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American Viking · 06-20-2009, 01:25 AM

yeah, my jaw dropped at the very first post. Why did you go about changing all that stuff right off the get-go? One thing you gotta know about tubes is that if the socket fits, it's interchangeable. 6550/KT88/EL34/5881/KT66 no matter what anyone tells you they will all work in the same amp as long as they can fit side by side. All the sockets are the same. In addition to that, modifications aren't necessarily required to make that switch. It all depends on the grade of tube and the window of adjustment combined with your plate voltage to get the desired bias.

So for starters, do you know how to get the proper readings for the equation on how to get the correct bias for the amp? Plate dissipation of desired tube divided by plate voltage of amp, take that number and multiply it by .7 peak performance. If you didn't create this formula then you aren't getting the proper bias. Also the plate dissipation will change depending on any tubes and resistors changed so it should be read every time the amp is biased.
The plate dissipation for all tubes is different as well...
35% for 6550/KT88
30% for 6L6/5881 (most there are some 25% but rare)
25% for EL34/KT77/E34L/6CA7
i forget the KT66 one, either 20% or 15%

Anyway, the only reason you should have to change any resistors in the amp is if the tube you are putting in is "hotter" or "colder" than the way the amp was set up. Like I said, this mainly depends on the tube first and foremost. If one pair rated at 20 is too cold and the amp won't get the tubes hot enough, then switching them out for a pair rated 10 or under may be able to be biased in the given window of the amp and no changes will be necessary.
You should never just change anything before you know what you've got to work with first. I don't think you should have to change anything at all in that amp. At least not right from the start. If anything would need to be switched it would only be the final bias resistor and nothing else. The tubes you're switching between are not so different that they should need any special work done to the amp.

Let us know more about what's going on so we can give you a hand. Hook us up with some numbers to work with, plate voltage, type of tubes, all that junk. Good luck dude.

06-20-2009, 01:25 AM	#12 (permalink)
American Viking Senior Member Join Date: Jan 2009 Location: RICHMOND, VIRGINIA Posts: 1,510	Re: JCM 900 6l6 to EL34 Conversion problems yeah, my jaw dropped at the very first post. Why did you go about changing all that stuff right off the get-go? One thing you gotta know about tubes is that if the socket fits, it's interchangeable. 6550/KT88/EL34/5881/KT66 no matter what anyone tells you they will all work in the same amp as long as they can fit side by side. All the sockets are the same. In addition to that, modifications aren't necessarily required to make that switch. It all depends on the grade of tube and the window of adjustment combined with your plate voltage to get the desired bias. So for starters, do you know how to get the proper readings for the equation on how to get the correct bias for the amp? Plate dissipation of desired tube divided by plate voltage of amp, take that number and multiply it by .7 peak performance. If you didn't create this formula then you aren't getting the proper bias. Also the plate dissipation will change depending on any tubes and resistors changed so it should be read every time the amp is biased. The plate dissipation for all tubes is different as well... 35% for 6550/KT88 30% for 6L6/5881 (most there are some 25% but rare) 25% for EL34/KT77/E34L/6CA7 i forget the KT66 one, either 20% or 15% Anyway, the only reason you should have to change any resistors in the amp is if the tube you are putting in is "hotter" or "colder" than the way the amp was set up. Like I said, this mainly depends on the tube first and foremost. If one pair rated at 20 is too cold and the amp won't get the tubes hot enough, then switching them out for a pair rated 10 or under may be able to be biased in the given window of the amp and no changes will be necessary. You should never just change anything before you know what you've got to work with first. I don't think you should have to change anything at all in that amp. At least not right from the start. If anything would need to be switched it would only be the final bias resistor and nothing else. The tubes you're switching between are not so different that they should need any special work done to the amp. Let us know more about what's going on so we can give you a hand. Hook us up with some numbers to work with, plate voltage, type of tubes, all that junk. Good luck dude. __________________ -I'd rather be a good liver than have one...