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Wurlitzer Upgrading Model to 3700/3800 amplifiers to more reliable/stable 3800 late series amplifier.
Last revision June 19, 2014


Upgrading Wurlitzer (American) amplifier from 556 (3700) to 556 Rev. 3 (3800)
This was a factory recommended improvement (originally two steps), amp runs cooler and is more stable!
The changes follow two procedures, depending on your starting point (check for 
missing/changed parts).
For the Wurlitzer 3700 amplifier and perhaps some early 3800s: Change your amplifier's two sub-PCBs from basic 556 (3700) to (3800) Revision 3 amp:

Parts needed:

2 X 0R5 @ 5W, 2 X 3K6 @ 1/2W, 1 X 1K @ 1/2W, 1 X 3K @ 1/2W, two chassis mount fuse holders
along with two 1.25A slo-blo fuses, 1 X 47/50V cap, 3 X PN2222A transistor, 1 X MPSA56 transistor,
1 X MPSA06 transistor


Instructions:
1) R17 & R18 changed to 0R5 at 5 watts if the original is 2W. 2) R23 & R24 changed from 470R to 3K6R. 3) R12 changed from 2KR to 1KR. 4) R8 changed from 2K7R to 3KR. 5) Add fuses to green wire to each board's output (edge connector pin 4) - 1.25A (1-1/4A) slo-blo. 6) Zener diodes Z2 & Z3 removed. 7) Remove R25 (1K8R) & R22 (1K8R). 8) R21 & R26 (4K7R) removed. 9) Verify that Positive lead of C1 is connected to base of Q1 (reversed from original placement). 10) Change C5 from to 47ufd/50V and that the positive lead is connected to ground. 11) Transistor Q4 (138789-2) changed to PN2222A 12) Change Q1 & Q2 from 142346 to 140622 (many boards have this done already) (original specs called for sub with 2N2919, however 2N2222 or PN2222 should work). 13) Change Q3 & Q11 from 142347 to 142623 (many boards have this done already) (sub with MPSA56). 14) Change Q5 & Q10 from 142348 to 142624 (many boards have this done already) (sub with MPSA06). 15) Change the lable on this board from 140617 to 142315 Rev.3. Note that the 2NXXXX or MPSAXX transistors installed face the OPPOSITE DIRECTION from the original 13/14XXXX parts
16) For C1, C4, C6, and C8 the sheet I have says to "Exchange Component Leads" - not sure what they mean by that, have to find original Tech Tip for clarity...


For the Wurlitzer 3800 amplifier including the 7500 Cabaret, 1050, and SONOTA
Change amp sub-PCBs from 556 "SI" (early 3800s) to 3800 Revision 3 amp - the S1 can be identified by checking if R21 & R26 are missing.

Parts needed:

1 X 1K @ 1/2W, 1 X 3K @ 1/2W, 2 X PN2222A, 1 X MPSA56, 1 X MSPA06, 1 X 47/50V cap.

Instructions:

1) Zener diodes Z2 & Z3 removed. 2) Remove R25 (1K8R) & R22 (1K8R). 3) R12 changed from 2KR to 1KR. 4) R8 changed from 2KR to 3KR. 5) Change Q1 & Q2 from 142346 to 140622 (many boards have this done already) (original specs called for sub with 2N2919, however 2N2222 or PN2222 should work). 6) Change Q3 & Q11 from 142347 to 142623 (many boards have this done already) (sub with MPSA56). 7) Change Q5 & Q10 from 142348 to 142624 (many boards have this done already) (sub with MPSA06). Note that the 2NXXXX or MPSAXX transistors installed face the OPPOSITE DIRECTION from the original 13/14XXXX parts! 8) Verify that Positive lead of C1 is connected to base of Q1 (reversed from original placement). 9) Change C5 from to 47ufd/50V and that the positive lead is connected to ground.
10) For C1, C4, C6, and C8 the sheet I have says to "Exchange Component Leads" - not sure what they mean by that, have to find original Tech Tip for clarity...
11) Label board 142315 Rev. 3.


That completes the changes! If you need clarification please send me a note! John :-#)#
 

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