Ted Breaux   (tabreaux@neworleans.com)
    Date:
         7/12/2000 5:14 AM
 Subject:
         'Authenticating' a Repop JTM45



Due to popular demand, I'll post it again. Going from memory, to get
a RI JTM45 to sound like an original, do the following in order:

(1) Change ALL tubes. Get a Chinese GZ34, Valve Art or GT
KT66s, and Ei or Tesla preamp tubes. Rebias the KT66s to around
40ma.

(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. This involves changing the 470k
mixing resistors to 270k, the value of at least one of the small value
caps near the Channel I volume pot, and maybe another resistor or
cap. You'll need to compare the schematics carefully. I don't have
my Marshall Book in front of me, so get your own. This book is a
must have for Marshall owners. Needless to say however, the small
parts swapping changes the tonal contour further.

(4) Change the output tranny to a Mercury Magnetics JTM45 type.
Their repro JTM45 tranny sounds spot-on. The amp sounds
smoother and has 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.

If this interests you remotely, please save these instructions as a
document somewhere on your computer. 

Ted B.