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JCM 2000 DSL tube conversion


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5/13/2000 3:35 PM
willie
JCM 2000 DSL tube conversion
Anyone have any experience with converting this amp from El-34 to KT-88? Its not laid out like the older amps. The bias needs to be increased a tad and the ot feedback tap needs to be connected for 4 ohm ops...BUT...this amp appears to defy this mod. The construction is quite different than its predessesors. Anyone done this mod? thanks
 
5/15/2000 3:15 PM
willie

The silence is deafening!!! Guess no one has ever moded EL-34 to KT-88 in a Newer Marshall amp....
 
5/15/2000 4:41 PM
Ray Ivers

Willie,  
 
Just FYI, when I see a seemingly simple question like yours about tube substitution, especially an OUTPUT tube substitution, here are some of the things that I consider before answering, in no particular order:  
 
a) Have I ever done it myself?  
b) heater current  
c) screen voltage and max dissipation  
d) plate voltage and max dissipation  
e) drive method, type of driver tube if any  
f) bias method/circuit, and ease of modifying same  
g) physical installation problems, if any  
h) known cooling issues  
i) output transformer plate-to-plate load impedance, power handling capacity, ultralinear taps if any, secondary taps available  
j) feedback circuits  
k) schematic available? If so, where?  
l) reason for the change?  
 
I'm sure I've left some things out. I just wanted to point out the above because sometimes it can take a little time to formulate an informed answer, one that most probably won't result in tube, amp, or owner destruction. Having said all this, I believe that the KT88 sub will work well in this amp (I have never substituted KT88's for EL34's in a Marshall DSL amp). The Gm of the two tube types appears to be almost identical (!), the heater current of the KT88 is slightly higher , plate and screen power handling is greater - all stuff you're probably aware of. I would say that the amp will sounder a little cleaner with more deep bass and high treble, due to the lower Rp of the KT88's. You will need to increase the negative bias voltage, as you know, but I would think the feedback arrangement can stay as-is to ensure that the Deep switches and presence control continue to function the same. A "JCM2000" schematic is available at:  
 
http://schematics.cjb.net/  
 
The (real) KT88 is my favorite high-power audio tube. I wish you the best of luck in your conversion. I will be back again on 5/23 - let me know if I can be of any further help. I would suggest component substitutions to get you your bias change and maximize bias stability, but as I may have mentioned before in other posts, I'm just not 100 percent sure what amplifier or amplifiers the "JCM2000" schematic mentioned above is for - sort of like having a "JCM800" or "JCM900" schematic, you know? Perhaps you might look it over and see if the values mentioned match what's in your amp - I would very much like to know what amp(s) this print is for.  
 
Ray Ivers  
R.A.G.E. Electronics
 
5/15/2000 5:03 PM
willie

Thanx Ray.....yes all the tube operating parameters were investigated.....this amp is not laid out like the older products and the schematic we downloaded is for a standard jcm2000 not the dsl...  
Actually, you'd think this would be a fairly simple job..but because we are very careful not to degrade the customers property by just "jumping in" and changing values....it has become a bit of a problem.  
We are in touch with Marshall and are hopeful of receiving some further guidance in how they would suggest we get a bit more voltage to the control grids without exceeding the little filter caps max working voltage.....really a decent schematic would be a God send....Thanks again...  
 
musicmenders, inc.
 
5/15/2000 6:04 PM
Ray Ivers

Willie,  
 
I don't know if you are dealing with Marshall U.K. or Korg USA, but there's a tech at Korg (516) 333-9100 named Mike who was quite helpful to me about a 5881-to-EL34 conversion question I had about a Marshall 6100 amp. Also, apparently all the post-JCM900 schematics are now available only as scan files accessable solely to authorized Marshall service centers. With previous model schematics, Laurie at the above number has been very helpful about copying and mailing them to me.  
 
These new multiple-stacked-board Marshalls are a pain to work on, even when you know in advance exactly what has to be done. I realize that in this day and age of multichannel/reverb/adj damping/adj power/adj series-parallel loop/MIDI some complexity is inevitable, but how I long for the turret-board days sometimes!  
 
Ray
 
5/15/2000 6:43 PM
Tonefactor

quote:
"Also, apparently all the post-JCM900 schematics are now available only as scan files accessable solely to authorized Marshall service centers."
 
 
Korg USA will fax or mail any Marshall schematic to anyone who asks. That's been my experience anyway.
 

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