Modding a KT88 2203 JCM back to EL34s

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JakeAC5253

JakeAC5253

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I recently picked up an '83 JCM800 2203 which came biased for KT88s and I've already received the quad of EL34s that I'd like to replace them with. I am fairly certain that to mod from EL34s to KT88s you have to adjust for more bias current and up the fuse rating a few levels. What I don't remember in order to reverse this is if any components have to actually be hard modified or if just adjusting the trimpot is enough to do it. Would I have to knock the fuse rating back down again to be safe? If you need more information let me know.
 
I wouldn't worry about the fuse. Turn trim pot all the way counterclockwise and try the EL34's. Sometimes an amp can be set up with enough range but usually not. Install the el34's but do not let the amp warm up much at all. Take it off of standby quickly and watch your bias meter. If the ma starts to rise quickly out of range shut down immediately. The resistor you need to change is the one in line with the first diode and a turret. You will need a higher value there. See what is there now and raise it in 10k increments. My amp is modded. It had a 10k ohm in that spot for 6550/KT88. I increased the value to 23k and it gave me a nice range for el34. I usually just snip it out and place a new one right from the diode to the turret in line in the circuit. This way you do not have to keep soldering until you get the right value. I use alligator clips. If your amp is stock you can probably just use this. This thread is good and all you need. :rock:

http://www.marshallheads.com/viewtopic.php?f=32&t=346
 
WOW! That's great, it even has pictures and everything. That's exactly what I was looking for, and yes, these resistors have been modified on my Marshall. I'm ordering carbon comp resistors to replace them with.

About the bias probe, I was between two things. The Eurotubes Pro One bias probe for 89 and the multimeter attachment for 25. I already have a MM, so I'd probably prefer to get the cheaper attachment, unless the Pro One is like more of a must have thing that does the job better or something. Any input?
 
JakeAC5253":3aozdbon said:
WOW! That's great, it even has pictures and everything. That's exactly what I was looking for, and yes, these resistors have been modified on my Marshall. I'm ordering carbon comp resistors to replace them with.

About the bias probe, I was between two things. The Eurotubes Pro One bias probe for 89 and the multimeter attachment for 25. I already have a MM, so I'd probably prefer to get the cheaper attachment, unless the Pro One is like more of a must have thing that does the job better or something. Any input?
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I just use an amp head probe with my DMV. Never had any problems from my bias being off. If the values are not stock you need to find a good medium on the one in the pic near the diodes which is most critical and the one in front of the bias trim pot. I have a 51k in that spot. I think stock is 47. I have a 27 in the range resistor spot. It allows me enough range to bias a hotter or colder quad in the amp. Once you make the change if you want to go back to 6550/6550 you can just piggyback another 27k over the new 27k you install. I'm sure you know KT88 are basically interchangeable as far as bias goes with 6550.
 
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