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JTM-45 Marshall plexi 6L6 VS. EL34


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4/21/1998 9:24 PM
Derrick JTM-45 Marshall plexi 6L6 VS. EL34
I have a '65-'66 Marshall JTM-45 that is running EL-34's. Someone told me that it is not a good idea to run them in that amp because it can blow the 5AR4 rect. tube. Is it true that it is not a good idea to run them in this amp? I will convert back to 6L6's in this amp soon anyhow and was also wondering if I need to change anything other than the bias resistor (from 220K for EL-34's, back to 150Kfor 6L6's)?
  
4/26/1998 3:37 PM
Ted Breaux
I have two very old JTM45s, both of which use the original GEC KT66 tubes, and sound great. When these became hard to come by, amny people converted these amps to use EL34s by changing bias resistors. Now, the GZ34 isn't quite up to a 50 watt amp, so regardless of what output tube you have in there, you will get some compression via sagging power at high volume levels. I haven't had a GZ34 fail yet, so I wouldn't be concerned about this. In fact, I like the effect. Now, since your amp with EL34s is similar to a plexi 50w Marshall, and you probably want it to sound more like a JTM45, you should strongly consider the new Tesla KT66, which is a great repro of the GEC. Otherwise, opt for a set of good 6L6. I haven't had to convert mine back, but I believe restoring the original bias resistor values will get you in the ballpark. You'll need to eye the power tubes when you get it running, and make sure you aren't running them to hot.  
 
Ted B.
  
4/28/1998 10:11 AM
Derrick
Thanks, Ted. Yes, I agree! I posted this before I realized they did not make a JTM-45 with "GZ-34 and EL-34's". My understanding is that they swiched over to EL-34's after they dropped the GZ34. I also heard they dropped the GZ34 because it wasn't quite sutable to power the EL-34's. (Hmm, I wonder?) Well, I am going with the origional design anyhow! Make Marcus proud! :-)  
 
-Derrick
  

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