I just bought a Audio Research D-90.

Usually a tube amp puts out the same wattage into any load, unlike a SS amp which doubles down until it runs out of current, often 4 ohms and in inexpensive amps somewhat above that. A few big tube amps even have 2 ohm taps though but their power is still the same, just more current.

Using 6L6's and 5881's will typically produce about half the power of a 6550. I've never owned your ARC amp - I was unaware that it would take 6l6/5881's, which along with EL 34 have a similar wattage rating, as do E34L's and 6L6GC's (about 5 wts more), all things not being equal. Don't assume they do, check with ARC, if you don't know for sure, and they will certainly require different bias points if they do. And that will vary as well.

Hope that helps a bit.

Thank you for you help. A.R. made 3 model's of D-90's. They are D-90 (4 pairs of 6550's),D-90-revised (4 pairs of 5581's and the D-90B witch also takes (4 pairs of 5881's.
I'm new to tubes,but I do have a C.J.45mv-a1 that I bought a few months ago. I've been rolling tube and driving tube sellers on ebay nuts with ?'s Is there a book out there what would explain the differant qualities of the tubes?
My Mac pre-amp has 2 sets of outputs, So I'm thinking of bi-amping with the C.J. for mids and highs and the A.R. for the lows.I've got several active x-overs so I thought what the he--.
Any input would be more then welcome. Thank you for your time and trouble,Bill

Very minor correction: while the D-90 put out 90 watts per channel into any load, the D-90B is listed by ARC as 80 watts per channel.

Housesound, I'm not exactly sure what you are looking for regarding tube differences. If you are looking for physical descriptions of tubes in general, and what uses to which they are typically put, do some Google research - there are a lot of books about, none of which I own so I can make no recommendation. Maybe someone else will sign in...... But while they will deal with design information I doubt that many (if any) will deal with tonal issues with specific tube type with any specificity.

Regarding the sonics of the tube used in your amp - for example while there will be a difference between an amp designed by ARC for a 6L6 type tube and an amp designed to use a 6550 they may sound more alike than not, i.e. the famous ARC house sound might dominate. If either amp had a provision for substituting in other types of tubes (as many non ARC amps do) the sonic differences would be much more pronounced. It is hard to pre-determine how any type of tube will sound (to you) when you change from that designated by the manufacturer.

I think it would be fair to say that normally you would find an amp with a 6L6/588l tube to be very linear and a little short on low frequency extension. Fairly uncolored. A 6550 whould have more 'balls' so to speak, a bigger sound, and judging by the 6550's I use a small bit of softness in the HF's, but very linear for the most part compared to some other large power tubes.

What strikes me about the different implementation of the tubes by ARC in these amps is that regardless of tube types they are producing similar power. This means to me that they are driving the 6L6's much closer to max than the 6550's. From that I would expect, at least, longer life from the 6550's, and fewer tube failures which can be a PITA with ARC amps.

Sorry I couldn't be of more/better help. BTW, a good source of information from a forum is the Tubes forum at AudioAsylum. Check it out.