Yeah just to add a few things, a 6550 is not a direct replacement for a KT88 although they are the same pinout and a tetrode.
Max plate voltage on a KT88 is 800V max plate voltage on a 6550 is 600V. So before substituting a KT88 for a 6550 you need to make sure you are within the safe operating area voltage wise, they also bias slightly differently so you really want have adjustable bias in the amp.
Also as mentioned, the KT88 has higher dissipation.
So,
NOT always safe to go from KT88 > 6550. Check plate voltage's first. I would be surprised if the plate voltage on a 275 is more than 600V but these things need to be checked!!
But,
SAFE to go 6550 > KT88
I swap between KT88's and 6550 in my guitar amps depending on what I feel like at the time and what tubes i have lying around, plate voltage is 480V, so all I do is check the bias which sometimes needs a tweak.
I am running GEC KT88's (no they are not for sale and no you can't try them...) in one amp and 6550's in the other at the moment, the difference I have found (in the same amp) is the 6550 has a stronger and more dense mid-range, great for dirty guitar sounds, but the KT88's has more extended frequency range, particularly in the top end which is nice and glassy which a rich "chime" to it....great for cleaner guitar sounds.
How that transposes to a HiFi amp I am not sure....But my 2cents is if the amplifier is designed for KT88's, then that's what I would be using hands down.
If I had a 275, I would just use the tubs McIntosh supply.....There's always theories about this and that, but personally, I would trust McIntosh.
My 2cents.....
I did a quick search and this is the only answer I could find that had any objectivity to it. The others were all about what the author heard. Which is neither bad nor good. I just wanted to see if there was any difference in design and there is. Here is what Bob Latino had to say about the differences. I've used both in my original issue 275. Both before and after a rebuild. The only 6550's I tried were Tung-Sol reissues and they sounded very good. Plus were the least expensive at the time. I was gifted a set of reissue Gold Lions and they are what I use now. I've used almost all the KT-88 brands out there and none have sounded bad to me. I must not be an audiophile as I can't say anything bad about any of them. OK, enough of my rambling here is what Bob had to say.
This has been brought up before and other's may have a different opinion but here is my take on the situation ..
1. KT88's can nearly always be substituted for 6550's and vice versa.
2. KT88's cost a little bit more than 6550's.
3. KT88's can stand a little more voltage than 6550's and *may* be more durable in amps with a very high B+ voltage.
4. As far as sound goes, IMHO, most 6550's have a "tubier" sound than KT88's with perhaps a little more mid-bass content. KT88's generally have a little more output at the very frequency extremes (top and bottom).
Which tube is best for your amp (and system) is probably dependent on your musical tastes and the components you have in your system. Neither tube is "always better" than the other ..
Bob
