Marshall JTM-45 and 62 Bluesbreaker Reissue Modifications

Posted on Feb 4, 2014

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 oth

Marshall JTM-45 and 62 Bluesbreaker Reissue Modifications
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er 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. 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. 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 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...

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