Retubing jmp50

[zrAC/DC]

I just recently bought a 1975 jmp 50 that has 6550 tubes. To my understanding in these amps you bias them up to el34 specs because the PT is designed for el34? Anyhow I'm not sure how long the power tubes have been in the amp (peavey labeled chinese 6550), so upon changing filter caps (appear fine, but 37 years old...) I am thinking about changing tubes, but was wondering if kt88's can be used in place of 6550 safely? I read that they draw more current which can put more strain on the PT but i've also read that you can use them so it just basically confused me. If you can use them how would they compare to the 6550? This amp has ALOT of drive, almost too much would kt88's help clear that up, or find different branded 6550, or should I swap out the 3 resistors that appear to have been changed and set it up for el34?

[DaveMcLain]

On my 1974 50 Marshall amps I've experimented with many different output tubes, 6V6, 6L6, KT66(several versions), EL34, 6550 and KT88. They all produce sounds that are somewhat different but I think you'll find that the greatest change is in the dynamics of the amp. With the 6550 or the KT88 the power amp portion of the amp tends to remain cleaner for a greater amount of volume before beginning to distort heavily where as something like an EL34 tends to be looser and begins to distort earlier and then in greater and greater amounts as the volume is turned up. In our experimentation using the Electro Harmonix KT88 we found them to be great for a tight rhythm guitar tone especially when playing quickly on low strings. They are really good for that and we seemed to feel that they were also better when using a humbucker equipped guitar vs a single coil in a Telecaster.

As long as you bias properly experimenting with different output tubes can be a good way to find some interesting tones.

[zrAC/DC]

If I don't have to change out any resistors I am thinking I'll try out a pair of the Shuguang treasure kt88's. First thing's first, I need to change my filter caps.