I just bought a Audio Research D-90.

It's said that it puts out 90 watts,X 2 @ 16 ohms. But what do'es it put out @ 4 and 8 ohms? And can anyone tell me the differance between the 6l6,6550 and 5881 tube's?
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.
Thanks for the info.What you said about the same tubes in differant amps having differant toneal quallity makes perfect sense.I had to hit myself in the head for being so dense.(By the way,please forgive my spelling.)Each tube is going to sound differant from one amp to another.I guess I did'nt think the ? though.I guess what I want to know is if one model of the tubes I mentioned has a better bass reproduction then the others.I'm looking at using this amp as a low end in a bi-amp set-up.(A.R.D-90)with a C.J.45-MV-a1 for the mids and highs.I've been into stereo's for over 30 years,but I'm very new to tubes. The C.J. is my first and the A.R.,my 2nd. So any input is a great help,Thank You,Bill
Bill - In response to your question ..."what I want to know is if one model of the tubes I mentioned has a better bass reproduction than the others?" I thought I answered that in my last post, but I'll try to put it in another way.

An amp designed around the use of a 6550 compared to an amp designed around a 6L6/5881 tube can and often will sound similar. In an amp designed around the use of 6550's but also allows the use of 6L6's will have the same differences only they will be more noticible. I described my perception of these probable differences in my third paragraph. Caveat, educated guesses perhaps, but they are just guesses!

Or, perhaps, are you just asking for a recommendation of which brand of tube to use in the ARC? As opposed to type? I do not know which of the models you have - does the model you have allow you to use either type of tube?

That is easy for 6550's - I would try SED 6550's first. I've used them for years and love their tone and ruggedness. They are often OEM tubes in ARC amps. If you want recommendations for 6L6's or 5881's, I can't help you. I've only used them in one amp and I never experimented. If your amp can use 6L6GC's though it is easy to recommend the SED 6L6GC, another excellent new production tube.

Hope that helps a bit more.....
Thanks for all your help.I ordered 2 sets of tubes today. 8-6L6gc's and 8-6550's.I thought I'd try them both and hear for myself the differents.I'm turning into quiet the tube junkie.My wife thinks I should start selling the ones I don't need on Ebay. She just doesn't get it.I just wanted to Thank You. If you or anyone else has any info, hints or insights that might help me in my new found quest for the perfect tube combo,please write. Thanks,Bill