1980 Marshall JMP 2203 100 watt with KT88s

[Irön/Beast]

I have an 1980 Marshall JMP 100 watt 2203 MKII Master volume amp with a full stack with G12H 30 watt speakers, the amp currently has EH EL34 tubes in them and i have had thoughts about changing to KT88s, most out for experimenting.

I know that it will probubly increase the headroom which is not bad imo. I'd like to try out KT88s to see if they sound better in my ears, and the tubes has to be good, i cannot afford NOS GEC KT88s and has to use modern tubes, now IF i will change to other tubes i will swich both pre and power section tubes whilst doing it. The KT88s should be as true buildwise and sound close to what a true KT88 would sound like, i know there are probubly no modern replica that sounds like the old GECs but at least great close reproduction tubes that is close will do it, AND that they will last. I think i've read from some people having issues with the tubes not keeping their bias, dunno is that low quality tubes then? Preamp tubes should be of similar quality(might as well go same brand?) The tubes cannot cost a fortune, then i might as well try to already obtain some old vintage if that's the case. What tubes can you suggest with all this in mind?

Will i need to change the transformer or will the tubes be to cold with the stock transformer to sing good? It is an 100 watt head afterall, and should have stock transformer from what i know. This is a tube swich afterall and i rather try to avoid having to mod the amp to make it sing good, they say that KT88s doesn't sing well in 50-100 watts amp since the bias and all that is to low to make the tubes hot enought, is that correct? I mean those 200 watt majors blows most modern KT88s from what i've heard, they have different transformers and higher voltage, but is the stock transformer in my amp not enought to make the tubes hot enough to sing great at really high volume? Can it be solved by having high bias or do i need to swich transformer for this? The majors pushes the tubes alot but hey that's alot more voltage going on so i have no idea if the transformer in my head will be able to press them high enough to make them sound great at high volume
Some say KT88s in Super Leads and 2203 jmps sound sterile or cold and whatever since the transformers doesn't push the tubes as good as the ones used in those 200 watt Majors. EL34s supposly are pressed great enought in these Super Lead and 2203 heads so that's no issue but doesn't KT88 take alot more power before they begin to sound just as great cranked?

If it's required to mod for another transformer and other complicated things i might just as well just skip the whole thing since it would become way to expensive.

Any advices?

Irön/Beast

[Roe]

just rebias, something that may require chaning the bias resistor

[DaveMcLain]

They will work similar to a 6550 which means it's easy to give them a try. Then if you feel that the tone works for YOU then it's the way to go. Bias will have to be adjusted and this will probably mean having to swap out a resistor or two in order to get the bias pot to "scale" into a usable range. Also, you might want to experiment with moving the feedback wire to a lower tap on the output side of the amp. I tried this and I felt that the 8 ohm tap was best but it's worth playing with just to see.

I've experimented with using the EH KT88 in a 50 watt Marshall that one of the guitar players in my band uses and he liked them a lot in a club that we play where the stage tends to be loud and muddy. He plays rhythm guitar straight into the amp with no effects or anything and he liked the clarity and punch. They are spectacular when playing a fast rhythm on the low strings. They were also very cool for a "turn down the guitar to clean it up sound". Ultimately the tone is not as smooth or "sweet" as other tube choices but in a real world situation they can be a good solution.

If you use an EH tube their 6550 and KT88 LOOK identical but I've never tested them to know if they are actually the same tubes or not.