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Marshall "Bluesbreaker" mods?


 :
10/3/1997 4:07 AM
Mike H
 Marshall "Bluesbreaker" mods?
Considering customisation options on Marshall Bluesbreaker (JTM45) reissue combo. (As a reissue it is wildly inaccurate but I picked it up cheap as a spare amp for messing about etc - I already have two early 70's 50w Super Leads).  
 
Existing line-up is : plexi-style 2 channel/4 input with one vol control per channel;  
usual Marshall shared tone stack;  
preamp=2x ECC83,tremolo=1x ECC83  
driver=1x ECC83,power=2x 6L6GC  
 
I am considering following options & wuold welcome any advice/suggestions:  
 
1. Convert output stage for EL34 instead of 6L6 (Bias adjustment only required ?)  
 
2. Install single master volume between phase splitter and output stage (1MegOhm stereo pot after decoupling caps?)  
 
3. Install Pentode/Triode mode switch in output circuit (seems straightforward?)  
 
4. Isolate tremolo circuit and utilise redundant ECC83 as extra gain stage for channel 2 and fit with own volume pot (not sure how much gain/headroom to aim for: suggested plate load/cathode R/C values anyone? or would a variable pot divider on plate load be more useful than a straight volume pot? possibly too ambitious? help?)  
 
None of the above would require drilling of chassis : the two additional pots would replace the redundant speed & intensity controls for the tremolo circuit; the pentode/triode switch can replace tremolo footswitch socket on rear of chassis; spare ECC83 from trem cct already on board! I have studied all the usual FAQ's and it mostly seems feasible but what are the pitfalls?  
 
Any advice/criticism greatly appreciated  
 
Mike.
  
10/13/1997 8:19 PM
Brendan
OK to convert to EL-34's, but check that pin 1 has no additional components attached (6L6 pin 1 is not connected to anything (its connected internally) and sometimes the pin is used as a convenient solder point for connecting other things. Absolutely adjust the bias.  
 
I don't like the master after the driver idea, I mean it can work, but all you are gaining is the driver's gain (and distortion, I know) before the MV. Have you thought about the effect of the bias voltage on the wiper of the pot? What will happen to the bias voltage if the pot is set to zero (wiper at ground potential? You will need isolating caps from the power tube direction as well as the driver stage.  
 
This mod as well as the others listed are not in line with the original Blues breaker, not that I'm a purist, it's your amp, and it's a reissue. What sound are you looking for?  
 
Regards,  
Brendan
  

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