resistors, and parts.
Manufacturers of vintage style loudspeakers and other products for the vintage equipment enthusiast.
Official Marshall Website
||Some relevant information here (and this site wouldn't be complete without it). Find out about other Marshall products, etc.
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JTM-45 Reissue Schematic
A schematic of the Marshall JTM-45 reissue head.
Original JTM-45 Schematic
Here you can compare your reissue to the real thing (or verify the
correctness of your real thing).
'62 Bluesbreaker Reissue Schematic
A schematic of the Marshall '62 Bluesbreaker reissue combo amp. As
recent reissues go, the main circuit difference between the JTM-45 head and the
Bluesbreaker combo, the latter includes tremolo.
||This is a much more readable schematic. It was created by
Brian Wallace for the Vintage Amps Bulletin Board at Plexi Palace.
||A nice scan of the schematic that's in Michael Doyle's
The History of Marshall (ISBN: 0793525098). Sent in by
Yet Another Schematic
||Sent in by Ake Ong.