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
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
- 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
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 same
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 Bluesbreaker reissue.
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
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
- 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
- Groove Tubes KT66HP are also recommended.
- At least one person has reported success with JJ/Tesla 6L6GC
power tubes biased around 35 ma.
- After installing KT-66 power tubes, rebias to around 40 ma
at idle (i.e. 70% plate dissipation).
- 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 ECC83 tubes.
- New Old Stock (or "NOS" as they're usually called) are the
preference here, if you can find them and/or afford them. Mullards
- 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
- 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 sound.
- Rectifier Tube
- Sovtek GZ34 seems to be the best one available.
- Speakers. 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 to go.
- Weber Alnico "Blue Dogs" (P12B). A great speaker at a more
Filter Capacitors.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
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
- 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" flavor.
- 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
- 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.
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Last update: Sun Jul 25 19:40:40 2004