1.The 5881s that come in these amps are a joke. Change ALL tubes. Get a Chinese GZ34, Valve Art or GT KT66s, and Ei or Tesla preamp tubes. Rebias the KT66s to around 40ma. Do not get Sovtek or Golden Dragon KT66s, as these are not KT66s and don't sound like KT66s. 

 2.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. 
           
3.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. 

1.Change (C3) 330uF at V1 cathode to 250uF. A low voltage (25v) electrolytic is fine. 
2.Change mix resistors (R16, R18) from 470k to 270k. 
3.Change Ch I mix resistor bypass cap (C6) from 220pF to 500pF. 
4.Change 100pF bright cap on Ch I volume pot to 500pF for a 'Lead' version JTM45.

According to Marshall literature, original model 1962 Bluesbreakers probably had no cap here ('Bass' configuration), although a 250pF or 100pF is possible in Marshall world. 
5.1k cathode resistor R15 on V2 was originally 820ohms. 
6.The .1uF cap at the Presence pot originally was connected from the pot wiper (center terminal) to ground. 
4.Change the output tranny to a Mercury Magnetics JTM45 type. Their repro JTM45 tranny sounds spot-on. The amp sounds smoother and has audibly better tonal dimension. FWIW, I believe Obsolete Electronics will have a JTM45 output tranny available soon. 
5.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 amp. 

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 also suggested. 

A random collection of mods, unsorted as of yet. Some elaborations on previously mentioned
          mods.

             1.Tubes. 
1.Power Tubes. The reissues come stock with Sovtek 5881 power tubes. Although some people like the sound of these tubes, most folks agree 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 probably 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 tube type. When replacing tubes of the same type, you almost always have to rebias, unless the replacement came from the same matched lot as the originals. Also note that KT-66 are very large tubes (physically), and
                    the Bluesbreaker Reissue may require some alterations to accommodate these large tubes. 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 ratio. 
Groove Tubes have been recommended, but you'd be best to go with the GEC or Valve Arts tubes. At least one person has reported success with JJ/Tesla 6L6GC power tubes biased around 35 ma. 

2.After installing KT-66 power tubes, rebias to around 40 ma at idle (i.e. 70% plate dissipation). 

3.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 probably 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 JJ/Tesla 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 and Telefunken 12ax7 preamp tubes definately won't dissapoint. 

Ei 12AX7. 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 tendency to be microphonic. 

JJ/Tesla 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.  


4.Rectifier Tube: Genuine Chinese GZ34 

2.Speakers. Regardless of what kind of speakers you decide on, you'll probably want them to be Alnico. The alternative is ceramic speakers, but virtually everyone agrees that Alnico is best for the JTM-45/Bluesbreaker. 1.12" 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 realistic price. 

3.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 of failure.

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.

4.Output Transformer. Install a Mercury Magnetics 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. The Mercury Magnetics output transformer is virtually identical to the original JTM-45 output transformers. 

             5.Small Parts Changes. 
1.Replace all carbon film resistors in the signal path with carbon composition. 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. 

2.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. 

3.Change (C3) 330uF at V1 cathode to 250uF---the correct value cap seems to make the amp less muddy. 

4.Change mix resistors (R16, R18) from 470k to 270k. This seems to give both channels noticeably more gain.
 
5.Change Ch I mix resistor bypass cap (C6) from 220pF to 500pF. Change 100pF bright cap on Ch I volume pot to 500pF for a 'Lead' version JTM45, or just remove the 100pF bright cap for BB. Addressing the issue of these capacitors is very important in giving the amp its old 'EQ' setting. These steps makes a very noticeable difference in the sound, almost as much as changing the tubes! 6.1k 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. 

6.Output Impedance. The JTM reissue output transformer has a primary of about 6.5k, which is about 65% higher than your typical two-tube Marshall iron. This might be because some of the very earliest JTMs had a similar unit, or because they simply wanted to further differentiate
the sound from the 50W reissue. Regardless, if you run the impedance selector set as marked, you'll not get more than 25W out of one, and the waveform will be squashed---typical of upward mismatch. If the plate voltage was higher than it is, it's likely that flyback arcing would be a problem. If you like the tone as it is, you won't hurt anything, most likely. But if you set the selector one notch higher than the load, i.e. set it at 16-ohms for an 8-ohm cabinet, your
amp will be happier, you'll see 40W or so, and the tone will be 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 "constricted."